Antioxidant, phytochemical, and therapeutic properties of medicinal plants: a review

ABSTRACT

Oxidation is an integral part of aerobic processes of life. It involves the transfer of electrons or hydrogen via a chemical reaction from a substance to an oxidizing agent leading to the production of free radicals. These free radicals which are highly reactive in turn initiate a chain of reactions that lead to cellular damage. The etiology of plethora diseases has been linked to the generation of free radicals beyond the body’s antioxidant capacity, leading to oxidative stress. Consequently, the focus of research has tilted toward plants which provide natural products rich in antioxidants capable of scavenging and distrupting the harmful effects of these free radicals. A large group of compounds produced by plants referred to as phytochemicals possessing high antioxidant properties have been seen to be helpful in tackling numerous diseases. This review covered the antioxidant potential of some plants with medicinal properties beneficial to people, industries, and health institutions who desire their potential benefits. A total of two hundred and fifty plants from the following families; Asteraceae, Combretaceae, Euphorbiaceae, Fabaceae, Lamiaceae, Moraceae and Malvaceae were reviewed. These plants exert important biological properties, such as anti-inflammatory, antioxidant, immunomodulatory, anticancer, and antimicrobial properties, among others.

KEYWORDS:

• Antioxidant activity
• phytochemicals
• medicinal plants
• free radicals
• antioxidant mechanisms

Introduction

Over the years, humans have been faced with various diseases, discomfort and struggles to antagonize it with various approaches.^[1,2] Amongst the numerous approaches employed in combatting ailments is the use of medicinal plants for the treatment of various diseases.^[3,4] Despite the development of various major therapies, the tilt toward herbal medicine is gaining momentum due to the rising concerns of the increasing toxicities associated with main line therapies.^[5–7] In recent times, the use of medicinal plants is considered as a complementary and alternative therapies in combination with other treatments.^[8]

These diseases are mostly linked to the production of free radicals.^[9] Free radicals are an essential part of aerobic life and metabolism.^[10] They are highly indispensable to any biochemical process and are implicated in the etiology of many diseases such as cancer,^[11] Alzheimer’s disease, Parkinson’s disease,^[12] inflammatory disease,^[13] lipid peroxidation,^[14] DNA damage,^[15] celiac disease,^[7] stroke,^[16] cardiovascular disease,^[17] protein oxidation,^[18] and diabetes.^[19–21]

Antioxidants protect cells from damage caused by free radicals. Antioxidants have been shown to slow down or prevent the oxidation of other molecules.^[22] They possess the ability to terminate chain reactions and inhibit oxidation reactions via the removal of radical intermediates and by becoming oxidized themselves.^[23,24] The body system is rich with substances that have the ability stop free radicals formation or limit their damage. These antioxidants can be sourced internally and externally.^[25] Internally made antioxidants are generated via the activity of body enzymes which includes superoxide dismutase (SOD), catalase (Cat) and Glutathione peroxidize.^[26] In contrast, they are sourced externally from foods containing vitamins A, E (alpha tocopherol),^[27] C (ascorbic acid),^[28] minerals,^[29] and polyphenols^[30,31] which are predominantly plant based.^[19]

Plants contain numerous antioxidants which help to confer protection against free radicals associated diseases.^[3,32] The antioxidant compounds are mostly produced in plants in the form of secondary metabolites. Phytochemicals can be literally referred to as ‘plant-chemicals.’ They are the non-nutritive chemical components of plants that possess numerous health benefits and disease prevention properties.^[33] The nutrients they contain are non-essential, i.e, they are not required by the body for sustaining life.^[34] These chemicals are produced by plants to sustain life which in turn confer health benefits to humans upon consumption.^[35,36] There are over a thousand known phytochemicals classified as primary or secondary constituents based on their role in plant metabolism.^[37]

Phytochemicals classified as primary constituents includes the common sugars, amino acids, chlorophyll’s, purines and pyrimidines of nucleic acids and proteins etc.^[5,38] Others classified as the secondary constituents are the chemicals consisting of alkaloids, flavonoids, terpenes, phenolics, lignans, plant steroids, curcumines, saponins, glucosides.^[38,39] Of these secondary constituents, phenolics are seen to be the most numerous consisting of 45% of the secondary phytochemical constituents of plants, terpenoids and steroids 27%, alkaloids 18% and others 10%.^[40] Phytochemicals possess nutraceutical importance.^[41] They are the bioactive constituents that maintain health and serve as a bridge between the food and pharmaceutical industries. Phytochemicals perform numerous functions.^[42] They possess unique pharmacological effects such as anti-inflammatory, antiplasmodic, anti-allergic, antioxidants, antibacterial, antifungals, chemopreventive, neuroprotective, hypotensive, antiaging, etc.^[43] They stimulate the immune system, block the formation of carcinogens, reduce oxidation, slow the growth rate of cancer cells, reduce inflammation, trigger apoptosis, prevent DNA damage, regulate hormones such as estrogen and inulin which excess levels are linked with increased risk of breast and colon cancer.^[44]

Polyphenols are major dietary phenolics comprising the polyphenols (hydrolysable and condensed tannins), phenolic acids (hydroxybenzoic and hydroxycinnamic acids) and flavonoids. Flavonoids are the most extensively studied group of polyphenols.^[38] The major dietary sources of polyphenols are legumes (pulses and beans), cereals (corn, barley, oats, sorghum, rice and wheat), nuts, oilseeds (rapseed, flaxseed, olive seeds and canola) beverages (fruit juices, tea, coffee, beer, wine and cocoa), fruits and vegetables.^[43] The subclasses of phenols include flavones, flavanols and minor flavonoids (flavanones and dihydroflavonols). They exhibit their antioxidant potentials by preventing the decomposition of hydroperoxide into free radicals and by inactivating free radicals. Flavonoids play important roles in preventing diseases associated with oxidative stress. It has the capacity to transport electrons to free radicals, inhibit oxidases, reduce radicals of alpha tocopherol, activate antioxidant enzymes and chelate metals.^[45] They help to block angiotensin converting enzyme (ACE) that raises blood pressure. They have also been found to block enzymens that produce estrogen implicated in breast cancer and inhibit cyclooxygenase which has been known to form prostaglandins.

Numerous methods can be applied in determining the antioxidant activities and capacities of various plants. These methods are broadly divided into two major categories; invitro and invivo.^[43] The invitro methods involves all assays carried out outside the living organism. They spectrophotometrically measure the reaction of antioxidants with chromogenic radicals such as 2,2ʹ-azino-bis-3-ethylbenzotiazolin-6-sulfonic acid (ABTS+), 2,2ʹ-diphenyl-1-picrylhydrazyl (DPPH) or its ability to reduce iron from +3 to +2 oxidations states (FRAP).^[46] In addition, scientific literature has witnessed a growing number of publications on the evaluation of the efficacy of medicinal plants which have made important contribution to the maintenance of health via numerous mechanisms. Despite this applaudible advancement, there still exist a paucity of information on the updated comprehensive compilation of promising medicinal plants from different flora.^[46,47]

The overall aim of this study was to comprehensively highlight the importance, uses, phytochemical content, isolated antioxidant compounds, antioxidant content (total phenol and total flavonoid contents) and the antioxidant activities (DPPH, FRAP) of selected medicinal plants with the aim of their short term or long-term development into future phytopharmaceuticals for the treatment or management of a wide range of diseases. Many medicinal plants Amaranthus hybridus L., Anarcadium occidentale L., Allium sativum, Vernonia amygdalina, Mangifera indica, Mondora myristica, Uvaria chamae, Xylopia aethiopica, Newbouldia laevia, Garcinia kola, Telifaira occidentalis, Carica papaya, Mucuna puriens, Ocimum gratissium, Thymus vulgaris, Rhizophora mangle, Zingiber officinale, Solanium aethiopicum L., Mentha arvensis, Anisopus mannii, were considered.

Medicinal plants with antioxidant potential

Tables 1 and 2 provide information on medicinal plants with antioxidant potential. Several reports have linked free radicals to the occurrence of several ailments like diabetes, cancer, cardiovascular and neurological diseases.^[57] Antioxidants, on the other hand, have the capacity to quench these free radicals sourced both exogenously and endogenously.^[125–127] Plants are naturally endowed with antioxidant and radical scavenging properties.^[128] There are several plant constituents that protect the cells from damage caused by free radicals. Plants with medicinal importance and antioxidants properties mainly have phenols and flavonoids as their main constituents. These constituents have the ability to scavenge these free radicals due to their structures.^[129]

Table 1. Antioxidant profiles of medicinal plants.

S/N	PLANTS	Plant Parts	Solvents	TPC (mg GAE/g DM)	TFC (mg QE)	DPPH (µg/ml)	FRAP (µg/ml)	REFERENCES
1.	Amaranthus hybridus L.	Leaf	Methanol	40.01	18.40	83.45	232.01	Adetutu et al.^[Citation48] Nana et al.^[Citation49] Amabye^[Citation50] Muniz-Marquez et al.^[Citation51] Karamac et al.^[Citation44]
			Hydroacetone	-	-	56.02	257.31
			Aqueous	-	-	42.31	245.16
		Seed	Methanol	31.20	-		-
		Whole plant	Aqueous	-	-	94.73	-
6.	Anarcadium occidentale L.	Leaf	Methanol	604.85	76.31	-	471.21	Duangjan et al.^[Citation52] Souza et al.^[Citation53] Baptista et al.^[Citation54]
			Aqueous	374.11	-	71.80	-
			Ethanol	402.61	-	89.80	-
			Butanol	-	-	7.77	305.61
			Ethyl acetate	-	-	5.66	402.12
			Dichloromethane	-	-	85.13	281.30
			Hexane	-	-	157.49	163.91
			Crude	-	-	9.94	303.82
		Stem bark	Methanol	660.52	76.86	-	-
			Ethyl acetate	-	-	358.18	-
		Cashew nut	Methanol	611.13	70.39	-	-
17.	Allium sativum	Whole plant	Methanol	70.18	-	-	-	Wani and Basir^[Citation55] Benkeblia^[Citation56]
			Ethanol	73.20	-	-	-
			Acetone	59.59	-	-	-
			Aqueous	49.81	-	-	198.67
		Bulb	Aqueous	25.70	12.67	24.02	-
			Ethanol	63.60	34.10	45.23	-
			Methanol	67.02	44.13	51.02	-
24.	Vernonia amygdalina	Leaf	Aqueous	70.64	-	340.22	-	Alara et al.^[Citation57] Imafidon et al.^[Citation58] Oluwaseun et al.^[Citation59] Habtamu and Melaku^[Citation60] Foo et al.^[Citation61]
			Ethanol	96.25	-	636.01	-
			Ethyl acetate	38.83	-	658.28	-
			Chloroform	-	-	-	54.10
			Acetone	681.70	23.70	-	91.60
29.	Mangifera indica	Leaf	Crude	230.00	131.01	-	-	Laulloo et al.^[Citation62] Prakash et al.^[Citation40]
			Ethanol	186.00	191.02	-	-
			Methanol	99.01	46.10	-	-
		Seed	Aqueous	-	-	41.20	59.68
30.	Mondora myristica	Seeds	Crude extract	14.78	41.2	-		Lawal et al.^[Citation3] Erukainure et al.^[Citation63]
			Aqueous	27.60	37.03	62.05	15.01
32.	Uvaria chamae	Leaf	Methanol	61.01	4.01	70.69		Nwakaego et al.^[Citation64] Iwu et al.^[Citation65]
		Roots	Methanol	69.31	3.01	76.13	-
			Aqueous	-	-	-	14.35
			Ethanol	-	-	-	9.00
36.	Xylopia aethiopica	Seed	Aqueous	375	492.88	19.01	492.88	Esekhiagbe et al.^[Citation66] Lawal et al.^[Citation3] Oso and Oladiji^[Citation67]
			Methanol	428.25		52.45	73.45
38.	Newbouldia laevia	leaf	Aqueous	91.49	2242	155.17	-	Lawal et al.^[Citation3] Habu and Ibeh^[Citation68]
			Ethanol	-	-	-	1225.05
40.	Garcinia kola	Seed	Aqueous	30.30	10.80		-	Ogunmoyole et al.^[Citation69] Okoko^[Citation70] Gulumian et al.^[Citation71]
			Ethanol	-	-	84.50	-
			Methanol	-	-	-	33.81
		leaf	Ethanol	45.20	29.20	163.50	-
44.	Telifaira occidentalis	leaf	Methanol	4932	-		-	Adetutu et al.^[Citation48] Lawal et al.^[Citation3]
			Aqueous	16.04	10.49	21.06	8.10	Jimoh^[Citation72]
46.	Carica papaya	Leaf	Aqueous	14.53	5.47	7.80	-	Swati and Shivam^[Citation73] Maisarah et al.^[Citation74] Sheneni et al.^[Citation75]
		Leaf	Ethanol	424.89	333.14		103.87
		Seed	Aqueous	30.32	59.54	1.00	-
		Fruit (Ripe)	Aqueous	272.66	92.95	6.50	-
		Fruit (Unripe)	Aqueous	339.91	59.54	4.30	-
51.	Mucuna puriens	Seed	Aqueous	46.44	2.25	98.70	-	Kavitha^[Citation76] Agbafor and Nwachukwu^[Citation77]
			Ethanol	-	-	96.75	41.40
			Acetone	-	-	98.05	-
			Petroleum ether	-	-	83.77	-
			Chloroform	-	-	61.04	-
56.	Ocimum gratissium	Leaf	Ethanol	3.60	-	79.90	51.20	Ganiyu^[Citation78] Igbinosa et al.^[Citation79] Gedikoglu et al.^[Citation80] Shukia et al.^[Citation81]
			Acetone	10.21	12.03	85.45	346.51
			Methanol	19.21	15.57	80.50	508.19
			Aqueous	12.02	11.50	78.50	159.83
		Root	Petroleum ether	3.42	-	-	-
			Diethyl ether	48.82	-	-	-
			Acetone	15.44	-	-	-
			Aqueous	8.42	-	-	-
64.	Thymus vulgaris	Whole plant	Methanol	14.59	7.29	36.77	30.88	Gedikoglu et al.^[Citation80] Patil et al.^[Citation82] Mahmoodi et al.^[Citation83]
			Ethanol	17.31	6.17	29.22	26.93
			Acetone	14.60	-	-	-
			Aqueous	12.01	-	159.59	3.25
68.	Rhizophora mangle	Leaf	Ethanol	63.73	110.10	15.04	-	Cruz et al.^[Citation84] Miranti et al.^[Citation85] Sanchez et al.^[Citation86]
		Bark	Ethanol	53.89	36.56	24.01	-
			Aqueous	-	-	-	31.90
		Root	Ethanol	59.86	35.16	21.43	-
72.	Zingiber officinale	Whole plant	Methanol	88.01	12.80	9.66	-	Lawal et al.^[Citation3] Gbasemzadeh et al.^[Citation87] Mao et al.^[Citation88]
			Ethanol	96.16	12.30	-	-
			Acetone	71.01	10.01	-	-
			Aqueous	65.27	9.34	-	-
		Leaf	Methanol	-	-	-	579.60
		Stem	Methanol	-	-	-	568.27
		Rhizome	Methanol	-	-	-	767.20
73.	Solanium aethiopicum L.	Fruit	Aqueous	3.89	27.59	65.04	54.96	Khatoon et al.^[Citation43] Eletta et al.^[Citation89]
74	Mentha arvensis	Leaf	Ethanol	75.90	44.14	56.04	-	Wani and Basir^[Citation55] Gaafar et al.^[Citation90]
			Methanol	60.20	29.83	47.13	-
			Aqueous	30.54	17.20	33.69	190.69
77.	Anisopus mannii	Whole plant	Methanol	53.20	40.01	93.59	100.01	Aliyu et al.^[Citation91]
			Ethyl acetate	59.60	58.01	93.71	111.32
			n-Butanol	56.40	57.03	91.02	120.17

Table 2. Isolated antioxidant compounds in medicinal plants.

S/N	Plants	Phytochemical content	Plant Parts	Solvents	Isolated Compounds	References
1.	Amaranthus hybridus L.	Phenols, flavonoids, phlobatannins, saponins, tannins, terpenoids, triterpernoids, courmarins, resins, balsams	leaf	-	Quercetin (flavonol), Ritin, Myricetin, Betalains	Sivakumar et al.^[Citation92]
2.	Anarcadium occidentale L.	Phenols, flavonoids, phlobatannins, saponins, tannins, terpenoids, triterpernoids, courmarins, resins, balsams	leaf	Methanol	Quercetin 3-O – α-D-glucopyranoside	Salehi et al.^[Citation93]
					Kaempferol −3-O-β-D-glucopyranoside	Gulumian et al.^[Citation71]
				Ethyl acetate	Agathisflavone (bliflavonoid)	Ajileye et al.^[Citation94]
					Quercetin 3-O- rutinoside	Ajileye et al.^[Citation94]
					Quercetin 3-O-rhamnoside	Ajileye et al.^[Citation94]
7.	Allium sativum	Flavonoids, phenols, phlobatannins, saponins, tannins, terpenoids, triterpenoids, coumarins, resins and balsam	Plant		Allicin, Diallyl sulfide (DAS), Diallyl disulfide (DADS), Diallyl trisulfide (DATS), S-allyl-cysteine, Alliin, E-ajoene, Z-ajoene, β-resorcyclic acid, pyrogallol, acrolein, phytocidin, scordinin gallic acid, rutin, protocatechuic acid, quercetin, B1-rhamnose, sativoside R!, sativoside, voghieroside D1.	Safar et al.^[Citation95] Safar et al.^[Citation95] Moutia et al.^[Citation96]
8.	Vernonia amygdalina	Flavonoids, phenols, phlobatannins, saponins, tannins, terpenoids, triterpenoids, coumarins, resins and balsam	leaf	Aqueous	Sesquiterpene lactones, Flavonoids (luteolin, luteolin 7-O-glucosides and luteolin 7-O glucuronide), Steroid glycosides, Vernonioside A, B, A1, A2, A3, B2, B3 and A4, Edotides.	Farombi and Owoeye^[Citation97]Gulumian et al.^[Citation71]
				Dicloromethane	Sesquiterpene lactones, Epivernadalol, elemanolide	Alara et al.^[Citation57]
10.	Mangifera indica	Phenols, flavonoids, saponins, steroids, tannins, anthraquinones and glucosides.		Ethanol and Methanol	Phenolic compounds (Protacatechic, Gallic acid, Methyl gallate, 2,5-Di-tert-butylphenol, Sodium gallate, Tetrahydroxy sodium benzoate, Derivatives of gallic acid, Derivatives of theogallin with on OH missing, theogallin)	Laulloo et al.^[Citation62] Palafox-Carlos et al.^[Citation98] Jahurul et al.^[Citation99] Kim et al.^[Citation100] Rasoanaivo et al.^[Citation101] Sani et al.^[Citation102] Ramirez et al.^[Citation103] Dorta et al.^[Citation104]
					Xanthones (Mangiferin, Isomangiferin, Mangiferin-3-methly ether, Mangiferin-6’-O-gallate)	Laulloo et al.^[Citation62]
					Flavonoids (Quercetin, Kaemferol, Rhamnetin, Quercetin carboxylic acid, Quercetin pentoside, Quercetin 3-O- rhamnoside, Quercetin 3-O- glucoside Epicatechin gallate hexamalonate, Rhamnetin hexoside)	Palafox-Carlos et al.^[Citation98] Dorta et al.^[Citation104]
					Benzophenones (Maclurin, Iriflophenone glucoside derivative, Iriflophenone 3-C-β-D-glucopyranoside, Maclurin 3-C-(6”-O-p-hydroxybenzoyl)β-D-glucoside, Iriflophenone tri-O-galloyl-glucoside)	Laulloo et al.^[Citation62] Gulumian et al.^[Citation71]
					Terpenoids (Lupeol, Mangieronic acid, Manglanostenoic acid, Cycloart-25-ene-3,24,27-triol, Cycloartane-3,24,25-triol)	Shailajan et al.^[Citation105] Laulloo et al.^[Citation62] Nguyen et al.^[Citation106] Ansari et al.^[Citation107]
					Gallotannins (Digalloyl glucoside, Tri-O-galloyl glucoside, Tetra-O-galloyl glucoside, Penta-O-gallose-glucose)	Laulloo et al.^[Citation62] Palafox-Carlos et al.^[Citation98] Jahurul et al.^[Citation99] Kim et al.^[Citation100] Rasoanaivo et al.^[Citation101]
					Ferulic acid Hexoside	Palafox-Carlos et al.^[Citation98]
11.	Mondora myristica	Phenols, flavonoids, tannins, alkaloids, phlobatannins, terpenoids, steroids, saponins and essential oils.			Catechin, phenol, Phenylacetic acid, Salicyclic acid, myrcene, cinnamic acid, Protocatechuic acid, carvacrol, gentisic acid, p-coumaric acid, vanillic acid, safole, eugenol, isoeugenol, gallic acid, methyl isoeugenol, methyly eugenol, elemicin, myristicin, caffeic acid, ferulic acid, syringic acid, piperic acid, sinapinic acid, daidzein, coumestrol, genistein, apigenin, naringenin chalcone, naringenin, shogaol, glycitein, kaempferol, luteolin, capsaicin, epicatechin, epigallocatechin, gingerol, myricetin, isorhamnetin, quercetin, 3-o-caffeoylquinic, chlorogenic acid, rosmarinic acid, curcumin, miquellanin, eriocitrin, rutin, papain, phenyl-6-o-malonyl-beta-D-glucoside, 4-o-methyl-epi-gallocatechin, Epi-gallocatechin-3O-gallate, lupeol and Eriocitrin.	Feyisayo and Oluokun^[Citation108]
12.	Uvaria chamae	Alkaloid, Flavonoid, Saponins, Steroids, Glycosides, tannins and phenols	Seed	Ethanol	Flavonoids, triterpernoids, phenols, afzeliindanone, uvafzelic acid, 2-hydroxydemethoxym at-teucinol and essential oils such as phytol, hexadecanoic acid.	Ofeimun et al.^[Citation109] Iwu et al.^[Citation65] Ita^[Citation110] Moukimou et al.^[Citation111]
13.	Xylopia aethiopica	Terpenoids, Phenols, flavonoids, cardiac glycosides., Alkaloids, saponins, tannins, phlobatannins, anthraquinones, steroids.	Seed		Essential oils such as α-pinene, β-pinene, 1,8-cineole, α-phellandrene, β-myrcene, Germacrene D, Caryophyllene oxide, α-terpinene, Camphene, Sabinene, α-eudesmol, β-eudesmol and α-cubebene.	Yin et al.^[Citation112] Biney et al.^[Citation113]
14.	Newbouldia laevia	Cyanogenic gylcosides, cardiac glycodies, steroid glycosides, saponins, tannins, alkaloids, amino acids, terpenoids, flavonoids carboxylic acids, aldehyde/ketones, ascorbic acid and anthracene derivatives.	leaf		Gallic acid, Catechin, Chlorogenic acid, Caffeic acid, ellagic acid, epicatechin, rutin, Verbacoside, Anthraquinone, isoquercetin, quercetin and Kaempferol.	Habu and Ibeh^[Citation68] Kolodziej^[Citation114]
15.	Garcinia kola	Phenols, flavonoids, tannins, alkaloids, phlobatannins, terpenoids, steroids, saponins.	Seed	Petroleum spirit	Cycloartenol, 24-methylene derivatives	Farombi and Owoeye^[Citation97] Gulumian et al.^[Citation71]
				Ethyl ether soluble fraction of the alcoholic extract	Apigenin-5,7,4’-trimethly ether, apigenin-4’-methlyether, fisetin, amen-flavone, kolaflavanone and GB1,Chromanols, garcinoic acid, garcinal, δ-tocotrienol
			Seed and Stem bark	Ethyl acetate soluble fraction of the acetone extract	C-3/8”-link biflavanone GB1, GB2, GB1a and Kolaflavanone, kolaviron.
18.	Telifaira occidentalis	Phenols, flavonoids, tannins, alkaloids, phlobatannins, terpenoids, steroids, saponins.	leaf	methanol	Flavonoids Kaempferol-3-O-rutinoside, Kaempferol	Aderogba et al.^[Citation115]
					Phenols Gallic acid, Catechin, chlorogenic acid, Caffeic acid, Ellagic acid, Epicatechin, Rutin, Quercetin, Isoquercetin	Jimoh^[Citation72]
20.	Carica papaya	Alkaloids, saponins, tannins, phlobatannins, anthraquinones, steroids, terpenoids, flavonoids, cardiac glycosides.	Seed	Acetone	Phenolic acid (Caffeic acid, ferulic acid, ρ-hyrdoxybenzoic acid, ρ-coumaric acid)	Kadiri et al.^[Citation116]
					Flavonoids (Kaempferol-3-glucoside, Quercetin-3-galactoside)
22.	Mucuna puriens	Flavonoids, cardiac glycosides. alkaloids, saponins, tannins, phlobatannins, anthraquinones, steroids, terpenoids.	Seeds		Tetrahydroisoquinoline (methylated and non-methylted), Phenolics, flavonoids	Gulumian et al.^[Citation71] Kavitha^[Citation76]
23.	Ocimum gratissium	Alkaloids, saponins, tannins, phlobatannins, anthraquinones, steroids, terpenoids, flavonoids, cardiac glycosides.	Plants Seeds Aerial plant part		Apigenin, Xanthomicrol, rutin, Kaempferol, luteolin-5-glycoside, 3-O-rutinoside, luteolin-7-glycoside, vitexin, isovitexin, quercetin-3-O-glucoside, isothymusin, vicenin-2, Thymol, eugenol, methyl chavical, gratissimol, alkaloids, tannins, flavonoids, p-cymene, terpene, trans sablene hydrate, euginol, 1,8-cineole, linalool, methyl chavicol, methyl eugenol, uronic acid, farnesene, oleonolic acid, T-cadinol, β-caryopophyllene, methyl isoeugeneol, α-humelene and β-elemene.	Bhavani et al.^[Citation117] Gulumian et al.^[Citation71]
24.	Thymus vulgaris	Phenols, flavonoids, tannins, alkaloids, phlobatannins, terpenoids, steroids, saponins and essential oils.	Whole plant	Ethanol Methanol	Gallic acid, 4-hydroxybenzoic acid, Chlorogenic acid, Syringic acid, Coumaric acid, Rutin, Benzoic acid, Cinnamic acid, Rosmarinic acid, Quercetin and essential oils such as eugenol, methyl chavical, gratissimol, alkaloids, tannins, flavonoids, p-cymene, terpene, trans sablene hydrate, euginol, 1,8-cineole, linalool, methyl chavicol, methyl eugenol, uronic acid, farnesene, oleonolic acid, T-cadinol, β-caryopophyllene, methyl isoeugeneol, α-humelene and β-elemene and thymol	Gedikoglu et al.^[Citation80]
25.	Rhizophora mangle	Alkaloids, saponins, tannins, phlobatannins, anthraquinones, steroids, terpenoids, flavonoids, cardiac glycosides.	Fruit		Caffeine, N,N-dimethyl-L-alanine, quercetin-3-O-galactopyranoside, dodecanoic acid, Quercetin, Taninn, procyanidin and prodelphinidin	Miranti et al.^[Citation85]
26.	Zingiber officinale	Phenols, flavonoids, cardiac glycosides. Alkaloids, saponins, tannins, phlobatannins, anthraquinones, steroids, terpenoids.			6-shogaol, Ginger oleoresin, Ginger phenylpropanoids, 6-gingerol-rich fraction, ginger extract, 6-dehydroshogaol, Zingerone, gingerenone, 10-gingerol, 10-shogaol, paradols and essential oils (β-bisabolene, α-curcumene, zingiberene, α-farnesene and β-sesquiphellandrene.).	Mao et al.^[Citation88] Romero et al.^[Citation118] Peng et al.^[Citation119] Chen et al.^[Citation120] Saiah et al.^[Citation121] Luettig et al.^[Citation122]
27.	Solanium aethiopicum L.	Terpenoids, Phenols, flavonoids, cardiac glycosides., Alkaloids, saponins, tannins, phlobatannins, anthraquinones, steroids.		Leaf Root Aerial parts	Quercetin, Kaempferol, rutin, isoquercetin, (1-3)-β-D-quinovopyranoside, solagenin 6-O-β-D-quinovo pyranoside, sitosterol, stigmasterol, campesterol and tetratriacontanoic acid	Zubaida et al.^[Citation123]
28.	Mentha arvensis	Alkaloids, saponins, tannins, phlobatannins, anthraquinones, steroids, terpenoids, flavonoids, cardiac glycosides.			Chlorogenic acid, Caffeic acid, Epicatechin, p-coumaric acid, Ferulic acid, Benzoic acid, Rutin, trans-cinnamic acid, quercetin, Kaempferol, Rosamarinic acid, lutein, riboflavin, violaxanthin, antheraxanthin, zeaxanthin, 13Z-β-Carotene, α-carotene, 9Z-β-carotene.	Park et al.^[Citation46]
29.	Anisopus mannii	Terpenoids, phenols, flavonoids, cardiac glycosides, alkaloids, saponins, tannins, phlobatannins, anthraquinones, steroids.			Phytol, phenols, flavonoids, 1,7-naphthyridine (an alkaloid named anisopusin), 5α-hydroxy-lup-20(29)-en-3β-yl eicosanoate,^[Citation6]-gingerdione,^[Citation6]-dehydrogingerdione and ferulic acid	Musa et al.^[Citation124]

Phenolic compounds are a group of secondary metabolites with highly effective free radical scavenging activity, inhibition of hydrolytic and oxidative enzymes and anti-inflammatory action.^[130] Flavonoids are a group of phenolic compounds with beneficial abilities ranging from their ability to scavenge a wide range of oxygen, chlorine and nitrogen species such as hydroxyl ions, peroxynitrous acid, superoxide, reactive oxygen, peroxylradicals^[131] and hypochlorious acid to their ability to chelate ions by decreasing the metal ions pro-oxidant capacity.^[132]

Antioxidant compounds have different polarity which accounts for the varying concentration obtained depending on the effect of solvent used in the extraction or isolation of these chemical entities. Solvents can be widely grouped into polar (ethanol, methanol, aqueous, etc) and nonpolar solvents (acetone, ethyacetate, hexane etc) depending on their ability to form ions in solution. The yield of the extraction process varies widely with high dependence on the solvent and the method of extraction employed.^[46]

In this study, we evaluated the total phenol, flavonoid, DPPH and FRAP activities of various plant parts of Amaranthus hybridus L., Anarcadium occidentale L., Allium sativum, Vernonia amygdalina, Mangifera indica, Mondora myristica, Uvaria chamae, Xylopia aethiopica, Newbouldia laevia, Garcinia kola, Telifaira occidentalis, Carica papaya, Mucuna puriens, Ocimum gratissium, Thymus vulgaris, Rhizophora mangle, Zingiber officinale, Solanium aethiopicum L., Mentha arvensis and Anisopus mannii using various polar and nonpolar solvents as shown in Table 1. Figures 1 and 2 show some of the mechanisms for antioxidant activities in live cell.

Figure 1. Antioxidant assays on live cell based on chemical stress inducers. (A) using H₂O₂ as stress inducer in catalase-like assay; The antioxidant activities can be obtained as the capability of inhibiting H₂O₂-induced cellular effects, e.g., DNA degradation or cell apoptosis. (B) Cell antioxidant assay using 2,2′-azobis(2-amidino propane) dihydrochloride (AAPH) as the stress inducer. 2′,7′-dichlorofluorescin diacetate is trapped inside the cell as 2′,7′-dichlorofluorescin, which could transform by peroxidation products into the fluorescent 2′,7′-dichlorofluorescein; The antioxidant effects are determined as the capacity of inhibiting AAPH-induced lipid peroxidation formation (Adapted from^[133]).

Figure 2. Photo-induced ROS production-based antioxidant/oxidant balance (AOP)1 assay, live cell antioxidant assay. (1) prior to photoinduction, there is massive removal of TO from cell by efflux transport proteins; (2) the initiation of photoinduction occurs via energy transfer from thiazole Orange (TO) to triplet state ‘molecular oxygen’ forming singlet oxygen, followed by free radicals (ROS); (3) The free radicals alter the efflux transport of TO and other functions of the cell; (4) TO’s massive entry triggers fluorescence emission increase. The effect can be measured as the capability of antioxidants to quench the production of ROS, ensuring TO is kept out of the cells, causing low fluorescence (Adapted from^[133]).

Iron (Fe) plays numerous vital roles in the body such as the transport of oxygen, cellular respiration, proper cell functioning and it is a co-factor in many enzymes.^[110] However, when in excess, Fe can catalyze Fenton reactions and hydroperoxide decomposition which are capable of accelerating oxidative stress.^[46] As a result, an important antioxidant property of any antioxidant compound is their reductive capacity in a Fe³⁺ – Fe²⁺ system. Reducing power of extracts measures their ability to donate electrons, serving as an important indicator of antioxidant capacity. DPPH assay has also been widely applied in numerous studies to evaluate the radical scavenging ability of various antioxidant compounds.^[80] The various extracts of the twenty medicinal plants showed varying reducing power potentials and DPPH scavenging activities. The results shown in Table 1 indicates that each of the plants possess unique FRAP and DPPH scavenging activities.

Amaranthus hybridus (Linn.)

Amaranthus hybridus (Linn.) is commonly known as spinach which is used majorly as a part of diet which could be consumed raw or cooked.^[92] It contains diverse medicinal and nutritional properties. These properties includes antidiabetic, anti-microbial, gastroprotective, anti-inflammatory, anti-malarial, antinoceceptive, cardioprotective, hepatoprotective and antioxidant effects.^[92] The antioxidant profile of this plant (Table 1) showed that the whole plant, leaf and seed parts of Amaranthus hybridus possesses high antioxidant activities. The total flavonoid content of the methanol extract of the leaf is 18.40 mg QE. The total phenol content of the methanol extract of the leaf (40.01 mg GAE/g DM) is higher than the seed (31.20 mg GAE/g DM). The aqueous extract of the whole plant exhibits highest DPPH scavenging activity, followed by the methanol extract of the leaf (83.45 µg/ml), hydroacetone extract (56.02 µg/ml) and Aqueous extract (42.31 µg/ml). The Ferric reducing power activity of the various leaf extracts is highest in the hydroacetone extract (257.31 µg/ml) followed by the aqueous extract (245.16 µg/ml) then the methanol extract (232.01 µg/ml). Its high DPPH and FRAP activities could be as a result of its flavonoid and phenol contents amongst other phytochemicals such as phlobatannins, saponins, tannins, terpenoids, triterpernoids, courmarins, resins and balsam. Other antioxidants present include quercetin (flavonol), rutin, myricetin, betalains in the plant^[92] (Table 2).

Anarcadium occidentale

Anarcadium occidentale L.is a member of the Anacardiaceae. This medicinal plant has a wide variety of biological activities ranging from anti-viral, anti-bacterial, anti-fungal and anti-inflammatory activities. The antioxidant profile of this plant (Table 1) showed that the leaf and stem bark possess antioxidant properties. The total phenol content is highest in its methanol extract of the stem bark (660.52 mg GAE/g DM), the cashew nut (611.13 mg GAE/g DM) and the leaf (604.85 mg GAE/g DM) followed by the other extracts of the leaves; ethanol (402.61 mg GAE/g DM) and aqueous (374.11 mg GAE/g DM) extracts, respectively.^[94] The total flavonoid content in the methanol extract of the leaf (76.31 mg QE), stem bark (76.86 mg QE) and cashew nut (70.39 mg QE) are similar. The antioxidant activities of the ethyl acetate extract of the stem bark and different extracts of the leaves varied extensively. The ethylacetate extract of the stem bark exhibit the highest DPPH scavenging activity of 358.18 µg/ml followed by the various extracts of the leaf. The hexane leaf extract exhibited the highest DPPH scavenging activity of 157.49 µg/ml followed by the ethanol (89.80 µg/ml), dichloromethane (85.13 µg/ml), aqueous (71.80 µg/ml), methanol (9.94 µg/ml), butanol (7.77 µg/ml) and ethyl acetate (5.66 µg/ml) respectively.^[94] This showed that hexane is the best extracting solvent compared to other solvents in determining the DPPH activity of this plant. The FRAP assay revealed that the methanol extract had the highest activity of (471.21 µg/ml) compared to other extracts; ethyl acetate (402.12 µg/ml), butanol (305.61 µg/ml), methanol (303.82 µg/ml), dichloromethane (281.30 µg/ml) and hexane (163.91 µg/ml).^[94]

The antioxidant activities of this plant parts could be as a result of the presence of various antioxidant compounds and phytochemical contents (Table 2). Particularly, the methanol extract maintained an overall high antioxidant property due to the presence of Quercetin 3-O – α-D-glucopyranoside and Kaempferol −3-O-β-D-glucopyranoside compounds of high antioxidant properties in its fraction.^[134] Also, the ethyl acetate fraction was found to contain Agathisflavone (bliflavonoid), Quercetin 3-O- rutinoside and Quercetin 3-O-rhamnoside antioxidant compounds.^[94]

Allium sativum L

Allium sativum L. belongs to the family of Amaryllidaceae. Its medicinal uses includes the treatment of atheroclerosis, hyperlipidemia, hypertension, diabetes mellitus, bacterial infections, cancer, fever, dyspepsia, intestinal worms and tuberculosis.^[38,135] The antioxidant profile (Table 1) showed that the whole plant and its bulb possesses significant antioxidant properties. The total phenol content was highest in the ethanol extract of the whole plant (73.20 mg GAE/g DM), followed by the methanol extract of the whole plant (70.18 mg GAE/g DM), methanol extract of the bulb (67.02 mg GAE/g DM), Acetone extract of the whole plant (59.59 mg GAE/g DM) and lastly the aqueous extracts of the whole plant (49.81 mg GAE/g DM) and bulb (25.70 mg GAE/g DM) respectively. The high phenol content of the methanol and ethanol extract shows that they are the most suitable solvents for extracting the phenols compounds present in the plant. The total flavonoid content of the bulb was shown to range from 44.13 (mg QE), 34.10 (mg QE) to 12.67 (mg QE) of the methanol, ethanol and aqueous extracts respectively. The DPPH scavenging activity of the bulb was highest in the methanol extract (51.02 µg/ml) followed by the ethanol extract (45.23 µg/ml) and least in the aqueous extract (24.02 µg/ml). The FRAP activity of the aqueous extract was seen to be very significant (198.67 µg/ml).^[136]

The antioxidant activities of this plant parts could be as a result of the presence of various antioxidant compounds and phytochemical contents (Table 2). This plant was found to contain allicin, diallyl sulfide (DAS), diallyl disulfide (DADS), diallyl trisulfide (DATS), S-allyl-cysteine, phytocidin, acrolein, alliin, E-ajoene, Z-ajoene, β-resorcyclic acid, pyrogallol, scordinin,^[95] gallic acid, rutin, protocatechuic acid, quercetin, B1-rhamnose, sativoside, voghieroside D1 compounds, all of which have high antioxidant properties.^[137,138] This further justifies its wide medicinal use and pharmacological importance as an anti-hypercholestrolemic, antihypertensive, antiviral, carminative, stimulant, cholagogue, tonic, blood purifier, febrifuge, rubifacient, antibiotic, anti-allergic, aphrodisiac, antifungal, diuretic, antiplasmodic, anticoagulant, antirheumatic, and immunostimulatory effects by enhancing mitogemic activity toward human peripheral blood lymphocytes, thymocytes and murine spenocytes.^[55,96]

Vernonia amygdalina

Vernonia amygdalina belongs to the Asteraceae family. It is commonly called bitterleaf due to its bitter taste. Its therapeutic uses include the treatment of dysentery, gastrointestinal tract disorders, diabetes,^[139,140] loss of appetite-induced ambrosia amongst others.^[141] The total phenol content of various extracts of the leaf of Vernonia amygdalina (Table 1) ranged from 681.70 mg GAE/g DM (Acetone extract) to 38.33 mg GAE/g DM (ethyl acetate extract). The polar solvents ethanol and aqueous extracts exhibited moderate total phenol content of 96.25 mg GAE/g DM and 70.64 mg GAE/g DM, respectively.^[58] The total flavonoid content of the acetone extract was 23.70 mg QE. The DPPH scavenging activity of the plant was highest in the ethyl acetate extract 658.28 (µg/ml) followed by the ethanol extract (636.01 µg/ml) and the aqueous extract (340.22 µg/ml).^[61] The FRAP assay revealed acetone to be a better extract (91.60 µg/ml) compared to chloroform (54.10 µg/ml).^[60] Overall, acetone exhibited a better extracting capacity when compared to other solvents used such as ethanol, chloroform, ethylacetate and aqueous extracts.^[59] The high antioxidant capacity of the leaves of Vernonia amygdalina can be greatly attributed to the presence of isolated antioxidant compounds which includes sesquiterpene lactones, flavonoids (luteolin, luteolin 7-O-glucosides and luteolin 7-O glucuronide), steroid glycosides, vernonioside A, B, A1, A2, A3, B2, B3 and A4, edotides as reported by Farombi and Owoeye.^[97] The plant contains complex active components such as essential oils, flavonoids, phenols, phlobatannins, saponins, tannins, terpenoids, triterpenoids, coumarins, resins and balsam etc.^[156] with antioxidant potentials.

Mangifera indica

Mangifera indica is a nutritional and medicinal important plant. It contains biomolecules from different plant parts such as stem bark, leaves, fruits and its seed kernels. It is used in the treatment of numerous diseases and has been found to possess antimicrobial properties.^[62] The total phenol content of the leaves of Mangifera indica was highest in the crude extract (230.00 mg GAE/g DM) followed by the ethanol extract (186.00 mg GAE/g DM) and methanol extract (99.01 mg GAE/g DM). The total flavonoid content was highest in the ethanol extract (191.02 mg QE), followed by the crude extract (131.01 mg QE) and least in the methanol extract (46.10 mg QE).^[40] The DPPH scavenging activity of the aqueous seed extract was 41.20 µg/ml, its FRAP activity was higher, 59.68 µg/ml.^[62] The overall high antioxidant activity could be accounted for as a result of the presence of a vast number of isolated antioxidant compounds which includes Phenolic compounds (Protacatechic, Gallic acid, Methyl gallate, 2,5-Di-tert-butylphenol, Sodium gallate, Tetrahydroxy sodium benzoate, Derivatives of gallic acid, Derivatives of theogallin with one OH missing, theogallin); Xanthones (Mangiferin, Isomangiferin, Mangiferin-3-methly ether, Mangiferin-6ʹ-O-gallate); Flavonoids (Quercetin, Kaemferol, Rhamnetin, Quercetin carboxylic acid, Quercetin pentoside, Quercetin 3-O- rhamnoside, Quercetin 3-O- glucoside Epicatechin gallate hexamalonate, Rhamnetin hexoside); Benzophenones (Maclurin, Iriflophenone glucoside derivative, Iriflophenone 3-C-β-D-glucopyranoside, Maclurin 3-C-(6”-O-p-hydroxybenzoyl)β-D-glucoside, Iriflophenone tri-O-galloyl-glucoside); Terpenoids (Lupeol, Mangieronic acid, Manglanostenoic acid, Cycloart-25-ene-3,24,27-triol, Cycloartane-3,24,25-triol); Gallotannins (Digalloyl glucoside, Tri-O-galloyl glucoside, Tetra-O-galloyl glucoside, Penta-O-gallose-glucose) and Ferulic acid Hexoside as shown in Table 2. It contains some phytochemical constituents such as phenols, flavonoids, saponins, steroids, tannins, anthraquinones and glucosides.^[62,157]

Monodora myristica

Monodora myristica belongs to the family of Annonaceae. It is commonly called Orchid flower tree (English), ‘Ehuru’ (Igbo), ‘Abo lakoshe’ (Yoruba), ‘Ehinawosin’ (Ikale), ‘Uyengben’ (Bini), ‘Fausse noix de muscade’ (French).^[142,143] Every part of the tree was economically and medicinally important. The seeds of Monodora myristica are used as spices or condiments.^[144] They are used in the treatment of sores, constipation^[145] and as a stimulant with palm oil. The total phenol content, flavonoid content, DPPH scavenging activity and FRAP activity can be found in Table 1. The total phenol content of the seeds (Table 1) is highest in the aqueous extract (27.60 mg GAE/g DM) and lower in the crude extract (14.78 mg GAE/g DM). The total flavonoid content was highest in the crude extract (41.20 mg QE) and lower in the aqueous extract (37.03 mg QE).^[63] The DPPH scavenging and FRAP activity of the aqueous seed extract was 62.05 µg/ml and 15.01 µg/ml respectively.^[3] The phytoconstituents of the seed includes essential oils, phenols, flavonoids, tannins, alkaloids, phlobatannins, terpenoids, steroids and saponins.^[146] Table 2 also shows the profile of isolated antioxidant compounds from the seed extract of Monodora myristica. The content includes Catechin, phenol, Phenylacetic acid, Salicyclic acid, myrcene, cinnamic acid, Protocatechuic acid, carvacrol, gentisic acid, p-coumaric acid, vanillic acid, safole, eugenol, isoeugenol, gallic acid, methyl isoeugenol, methyly eugenol, elemicin, myristicin, caffeic acid, ferulic acid, syringic acid, piperic acid, sinapinic acid, daidzein, coumestrol, genistein, apigenin, naringenin chalcone, naringenin, shogaol, glycitein, kaempferol, luteolin, capsaicin, epicatechin, epigallocatechin, gingerol, myricetin, isorhamnetin, quercetin, 3-o-caffeoylquinic, chlorogenic acid, rosmarinic acid, curcumin, miquellanin, eriocitrin, rutin, papain, phenyl-6-o-malonyl-beta-D-glucoside, 4-o-methyl-epi-gallocatechin, Epi-gallocatechin-3O-gallate, lupeol and Eriocitrin.^[108] These numerous bioactive compounds which accounts for its antiemetic, aperients, stimulant, stomachic, tonic functions.^[147] They impart stimulating properties when added to medicines. It also possesses the potential of reducing coronary heart disease when consumed due to its high unsaturated fatty acid content.^[148]

Uvaria chamae

Uvaria chamae belongs to the Annonaceae family. It is widely distributed in the eastern and southern parts of Nigeria. Its local names include “gbogbonishe” in Yoruba, “Umimi ofia” in Igbo and “Osu-umimi” in Ukwani.^[110,149] It is used for the treatment of cough, infections of the liver, kidney and bladder, vaginal tumor, jaundice and gonorrhea.^[65,150] The total phenol, total flavonoid, DPPH scavenging and FRAP activity of various extracts of the leaves and roots of the plant were shown in Table 1. The total phenol content of the methanol extract of the roots and leaves were 69.31 mg GAE/g DM and 61.01 mg GAE/g DM respectively.^[111] The total flavonoid contents of the methanol extracts of the leaves and roots are 4.01 mg QE and 3.01 mg QE respectively. The DPPH scavenging activity of the methanol extracts of the leaves and roots are 70.69 µg/ml and 76.13 µg/ml respectively. The FRAP activity of the roots of the plant are higher in the aqueous extract (14.35 µg/ml) compared to the ethanol extract (9.00 µg/ml). The antioxidant potential of the plant could be accounted for by its phytochemical constituents such as flavonoids, triterpernoids, phenols, afzeliindanone, uvafzelic acid, 2-hydroxydemethoxym at-teucinol. As shown in Table 2, these compounds account for its antioxidant activities.^[65,109]

Xylopia aethiopica

Xylopia aethiopica belongs to the family of Annonaceae. Its common name is Ethiopian pepper seed. Its local names are Sesedo (Yoruba) and Uda (Igbo). It is a small tree plant whose fruits are used as spice and as an appetite stimulant.^[3] The plant extracts possess antifungi, ant-inflammatory, antioxidant and hepatoprotective properties.^[3,66] The total phenol, total flavonoid, DPPH scavenging and FRAP activity of various extracts of the leaves and roots of the plant were shown in Table 1. The total phenol content of the methanol extract (428.25 mg GAE/g DM) is higher than the aqueous extract (375.00 mg GAE/g DM). The total flavonoid content of the aqueous seed extract is 492.88 mgQE. The DPPH scavenging activity of the methanol extract (52.45 µg/ml) is significantly higher than the aqueous extract (19.01 µg/ml). However, the FRAP activity of the aqueous seed extract (492.88 µg/ml) is significantly higher than the methanol extract (73.45 µg/ml).^[67] These antioxidant properties could be attributed to the presence of various essential oil constituents with antioxidant potentials as shown in Table 2. These includes α-pinene, β-pinene, 1,8-cineole, α-phellandrene, β-myrcene, Germacrene D, Caryophyllene oxide, α-terpinene, Camphene, Sabinene, α-eudesmol, β-eudesmol and α-cubebene.^[67]

Newbouldia laevis

Newbouldia laevis (P. Beauv) belongs to the family Bignoniaceae and grows to a height of about 7–8 meters. Its therapeutic effect has been described on many diseases where oxidative stress is implicated. It has antimalarial, antimicrobial, antidiabetic, anti-inflammatory and antioxidant properties.^[151,152] It is used in the treatment of epilepsy, and convulsion in children.^[68,153] Newbouldia laevis can inhibit lipid peroxidation, has high vitamin C content, significant antioxidant activity and high phenol content. The plant leaf also scavenged free radicals as tested by DPPH scavenging assay and hydrogen peroxide assay.^[2] The total phenol, total flavonoid, DPPH scavenging and FRAP activity of various extracts of the leaves and roots of the plant were shown in Table 1. The total phenols, flavonoids and DPPH scavenging activities of the aqueous leaf extracts were 91.49 mg GAE/g DM, 2242 mgQE and 155.17 µg/ml. The FRAP activity of the extract of the leaf is 1225.05 µg/ml. These antioxidant properties could be attributed to the presence of various isolated antioxidant compounds in the plant (Table 2) such as Gallic acid, Catechin, Chlorogenic acid, Caffeic acid, ellagic acid, epicatechin, rutin, Verbacoside, Anthraquinone, isoquercetin, quercetin and Kaempferol.^[154]

Garcinia kola

Garcinia kola belongs to the family of Clusiaceae. The seed has a characteristic bitter taste and is highly valued as a stimulant. The seeds are extensively used in the treatment of diarrhea, bronchitis and throat infections.^[97] They possess the numerous pharmacological properties acting as an antipugative, antinephrotoxic, antimicrobial, an aphrodisiac and an antioxidant.^[71,97] The total phenol, total flavonoid content, DPPH scavenging and FRAP activities of various extract of the seed and leaf parts of the plant can be seen in Table 1. The total phenol content of the ethanol extract of the leaf (45.20 mg GAE/g DM) was higher than that of the aqueous extract of the seed (30.30 mg GAE/g DM). Same trend was observed with the total flavonoid content of the ethanol and aqueous extracts of the leaf (29.20 mgQE) and seed (10.80 mgQE) respectively, likewise the DPPH scavenging activity of the ethanol leaf extract (163.50 µg/ml) and the ethanol seed extract (84.50 µg/ml) of which the latter was found to be higher.^[69,70] The FRAP activity of the methanol extract of the seed is 33.81 µg/ml.^[70,97] These antioxidant potentials could be as a result of the presence of several isolated antioxidant compounds in the plant parts (Table 2), these compounds include Cycloartenol, 24-methylene derivatives, Apigenin-5,7,4ʹ-trimethly ether, apigenin-4ʹ-methlyether, fisetin, amen-flavone, kolaflavanone and GB1, Chromanols, garcinoic acid, garcinal, δ-tocotrienol, C-3/8ʹʹ-link biflavanone GB1, GB2, GB1a and Kolaflavanone and kolaviron.^[97]

Telifairia occidentalis

Telifairia occidentalis belongs to the family of Cucurbitaceae. It is also called fluted pumpkin, Ugu (Igbo). It is highly consumed due to its nutritional and medicinal values.^[72] Its therapeutic uses include the treatment of hypercholesterolemia, anemia, diabetes,^[155] impaired immune system, chronic fatigue and liver problems. Its phytoconstituents includes essential oils, flavonoids, phenols, phlobatannins, saponins, tannins, terpenoids, triterpenoids, coumarins, resins and balsam etc.^[72] The total phenol, total flavonoid content, DPPH scavenging and FRAP activities of various extract of the leaf of the plant are shown in Table 1. The total phenol content of the methanol and aqueous extracts of the leaves are 4932 mg GAE/g DM and 16.04 mg GAE/g DM respectively. The total flavonoid content, DPPH scavenging activity and the FRAP activity of the aqueous leaf extract are 10.49 mgQE, 21.06 µg/ml and 8.10 µgrespectivelyvely. Table 2 shows the isolated antioxidant compounds found in the methanol extract of the leaves, these include kaempferol-3-o-rutinoside, kaempferol, gallic acid, catechin, chlorogenic acid, caffeic acid, ellagic acid, epicatechin, rutin, quercetin, isoquercetin.^[72] These compounds accounts for the antioxidant activities of Telifairia occidentalis.^[48]

Carica papaya

Carica papaya belongs to the Carica family. The fruits, leaves and seeds are all highly beneficial for both nutritional and medicinal purposes.^[75] This plant also has wide spread use in the treatment of a number of oxidative stress related diseases. The total phenol, total flavonoid content, DPPH scavenging and FRAP activities of various extract of the leaf of the plant are shown in Table 1. The total phenol content of the various plant extracts followed the order of ethanol leaf extract (424.89 mg GAE/g DM), the aqueous unripe fruit extract (339.91 mg GAE/g DM), the aqueous ripe fruit extract (271.66 mgGAE/gDM), the aqueous seed extract (30.32 mg GAE/g DM) and the least being the aqueous leaf extract (14.53 mg GAE/g DM). The total flavonoid content followed a similar trend with the ethanol leaf extract as the highest (333.14 mgQE), followed by the aqueous ripe fruit extract (92.95 mg QE), the aqueous extract of the unripe fruit (59.54 mgQE), seed (59.54 mg GAE/g DM) and the aqueous leaf extract as the least (5.47 mgQE). The DPPH scavenging activity of the aqueous extracts of the seed, ripe fruit and unripe fruit were 1.00 µg/ml, 6.50 µg/ml and 4.30 µg/ml. The FRAP activity of the ethanolic leaf extract was 103.87 µg/ml. This antioxidant potential can be accounted for by the presence of diverse isolated antioxidant compounds and phytochemical constituents in the plant (Table 2), including caffeic acid, ferulic acid, ρ-hyrdoxybenzoic acid, ρ-coumaric acid, kaempferol-3-glucoside, quercetin-3-galactoside.^[116]

Mucuna puriens

Mucuna puriens belongs to the Fabaceae family. It is a widely consumed legume with numerous associated medicinal benefits aside from its nutritional importance. It has mostly been used in the management of numerous inflammation and oxidative stress related diseases.^[76] The total phenol content of the aqueous extract of the seed of the plant is 46.44 mg GAE/g DM, the total flavonoid content is 2.25 mg QE. The DPPH scavenging activity of the various extracts are 98.70 µg/ml, 96.75 µg/ml, 98.05 µg/ml, 83.77 µg/ml and 61.04 µg/ml for the aqueous, ethanol, acetone, petroleum ether and chloroform extracts respectively. The FRAP activity of the ethanol seed extract is 41.40 µg/ml. Its antioxidant activity could be attributed to its bioactive compounds such as tetrahydroisoquinoline (methylated and non-methylted), phenolics and flavonoids shown in Table 2.^[71]

Ocimum gratissium

Ocimum gratissium belongs to the family of Lamiaceae. It is a wild herb commonly known as African Basil. It is a native to tropical Africa with hairy leaves, scented flowers and a distinct mint flavor.^[117,158] It possesses a high antioxidant activity which accounts for its hepatoprotective property.^[158] The total phenol, total flavonoid content, DPPH scavenging and FRAP activities of various extract of the leaf of the plant are shown in Table 1. The total phenol content of various extracts of the leaf were 19.21 mg GAE/g DM (methanol extract); 12.02 mg GAE/g DM (aqueous extract); 10.21 mg GAE/g DM (Acetone extract) and 3.60 mg GAE/g DM (ethanol extract). The total phenol contents of the roots were reported in values of 48.82 (mg GAE/g DM), 15.44 mg GAE/g DM (Acetone), 8.42 mg GAE/g DM (Aqueous) and 3.42 mg GAE/g DM (Petroleum ether). The total flavonoid contents of the leaf extracts were reported in values of 15.57 mgQE (methanol), 12.03 mgQE (acetone) and 11.50 mgQE (aqueous). The DPPH scavenging activity of the leaf extracts were 85.45 µg/ml (acetone extract), 80.50 µg/ml (methanol extract), 79.90 µg/ml (ethanol extract) and 78.50 µg/ml (aqueous extract). The FRAP activity of the leaf extracts were highest in the methanol extract (508.19 µg/ml), followed by the acetone extract (346.51 µg/ml), the aqueous extract (159.83 µg/ml) and the least was the ethanol extract (51.20 µg/ml).^[80,117] The antioxidant potentials of this plant could be accounted for by the presence of antioxidant compounds in its plant parts as shown in Table 2, these include thymol, eugenol, methyl chavical, gratissimol, alkaloids, tannins, flavonoids, p-cymene, terpene, trans sablene hydrate, euginol, 1,8-cineole, linalool, methyl chavicol, methyl eugenol, uronic acid, farnesene, oleonolic acid, T-cadinol, β-caryopophyllene, methyl isoeugeneol, α-humelene and β-elemene.^{[79–81,117]}

Thymus vulgaris

Thymus vulgaris belongs to the Lamiaceae family. It is a commonly known as thyme. The leave is the part of the part with a high antioxidant potential due to its major phytoconstituents, flavonoids. The part of the plant mostly used are the leaves. It is widely used as a drug, perfume, home remedy, spice and insecticide. Its medicinal properties include antispasmolytic, antibacterial, antifungal, secrotolytic, expectorant, antitussive and antlemintic.^[136,159] Table 1 shows the total phenol, total flavonoid content, DPPH scavenging and FRAP activities of various extract of the leaf of the plant. The total phenol contents of the whole plant extracts have been shown to be 14.59 mg GAE/g DM (methanol extract), 17.31 mg GAE/g DM (ethanol extract), 14.60 mg GAE/g DM (acetone extract) and 12.01 mg GAE/g DM (aqueous extract). The total flavonoid contents of the methanol and ethanol extracts are 7.29 mgQE and 6.17 mgQE respectively. The DPPH scavenging activities of the plant extracts are 36.77 µg/ml and 29.22 µg/ml for the methanolic and ethanolic extracts respectively. Also, the FRAP activities of the plant extracts were 30.88 µg/ml and 26.93 µg/ml for the methanolic and ethanolic extracts respectively.^[80] The antioxidant properties exhibited in Table 1 could be accounted for by the presence of a wide array of active compounds with high antioxidant potentials found in the plant (Table 2). These bioactive compounds includes Gallic acid, 4-hydroxybenzoic acid, Chlorogenic acid, Syringic acid, Coumaric acid, Rutin, Benzoic acid, Cinnamic acid, Rosmarinic acid, Quercetin and essential oils such as eugenol, methyl chavical, gratissimol, alkaloids, tannins, flavonoids, p-cymene, terpene, trans sablene hydrate, euginol, 1,8-cineole, linalool, methyl chavicol, methyl eugenol, uronic acid, farnesene, oleonolic acid, T-cadinol, β-caryopophyllene, methyl isoeugeneol, α-humelene and β-elemene and thymol.^[80]

Rhizophora mangle

Rhizophora mangle belongs to the Rhizophoraceae family. The plant bark extract has been shown to contain polyphenols, carbohydrates and sterols and exhibits hydroxyl radicals scavenging activity.^[25,84] Table 1 shows the total phenol, total flavonoid content, DPPH scavenging and FRAP activities of various extract of the leaf of the plant. The total phenol contents of the plant extracts are 63.73 mg GAE/g DM (ethanol leaf extract), 53.89 mg GAE/g DM (ethanol stem bark extract) and 59.86 mg GAE/g DM (ethanolic root bark extract). The total flavonoid contents are 110.10 mgQE (ethanol leaf extract), 36.56 mgQE (ethanol stem bark extract) and 35.16 mgQE (ethanol root bark extract). The DPPH scavenging activities of the plant are 15.04 µg/ml (ethanol leaf extract), 24.01 µg/ml (ethanol stem bark extract) and 21.43 µg/ml (ethanol root bark extract). The FRAP activity of the aqueous extract of the plant bark is 31.90 µg/ml.^[84] Table 2 shows the profile of isolated antioxidant compounds present in the plant fruits as well as their phytochemical constituents. These include caffeine, N,N-dimethyl-L-alanine, quercetin-3-O-galactopyranoside, dodecanoic acid, quercetin, tannin, procyanidin and prodelphinidin.^[85]

Zingiber officinale

Zingiber officinale belongs to the family of Zingiberaceae. It is a commonly used spice and herb. The roots are used in the treatment of numerous diseases such as headaches, nausea, cold and emesis.^[88] The total phenol content of various extracts of the whole plant shows that the ethanol extract showed the highest phenol content of 96.16 mg GAE/g DM followed by the methanol extract 88.01 mg GAE/g DM, the acetone extract 71.01 mg GAE/g DM, and the least being the aqueous extract 65.27 mg GAE/g DM. The total flavonoid content of the whole plant showed that the ethanol (12.30 mg QE) and methanol extract (12.80 mg QE) have higher activities, followed by the acetone extract (10.01 mg QE) and then the aqueous extract (9.34 mg QE).^[3] The DPPH scavenging activity of the methanol extract of the whole plant was 9.66 µg/ml. The FRAP activity of the methanol extracts of the leaf, stem and rhizome was found to be 579.60 µg/ml, 568.27 µg/ml and 767.20 µg/ml respectively.^[136] The high antioxidant property of this plant can be accounted for by the presence of numerous isolated antioxidant compounds in the plant parts such as 6-shogaol, ginger oleoresin, ginger phenylpropanoids, 6-gingerol-rich fraction, ginger extract, 6-dehydroshogaol, Zingerone, gingerenone, 10-gingerol, 10-shogaol, paradols and essential oils.^[88] The essential oil components of this plant include β-bisabolene, α-curcumene, zingiberene, α-farnesene and β-sesquiphellandrene.^[121]

Solanium aethiopicum L

Solanium aethiopicum L. Belongs to the Solanaceae family. It has diverse uses, ranging from weight reduction to the treatment of a vast number of diseases such as asthma, allergy, skin infections, swollen joint pains, constipation, dyspepsia, gastroesophageal reflux amongst others.^[89] It equally has several nutritional importance and uses especially in soup and stew making.^[43] Table 1 shows the total phenol, total flavonoid content, DPPH scavenging and FRAP activities of various extract of the leaf of the plant. The total phenol content, total flavonoid content, DPPH scavenging activity and FRAP activity of the aqueous fruit extract of the plant were 3.89 mg GAE/g DM, 27.59 mgQE, 65.04 µg/ml and 54.96 µg/ml respectively.^[43] The various antioxidant compounds present in this plant parts that accounts for their antioxidant potential are shown in Table 2. They include quercetin, kaempferol, rutin, isoquercetin, (1-3)-β-D-quinovopyranoside, solagenin 6-O-β-D-quinovo pyranoside, sitosterol, stigmasterol, campesterol and tetratriacontanoic acid.^[43,123]

Mentha arvensis

Mentha arvensis belongs to the Lamiaceae family. It is widely used with numerous nutritional and medicinal uses as a spice and in the treatment of cancer, diabetes, chronic inflammation and other numerous oxidative stress related diseases.^[3] The total phenol, total flavonoid content, DPPH scavenging and FRAP activities of various extract of the leaf of the plant can be seen in Table 1. The total phenol contents of the leaf extracts are shown to be 75.90 mg GAE/g DM, 60.20 mg GAE/g DM and 30.54 mg GAE/g DM for the ethanol, methanol and aqueous extracts respectively. The total flavonoid contents of the leaf extracts are shown to be 44.14 mgQE, 29.83 mgQE and 17.20 mgQE for the ethanol, methanol and aqueous extracts respectively. Also, the DPPH scavenging activity of the leaf extracts are 56.04 µg/ml, 47.13 µg/ml and 33.69 µg/ml for the ethanolic, methanolic and aqueous extracts respectively. The FRAP activity of the aqueous leaf extract is 190.69 µg/ml. The antioxidant properties may be accounted for by the presence of antioxidant compounds in the plant part as shown in Table 2, which includes Chlorogenic acid, Caffeic acid, Epicatechin, p-coumaric acid, Ferulic acid, Benzoic acid, Rutin, trans-cinnamic acid, quercetin, Kaempferol, Rosamarinic acid, lutein, riboflavin, violaxanthin, antheraxanthin, zeaxanthin, 13Z-β-Carotene, α-carotene, 9Z-β-carotene.^[46] It also contains numerous phytochemicals such as anthocyanins, phenyl propanoids, saponins, phenols etc., all of which several health benefits such as anti-hepatotoxic, antitumor, antimicrobial, and inflammatory activities.

Anisopus mannii

Anisopus mannii belongs to the family of Asclepladaceae. It is used in the treatment of diarrhea, diabetes and pile.^[124] The total phenol, total flavonoid content, DPPH scavenging and FRAP activities of various extract of the leaf of the plant can be seen in Table 1. The total phenol, flavonoids, DPPH scavenging and FRAP activities of the methanol, ethylacetate and n-butanol extracts of the whole plant were reported in many studies. The values were 53.20 mg GAE/g DM, 40.01 mgQE, 93.59 µg/ml and 100.01 µg/ml respectively for the methnolic extract; 59.60 mg GAE/g DM, 58.01 mgQE, 93.71 µg/ml and 111.32 µg/ml respectively for the ethyl acetate extract and 56.40 mg GAE/g DM, 57.03 mgQE, 91.02 µg/ml and 120.17 µg/ml respectively for the n-Butanol extract. Table 2 also shows the array of bioactive compounds found in the plants which includes Phytol, phenols, flavonoids, 1,7-naphthyridine (an alkaloid named anisopusin), 5α-hydroxy-lup-20(29)-en-3β-yl eicosanoate,^[6]-gingerdione,^[6]-dehydrogingerdione and ferulic acid.^[91,124] The recent studies on medicinal plants, their phytochemical and biological properties are summarized in Table 3.

Table 3. Recent studies done on medicinal plants, their phytochemical and biological properties.

Title of study	Type of study	Objectives	Findings and conclusion	References
“Indigenous Uses, Phytochemical Analysis, and Anti-Inflammatory Properties of Australian Tropical Medicinal Plants”	Review	The study reviewed over 78 medicinal plants used against many conditions, including inflammation/inflammatory-related conditions.	The study reported that crude extracts or isolated pure compounds from at least 45 species of plants studied showed important biological properties, such asanti-inflammatory properties. Over 83 phytochemicals were associated with many anti-inflammatory actions. Betulinic acid, Hispidulin, Alphitolic acid, Malabaric acid, etc. reduced cyclooxygenase enzymes (COX-1 and 2) and proinflammatory cytokines.	Yeshi et al.^[Citation160]
“Biological properties and polyphenols content of Algerian Cistus salviifolius L. aerial parts”	Research	The study aimed to evaluate the bioactive compounds such as polyphenols in Cistus salviifolius L. and their in vitro and in vivo biological properties	The study reported that the extracts from Cistus salviifolius L. have antioxidant, antibacterial, antifungal, and anti-inflammatory properties. Many phytochemicals such as vanillic acid, orientin, gallic acid, quercetin, tannins, etc. were reported to be present in these extracts	Boubekeur et al.^[Citation161]
“Changes in Physicochemical, Free Radical Activity, Total Phenolic and Sensory Properties of Orange (Citrus sinensis L.) Juice Fortified with Different Oleaster (Elaeagnus angustifolia L.) Extracts”	Research	The study characterized the changes in the chemical, physical, biological, and sensory properties of Citrus sinensis L. Juice fortified with Oleaster Elaeagnus angustifolia L. extracts	The study found that the aqueous oleaster extracts (20% and 25%) caused significant physicochemical differences in the juice, while both phenolic compounds and free radical activity significantly increased after 30 days	Sarvarian et al.^[Citation162]
“Natural Antioxidants in Foods and Medicinal Plants: Extraction, Assessment and Resources”	Review	This review described the extraction methods, assessment, and sources of antioxidants and other phytochemicals in foods and medicinal plants	The study provides wide-ranging information on the green technologies for extracting natural antioxidants, assessing their antioxidant activities at cellular and chemical levels, and their sources from medicinal plants and foods.	Xu et al.^[Citation163]
“Effect of Extraction Methods on Polyphenols, Flavonoids, Mineral Elements, and Biological Activities of Essential Oil and Extracts of Mentha pulegium L.”	Research	The study assessed the in vitro antioxidant, antibacterial, antifungal, anti-inflammatory, and chemical properties of essential oils, phenols, and flavonoids in Mentha pulegium L.	The study demonstrated the in vitro antioxidant, antibacterial, antifungal, anti-inflammatory, and chemical properties of essential oils, phenols, and flavonoids in Mentha pulegium L.	Messaoudi et al.,^[Citation164]
“Live Cell Assays for the Assessment of Antioxidant Activities of Plant Extracts”	Review	Reviewed the live assays of antioxidants in plants	Described the mechanisms of live cell assays of antioxidants in plants	Furger et al.^[Citation133]
“Physicochemical, antioxidant, and polyphenolic attributes of microencapsulated freeze-dried kinnow peel extract powder using maltodextrin as wall material”	Research	The study investigated the physicochemical, polyphenolic, and antioxidant properties of encapsulated lyophilized powder of kinnow peel extract with maltodextrin wall material.	The research showed that the maltodextrin obtained from broken rice starch is a promising microencapsulation wall material for powder from citrus peel extract	Rafiq et al.^[Citation165]
“Roles of Medicinal Plants in the Diagnosis and Treatment of Eumycetoma”	Chapter	The chapter aimed to throw light on the medicinal plants and phytochemicals that can be used to treat Eumycetoma, one of the neglected tropical diseases	The chapter summarized that some medicinal plants such as Nigella sativa, Boswellia papyrifera, Acacia nilotica, Moringa oleifera, etc., can be used to treat eumycetoma. Phytochemicals such as stigmatriene, beta-amyrin, sitosterol, beta-amyrone, etc. Were also identified to be effective in treating the disease	Awuchi et al.^[Citation166]
“Bioactive Compounds Effective Against Type 2 Diabetes Mellitus: A Systematic Review”	Systematic review	This systematic review aimed to provide information on the bioactive compounds and medicinal plants effectiv for the treatment of type 2 diabetes mellitus	The study reported that rosmarinic acid, cyanidin, quercetin, kaempferol, rutin, luteolin, ellagic acid, catechin, etc. were among the mostly used bioactive compounds for the treatment of diabetes. The study also cited the sources of these compounds, including their medicinal plant sources	Egbuna et al.^[Citation138]

Methods for the extraction of bioactive compounds from plants

Many conventional and non-conventional methods have been employed to extract important phytochemicals from plants. Their efficiency mostly depends on factors such as nature of plant matrix, critical input parameters, scientific expertise, chemistry of bioactive compounds, etc.^[125,167] Solvent extraction is mostly used. The extraction can progress in four stages via: penetration of solvent into the matrix; dissolving of solute dissolves in the solvent; diffusion of solute is out of the matrix; and, collection and purification of extracted solutes.^[167,168] Factors that improve the solubility and diffusivity in these stages will facilitate the extraction process. The solvent properties, the ratio of solvent to solid, materials particle size, extraction duration, and extraction temperature also affect the efficieny of extraction.^[168,169] Many methods have been employed to improve the yield of bioactive compounds from plant materials, including enzyme digestion, marceration, ultrasound, pulsed electric field, ohmic heating, accelerated solvents, extrusion, microwave heating, supercritical fluids, etc.^{[168,170,171]} Solid-phase extraction, liquid-liquid extraction, and solid-phase micro-extraction are considered conventional/traditional methods, while microwave-assisted extraction (MAE), ultrasound-assisted extraction (UAE), pulsed electric field (PEF), instant controlled pressure drop (DIC), supercritical fluid extraction (SFE), etc., have been employed as cost effective and environmentally friendly alternatives.^[171] Hydrodistillation has also been employed as a traditional method for extracting essential oils and bioactive compounds from plant materials.^[38,137] Hydrodistillation does not make use of organic solvents and can be done before dehydrating plant materials.

Conclusion

The study evaluated the major biological properties of many medicinal plants with the aim of understanding their therapeutic uses and potential antioxidant properties. The overall biological properties, especially antioxidant strengths, of the plants were extensively studies. The medically significant plants were shown to possess high antioxidant capacity when compared to synthetic antioxidants. These plants have high phenolic and flavonoid contents alongside high DPPH and FRAP activities. They were also shown to possess some active compounds with high antioxidant and other biological activities. Systematic investigations of these antioxidant plants in vitro and in vivo studies using same experimental methods of analysis and solvents are needed to enable an overall ranking of the plants in order of their antioxidant capacities.

Data availability

Additional data will be made available on request

Acknowledgments

Authors are thankful to University of Ibadan, Ibadan, Nigeria, and Kampala International University, Kampala, for providing the facilities used for this study.

Disclosure statement

All Authors have affirmed zero conflict of interest.

Additional information

Funding

The authors have no funding to report.