Metabolism of phenolics in coffee and plant-based foods by canonical pathways: an assessment of the role of fatty acid β-oxidation to generate biologically-active and -inactive intermediates

Figures & data

Figure 1. Generalized Structures. Note that with reference to β-oxidation the double bond in the ω-phenyl-alkenoic acid 2 is always at C2, the ω-phenyl-hydracrylic acid 3 side-chain hydroxyl is always at C3 whatever the overall side chain length, and n may be zero.

Figure 2. a) Generalized fatty acid β-oxidation scheme occurring in peroxisomes and mitochondria. b) Isomerase can inter-convert cis-3- and trans-2-enoyl-CoAs associated with conventional fatty acids, but it cannot accommodate the trans-2-enoyl-CoA geometric isomer of 3-phenyl-alkenoic acids because the side chain is too short to permit the necessary isomerization.

Figure 3. Outline metabolism of unsubstituted ω-phenyl-alkanoic acids.

Figure 4. Synthetic ω-phenyl-alkanoic acids used by Dakin.

Figure 5. Outline metabolism of C₆–C₄ ω-phenyl-alkanoic acids.

Figure 6. Structures of C₆–C₃ positional isomers. That on the left is a potential β-oxidation substrate. That on the right is not.

Figure 7. Outline metabolism of [6]-Gingerol by rats. The precise pathways have not been elucidated but appear to be a mixture of ω-, α- and β-oxidation, and phase-2 conjugation.

Figure 8. Outline metabolism of 3′,5′-dihydroxy-ω-phenyl-alkanoic acids.

Figure 9. Outline metabolism of 2′-hydroxy-substituted ω-phenyl-alkanoic acids.

Figure 10. Outline metabolism of 3′-hydroxy-substituted ω-phenyl-alkanoic acids.

Figure 11. Outline metabolism of 4′-hydroxy-substituted ω-phenyl-alkanoic acids.

Figure 12. Outline metabolism of n-nonyl-phenol and n-octyl-phenol by Rainbow trout and Mosquitofish.

Figure 13. Outline metabolism of 2′-methoxy-substituted ω-phenyl-alkanoic acids.

Figure 14. Miscellaneous metabolites.

Figure 15. Outline metabolism of 4′-methoxy-substituted ω-phenyl-alkanoic acids.

Figure 16. Outline metabolism of 3′,4′-dihydroxy-substituted ω-phenyl-alkanoic acids. For subsequent metabolism of 3′-hydroxyphenyl-substituted metabolites see Figure 10.

Figure 17. Outline metabolism of 3′-methoxy-4′-hydroxy-substituted ω-phenyl-alkanoic acids.

Figure 18. Outline metabolism of 3′-hydroxy-4′-methoxy-substituted ω-phenyl-alkanoic acids.

Figure 19. Outline human and rat metabolism of 3′,4′-dimethoxy-ω-phenyl-alkanoic acids.

Figure 20. Outline metabolism of 3′,4′,5′-tri-substituted-ω-phenyl-alkanoic acids associated with flavanols.

Figure 21. Outline metabolism of 3′,4′-methylenedioxy-substituted-ω-phenyl-alkanoic acids.

Figure 22. Outline metabolism of preexisting 3′,4′,5′-tri-substituted-ω-phenyl-alkanoic acids.

Figure 23. Proposed transformation of 4-hydroxy-5-(phenyl)pentanoic acids to 3-hydroxy-5-(phenyl)pentanoic acid enantiomers.

Figure 24. Proposed transformation of 4-hydroxy-5-(phenyl)pentanoic acids to phenylacetic acids.

Figure 25. Metabolism of phenolic acids by cytosolic and mitochondrial enzymes. ω-Phenyl-alkanoic and alkenoic acids can be conjugated in the cytosol or can enter mitochondria. The distribution between reactions depends on the chain length and is defined by the individual rate constants (k).

Figure 26. Structure of cinnamic acids tested in rats for modulation of lipid metabolism.

Table 1. The less common phase-2 conjugations of phenolic acids.

Glycine	Glutamine	Taurine	I-Carnitine	Acyl-glucuronic acid
Data from human studies
Phenylacetyl	Phenylacetyl	Phenylacetyl^e	Benzoyl^b	Benzoyl
4′-Hydroxyphenylacetyl	4′-Hydroxyphenylacetyl		Phenylacetyl^c	Hippuroyl (tentative)
3-(Phenyl)propanoyl^a	4′-Methoxyphenylacetyl		3-(Phenyl)propanoyl^a	Phenylacetyl
3-hydroxy-3-(phenyl)propanoyl^d	4-(Phenyl)butanoyl		3-(3′,5′-dihydroxyphenyl)propanoyl^f	4′-Methoxyphenylacetyl
Cinnamoyl^d				Cinnamoyl
4′-Methoxycinnamoyl				4-(Phenyl)butanoyl
3′-Methoxy-4′-hydroxycinnamoyl
3′-Hydroxy-4′-methoxycinnamoyl
Data from studies of other mammalian species^f
3′-Hydroxycinnamoyl
4′-Hydroxycinnamoyl
2′-methoxycinnamoyl
2′,4′,5′-Trihydroxycinnamoyl
3′,4′-Dimethoxycinnamoyl
3-(3′,4′,5′-Trimethoxyphenyl)propanoyl
3-(2′-Hydroxyphenyl)propanoyl

^aReported only in connection with MCAD deficiency

^bReported only following sodium benzoate therapy for non-ketotic hyperglycinaemia or hyperammonemia

^cReported only in connection with phenylketonuria

^dNow known to be a normal human metabolite at very low concentrations and reported tentatively at elevated concentrations in connection with long chain mitochondrial 3-hydroxy-acyl-CoA-dehydrogenase deficiency

^eAt a very high dose (588 μmol/kg)

^fAnimal metabolism differs from human metabolism and the doses used were very high relative to the likely human exposure and the relevance of these observations for humans is unclear.

Table 2. Excretion, corrected for placebo, of hesperetin Phase II conjugates and 3′-hydroxy-4′-methoxyphenyl-substituted metabolites after consumption by healthy volunteers of 111 µmol (N = 15) or 348 µmol (N = 12) hesperetin in orange juice hesperetin glycosides in orange juice.

Volunteer number	1	2	3	4	5	6	7	8	9	10	11	12	13	14	15	16	17	18	19	20	21	22	23	24	25	26	27
Hesperetin intake (µmol)	348	348	348	111	111	111	111	111	111	111	111	111	111	111	111	111	348	111	348	348	348	111	348	348	348	348	348
Hesperetin Phase II conjugates excreted (µmol/24 hours)	141.83	100.34	117.89	64.84	19.60	56.86	52.66	51.89	41.50	69.09	21.02	11.93	52.17	15.90	57.10	33.27	25.54	21.11	79.02	41.22	37.22	12.40	51.01	25.92	70.71	58.35	82.60
Nett delivery of hesperetin to colon (µmol/24 hours)	206.17	247.66	230.11	46.16	91.40	54.14	58.34	59.11	69.50	41.91	89.98	99.07	58.83	95.10	53.90	77.73	322.46	89.89	268.98	306.78	310.78	98.60	296.99	322.08	277.29	289.65	265.40
3-(3′-hydroxy-4′-methoxyphenyl) propanoic acid excreted (µmol/24 hours)	n.d.	5.82	6.42	n.d.	6.51	2.57	2.94	2.77	1.54	0.72	2.51	8.31	8.10	3.94	0.09	0.58	4.31	0.95	4.16	6.85	6.30	n.d.	3.00	5.74	7.05	6.72	n.d.
3′-hydroxy-4′-methoxycinnamic acid excreted (µmol/24 hours)				n.d.	n.d.	0.31	0.06	n.d.	n.d.	n.d.	n.d.	0.07	0.04	0.05	0.17	0.10		0.19				0.04
3-hydroxy-3-(3′-hydroxy-4′-methoxyphenyl)propanoic acid excreted (µmol/24 hours)	n.d.	n.d.	n.d.	n.d.	n.d.	n.d.	n.d.	n.d.	0.85	3.79	7.61	8.28	12.28	16.88	22.46	23.65	51.19	55.55	68.14	80.75	88.81	92.18	94.72	96.54	101.00	101.38	186.00
Notes: n.d. = not detectable. Blank cells = metabolite not sought.

Table 3. Excretion by rats and humans of each subgroup of C₆–C₅ flavanol metabolites expressed as a percentage of the total C₆–C₅ metabolites.

	Rats			Humans (Castello et al. 2018)
	[2-^14C]-(–)-epicatechin	Red wine extract	Grape seed phenolics	Cocoa	Cocoa enriched in flavanols	Apples	Grape pomace
Conjugated (phenyl)-γ-valerolactones	23.9	38.8	14.8	83.9	79.1	97.7	99.7
Free (phenyl)-γ-valerolactones	21.2			0.94	2.1		0.18
Conjugated 4-hydroxy-5-(phenyl) pentanoic acids	54.9	18.7	25.5	15.2	18.7	1.9	0.11
Free 4-hydroxy-5-(phenyl)pentanoic acids		0.12	0.15	n.d.	n.d.
Conjugated 5-(phenyl)pentanoic acids		41.9	59.3
Free 5-(phenyl) pentanoic acids		0.3	0.26

Table 4. 24-Hour excretion of 3′-hydroxy-4′-methoxyphenyl-substituted C₆–C₃ metabolites by volunteers.

Commodity	Intake	Excretion of 3′-hydroxy-4′methoxyphenyl-substituted C6–C3 metabolites			Reference
		Number of metabolites quantified	Mean Flux (μmol/24 hours)	Maximum flux (μmol/24 hours)^b
Artichoke	8.62 mmol 3′,4′-dihydroxycinnamic acid equivalents	4 phase-2 conjugates 82% + the cinnamic acid 18%	54.8		Dominguez-Fernandez et al. 2022
Coffee	4.4 mmol 3′,4′-dihydroxycinnamic acid equivalents	4 phase-2 conjugates	49.5		Erk et al. 2012
Coffee	2.15 mmol 3′,4′-dihydroxycinnamic acid equivalents	4 phase-2 conjugates	32.7		Erk et al. 2012
Coffee	1.01 mmol 3′,4′-dihydroxycinnamic acid equivalents	4 phase-2 conjugates	19.9		Erk et al. 2012
Coffee	0.75 mmol 3′,4′-dihydroxycinnamic acid equivalents	2 phase-2 conjugates	8.1		Stalmach, Williamson, and Crozier 2014
Coffee	0.54 mmol 3′,4′-dihydroxycinnamic acid equivalents	2 phase-2 conjugates	7.5		Stalmach, Williamson, and Crozier 2014
Coffee	0.35 mmol 3′,4′-dihydroxycinnamic acid equivalents	2 phase-2 conjugates	5.3		Stalmach, Williamson, and Crozier 2014
Coffee	0.19 mmol 3′,4′-dihydroxycinnamic acid equivalents	5 phase-2 conjugates	15.25		Gomez-Juaristi et al. 2018b
Maté	0.23 mmol 3′,4′-dihydroxycinnamic acid equivalents	4 phase-2 conjugates 88% + the cinnamic acid 28 12%	10.58		Gomez-Juaristi et al. 2018a
Apple (white fleshed)	0.076 mmol 3′,4′-dihydroxycinnamic acid equivalents	5 phase-2 conjugates 75.5% + the phenylpropanoic 44 1.3% and cinnamic acids 28 23.2%	6.30	18.8	Rubió et al. 2021
Apple (red fleshed)	0.195 mmol 3′,4′-dihydroxycinnamic acid equivalents	5 phase-2 conjugates 88% + the phenylpropanoic 44 6.3% and cinnamic acids 28 5.7%	9.25	25.4	Rubió et al. 2021
Orange juice	0.287 mmol hesperetin (nett)^a	Phenyl-hydracrylic acid 89 (94%) and phenyl-propanoic acid 44 (6%)^c	76.8 (72 phenyl-hydracrylic acid 89)^c	186 (186 phenyl-hydracrylic acid 89)	Pereira-Caro et al. 2014
Orange juice	0.079 mmol hesperetin (nett)^a	Phenyl-hydracrylic acid 89 (57%), phenyl-propanoic acid 44 (21%) and cinnamic acid 28 (21%)^c	19.2 (11.0 phenyl-hydracrylic acid 89)^c	92.2 (92.1 phenyl-hydracrylic acid 89)	Pereira-Caro et al. 2015
Orange juice	0.079 mmol hesperetin (nett)^a	Phenyl-hydracrylic acid 89 (65%), phenyl-propanoic acid 44 (18%) and cinnamic acid 28 (17%)^c	26.0 (17.0 phenyl-hydracrylic acid 89)^c	–	Pereira-Caro et al. 2015
Orange juice	0.24 mmol hesperetin (nett)^a	3 phase-2 conjugates 37.5% + the phenyl-hydracrylic acid 89 62.5%	26.0 (17.0 phenyl-hydracrylic acid 89)		Pereira-Caro et al. 2017
Orange juice	0.24 mmol hesperetin (nett)^a	3 phase-2 conjugates 32.9% + the phenyl-hydracrylic acid 89 67.1%	29.9 (18.5 phenyl-hydracrylic acid 89)		Pereira-Caro et al. 2017

a The intake of hesperetin glycosides has been corrected for mean excretion of hesperetin phase-2 conjugates.

b Value for the volunteer with the greatest excretion.

c Quantified by GC–MS after deconjugation with β-glucuronidase and sulfatase.

Table 5. A summary of the human metabolism of the major classes of ω-phenyl-alkanoic acids 1 and phenyl-alkenoic acids 2.

Origin of C6–C3 ω-phenyl-alkanoic 1 and / or ω-phenyl-alkenoic acids 2				Labeled study (volunteers or human gut microbiota)	Fate of the C6–C3 metabolite
Phenyl-ring substituents	Present preformed in the diet	Produced by gut microbiota metabolism	Produced by endogenous ω-oxidation	References	Phase-2 methylation (M), sulfate (S), or glucuronide (G) conjugation	C6–C3-glycine conjugate	Hepatic “reverse” hydrogenation	3-(Phenyl)-hydracrylic acid 3 excreted	Benzoic acid (B) and / or benzoyl-glycine (BG) excreted
None	No	Yes		Hoskins, Holliday, and Greenway 1984	Ester glucuronide	Yes	Yes ^c	Yes	Yes (B, BG)
2′-hydroxy	Yes, minor	No			No	Yes			Yes (B, BG)
3′-Hydroxy ^a	No	Yes		Borges et al. 2018; Baba et al. 1981; Hackett et al. 1983; Ottaviani et al. 2016.	Tentative report of S and G of the hydracrylic acid 58	No		Yes	Yes (B, BG)
4′-Hydroxy ^a	Yes	Yes	Yes, minor	Chen et al. 2018; Chen et al. 2019	Yes (S, G)	No	Yes ^d	Yes	Yes (B, BG)
4′-Methoxy	No	Yes	Yes, minor	Sangster et al. 1987	No	Yes			Minor (B, BG)
3′,4′-Dihydroxy ^a,b	Yes, major	Yes	Extremely limited	Hackett et al. 1983; Ottaviani et al. 2016	Major (M, S, G)	No	Yes ^e	No	Minor (B)
3′-Methoxy-4′-hydroxy	Yes	Yes	Minor		Major (S, G)	Yes	Yes ^e	Requires confirmation	Minor (B)
3′-Hydroxy-4′-methoxy	No	Yes			Major (S, G)	Yes		Yes, major	Minor (B)
3′,4′-Dimethoxy	Yes				No	Yes	Yes ^f		No
3′,4′,5′-Trihydroxy ^a^,b	No				Only C6–C5 (M, S)	No			No
3′,5′-Dihydroxy^a^,b	No	Yes	Yes		Yes (S, G)	No		No	Yes (B, G)

a C₆–C₅ precursors yield significant amounts of 5-(phenyl)-γ-valerolactones which are not β-oxidation substrates.

b C₆–C₃ and C₆–C₅ precursors, with the exception of the alkyl-resorcinols, are susceptible to gut microbiota dehydroxylation.

c In mitochondria (enoyl-CoA reductase) and microsomes (Zhao et al. 2019).

d In mitochondria (Ranganathan and Ramasarma 1974).

e Ileostomists (Stalmach et al. 2010).

f Volunteers within 1 hour of consuming 3′,4′-dimethoxycinnamic acid (Farrell et al. 2012).

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