Abstract
A new series of isoquinoline urea/thiourea derivatives (1–11) were synthesized, and their inhibitory effects on tyrosinase were evaluated. Isoquinoline urea/thiourea derivatives were obtained as a result of the reaction of 5-aminoisoquinoline with isocyanates or isothiocyanates. The result showed that all the synthesized compounds inhibited the tyrosinase enzyme activity. Among the compounds synthesized, 1-(4-chlorophenyl)-3-(isoquinolin-5-yl)thiourea (3) was found to be the most active one (Ki = 119.22 μM), and the inhibition kinetics analyzed using Lineweaver–Burk double reciprocal plots revealed that compound 3 was a competitive inhibitor. We also calculated HOMO-LUMO energy levels, some selected the synthesized compounds (1, 4, 11, 3, 6, 2) using Gaussian software.
Keywords::
Introduction
Urea derivatives are very useful molecules with a wide range of applications in agrochemicals, petrochemicals, and pharmaceuticals (Gallou et al. Citation2005). Some urea derivatives are known as the most useful classes of anticancer agents, in particular, because of their tyrosine kinase inhibitory properties (Keating and Santoro Citation2009, Nakamura et al. Citation2006, Dumas et al. Citation2004). Heterocyclic urea derivatives have inhibitory activity against most enzymes (Li et al. Citation2009). Recently, it is demonstrated that some substitute urea derivatives have the ability to inhibit HIV protease enzyme (Gabriele and Salerno Citation2004, Zheng and Li Citation2007, Chiarotto and Feroci Citation2003). Thiourea is a form in which the oxygen atom of its urea compound is replaced by a sulfur atom. They are structurally very similar to each other but showed differences in terms of features. Thiourea derivatives are effective as antithyroid, anthelmintic, anti-phenoloxidase, antituberculous, hypnotic, anesthetic, antibacterial, insecticide, and rodenticide (Schroeder Citation1955).
Tyrosinase, sometimes referred to as phenol oxidase, catecholases, phenolase, catechol oxidase, or even polyphenol oxidase, is considered to be an o-dipenol (Okot-Kotber et al. Citation2002). Tyrosinase (EC 1.14.18.1), a multifunctional copper-containing enzyme, is widely distributed in nature. It catalyzes two distinct reactions of melanin synthesis: a hydroxylation of monophenols to o-diphenols (monophenolase activity) and an oxidation of o-diphenols to o-quinones (diphenolase activity), both using molecular oxygen (Yan et al. Citation2009). Tyrosinase is responsible not only for melanization in animals, but also for browning in plant. The unfavorable enzymatic browning of fruits and vegetables generally results in a loss of nutritional and commercial value (Friedman Citation1996). An important group is constituted by the compounds structurally analogous to phenolic substrate, which generally show competitive inhibition with respect to this substrate, although it can vary depending on the enzyme source and the substrate used (Vamos-Vigyazo Citation1981).
Synthesis of urea and thiourea derivatives is very important in synthetic chemistry for the reasons given above. In this study, 11 new derivatives of aminoisoquinoline connected to aliphatic and aromatic groups with urea were synthesized and their activity was investigated against tyrosinase enzyme.
Materials and methods
General procedure of the isoquinoline urea/thiourea derivatives
Chemicals and solvents used in the study were obtained from Sigma-Aldrich and Merck and were used without further purification. Melting points were measured on Barnstead/Electrothermal 9200 melting point apparatus. 1H and 13C NMR spectra were recorded on a Varian Mercury NMR at 300 and 75 MHz instrument in DMSO-d6, respectively.
Synthesis of carbazole-substituted isoquinoline urea and isoquinoline thiourea derivatives was carried out according to and , respectively.
Scheme 1. Synthesized urea/thiourea derivatives.
Scheme 2. Synthesized N-aryl,N’-alkyl urea derivatives.
1-(isoquinolin-5-yl)-3-phenylurea (1): Phenylisocyanate (6.94 mmol, 0.826 g) was dissolved in toluene (10 ml) and added dropwise in the stirred solution of 5-aminoisoquinoline (6.94 mmol, 1 g) in tetrahydrofuran (15 ml). The reaction mixture was stirred at 60°C overnight. The precipitate was collected by filtration, washed with toluene and acetone (5 ml), and dried under vacuum at 40°C (Demir et al. Citation2012a). Yield 95%; mp: 251–253°C; 1H NMR (300 MHz, DMSO-d6): δ(ppm) = 6.99–7.04 (1H,t), 7.30–7.35 (2H, t), 7.51–7.54 (2H, d), 7.63–7.69 (1H, t), 7.81–7.84 (1H, d), 7.97–7.99 (1H,d), 8.30–8.33 (1H,d), 8.91 (1H,s), 9.09 (1H,s), 9.32 (1H,s); 13C NMR (75 MHz, DMSO-d6): δ(ppm) = 115.06, 118.89, 121.03, 122.76, 128.24, 128.65, 129.39, 129.60, 134.45, 140.19, 143.31, 153.40, 153.43
1-(isoquinolin-5-yl)-3-(4-methoxyphenyl) thiourea (2): The experimental procedure is the same as described in the first molecule. Yield 60%; mp: 179–181°C; 1H NMR (300 MHz, DMSO-d6): δ(ppm) = 3,73 (3H,s); 6.86–6.92 (2H,dd); 7.27–7.36 (2H,dd); 7.63–7.70 (1H,m); 7.75–7.78 (1H,dd); 7.82–7.85 (1H,dd); 8.00–8.05 (1H,t); 8.51–8.58 (1H,dd); 9.32 (1H,s); 9.39 (1H,s); 9.99 (1H,s). 13C NMR (75 MHz, DMSO-d6): δ(ppm) = 114.31 (2C), 114.42, 116.85, 126.80 (2C), 127.12, 129.66, 129.97, 132.90, 135.20, 143.71, 153.23, 157.18, 157.45, 180.85.
1-(4-chlorophenyl)-3-(isoquinolin-5-yl) thiourea (3): The experimental procedure is the same as described in the first molecule. Yield 92%; mp: 117–119°C; 1H NMR (300 MHz, DMSO-d6): δ(ppm) = 7.38–7.41 (2H,d), 7.54–7.57 (2H,d), 7.67–7.72 (1H,t), 7.78–7.79 (1H,d), 7.79–7.81 (1H,d), 8.04–8.07 (1H,d), 8.54–8.56 (1H,d), 8.36 (1H,s), 9.98 (1H,s), 10.02 (1H,s); 13C NMR (75 MHz, DMSO-d6): δ(ppm) = 116.81, 126.41, 126.99, 127.94, 129.00, 129.25, 129.65, 129.96, 133.09, 135.07, 139.09, 143.65, 153.24, 182.06
1-(3,4-dichlorophenyl)-3-(isoquinolin-5-yl) urea (4): The experimental procedure is the same as described in the first molecule. Yield 94%; mp: 244–246°C; 1H NMR (300 MHz, DMSO-d6): δ(ppm) = .35–7.38 (1H,d), 7.54–7.57 (1H,d), 7.64–7.69 (1H,t), 7.85–7.88 (1H,d), 7.93–7.95 (1H,d), 7.95 (1H,s), 8.21–8.24 (1H,d), 8.58–8.60 (1H,d), 9.00 (1H,s), 9.32 (1H,s), 9.36 (1H,s); 13C NMR (75 MHz, DMSO-d6): δ(ppm) = 115.17, 119.02, 119.98, 121.91, 123.38, 124.02, 128.14, 129.04, 129.36, 131.30, 131.82, 138.98, 140.43, 143.36, 153.29, 153.40
1-(2,4-dichlorophenyl)-3-(isoquinolin-5-yl) thiourea (5): The experimental procedure is the same as described in the first molecule. Yield 91%; mp: 186–188°C; 348; 1H NMR (300 MHz, DMSO-d6): δ(ppm) = 7.39–7.43 (1H,d), 7.58–7.61 (1H,d), 7.66–7.71 (1H,t), 7.68–7.69 (1H,d), 7.78–7.79 (1H,d), 7.81 (1H,s), 8.04–8.07 (1H,d), 8.54–8.56 (1H,d), 9.33 (1H,s), 9.60 (1H,s), 10.12 (1H,s); 13C NMR (75 MHz, DMSO-d6): δ(ppm) = 116.84, 127.29, 128.02, 128.21, 129.65, 129.69, 130.32, 132.03, 132.08, 132.54, 133.19, 134.85, 136.38, 143.64, 153.19, 182.82
1-(4-fluorophenyl)-3-(isoquinolin-5-yl) thiourea (6): The experimental procedure is the same as described in the first molecule. Yield 92%; mp: 2184–186°C; 1H NMR (300 MHz, DMSO-d6): δ(ppm) = 7.15–7.21 (2H,d), 7.47–7.52 (2H,d), 7.67–7.72 (1H,t), 7.77–7.79 (1H, d), 7.79–7.81 (1H,d), 8.04–8.07 (1H,d), 8.54–8.56 (1H,d), 9.35 (1H,s), 9.86 (1H,s), 9.93 (1H,s); 13C NMR (75 MHz, DMSO-d6): δ(ppm) = 115.63, 115.92, 116.86, 126.95, 127.30, 127.41, 127.95, 129.66, 130.01, 133.17, 135.16, 136.33, 136.37, 143.58, 153.20, 182.34
1-(isoquinolin-5-yl)-3-(4-nitrophenyl) urea (7): The experimental procedure is the same as described in the first molecule. Yield 93%; mp: 266–268°C; 1H NMR (300 MHz, DMSO-d6): δ(ppm) = 7.66–7.74 (1H,t), 7.76–7.78 (2H,d), 7.88–7.90 (1H,d), 7.95–7.97 (1H,d), 8.21–8.24 (2H,d), 8.24–8.27 (1H,d), 8.60–8.62 (1H,d), 9.11 (1H,s), 9.34 (1H,s), 9.78 (1H,s); 13C NMR (75 MHz, DMSO-d6): δ(ppm) = 115.16, 118.22, 122.14, 123.69, 125.90, 125.99, 128.16, 128.88, 129.11, 133.74, 143.46, 146.83, 153.00, 153.44
1-butyl-3-(isoquinolin-5-yl) urea (8): The experimental procedure is the same as described in the first molecule. Yield 94%; mp: 179–181°C; 1H NMR (300 MHz, DMSO-d6): δ(ppm) = 0.89–0.94 (3H,t), 1.31–1.39 (2H,m), 1.42–1.49 (2H,q), 3.13–3.19 (2H,q), 6.59–6.63 (1H,t), 7.56–7.62 (1H,t), 7.71–7.73 (1H,d), 7.92–7.94 (1H,d), 8.29–8.32 (1H,d), 8.53–8.55 (1H,d), 8.62 81H,s), 9.26 (1H,s); 13C NMR (75 MHz, DMSO-d6): δ(ppm) = 14.37, 20.23, 32.44, 40.93, 115.03, 119.84, 121.67, 128.04, 128.27, 129.40, 135.31, 143.01, 153.34, 155.99
1-(isoquinolin-5-yl)-3-propylurea (9): The experimental procedure is the same as described in the first molecule. Yield 93%; mp: 192–194°C; 1H NMR (300 MHz, DMSO-d6): δ(ppm) = 0.90–0.95 (3H,t), 1.46–1.54 (2H,m), 3.09–3.16 (2H,q), 6.62–6.66 (1H,t), 7.52–.62 (1H,t), 7.71–7.74 (1H,d), 7.93–7.95 (1H,d), 8.28–8.31 (1H,d), 8.53–8.56 (1H,d), 8.64 (1H,s), 9.27 (1H,s); 13C NMR (75 MHz, DMSO-d6): δ(ppm) = 12.06, 23.61, 41.64, 115.01, 119.76, 121.60, 128.23, 129.39, 135.34, 143.02, 153.33, 155.98
1-hexyl-3-(isoquinolin-5-yl) urea (10): The experimental procedure is the same as described in the first molecule. Yield 93%; mp: 162–164°C; 1H NMR (300 MHz, DMSO-d6): δ(ppm) = 0.86–0.89 (3H,t), 1.29–1.33 (6H,m), 1.45–1.49 (2H,q), 3.11–3.17 (2H,q), 6.59–6.62 (1H,t), 7.56–7.61 (1H,t), 7.70–7.73 (1H,d), 7.91–7.93 (1H,d), 8.27–8.30 (1H,d), 8.52–8.54 (1H,d), 8.61 (1H,s), 9.26 (1H,s); 13C NMR (75 MHz, DMSO-d6): δ(ppm) = 14.64, 22.81, 26.80 (2C), 30.32, 31.72, 115.00, 119.69, 121.58, 128.02, 128.26, 129.39, 135.35, 143.02, 153.35, 155.94
1-isopropyl-3-(isoquinolin-5-yl) urea (11): The experimental procedure is the same as described in the first molecule. Yield 92%; mp: 234–236°C; 1H NMR (300 MHz, DMSO-d6): δ(ppm) = 1.14–1.21 (6H,d), 3.78–3.85 (1H,m), 6.52–6.54 (1H,d), 7.70–7.72 (1H,d), 7.91–7.93 (1H,d), 8.30–8.33 (1H,d), 8.53 (1H,s), 8.53–8.55 (1H,d), 9.26 (1H,s); 13C NMR (75 MHz, DMSO-d6): δ(ppm) = 23.67 (2C), 41.79, 114.89, 119.43, 121.48, 127.89, 128.26, 126.39, 135.34, 143.03, 153.36, 155.21
Purification of tyrosinase
All purification steps were carried out at 25°C. The extraction procedure was adopted from Wesche-Ebeling & Montgomery (Wesche-Ebeling and Montgomery Citation1990). The bananas were washed with distilled water thrice to prepare the crude extract; 50 g of bananas was cut quickly into thin slices and homogenized in a Waring blender for 2 min using 100 ml of 0.1 M phosphate buffer, pH 7.3, containing 5% poly(ethylene glycol) and 10 mM ascorbic acid. After filtration of the homogenate through muslin, the filtrate was centrifuged at 15 000 × g for 30 min, and the supernatant was collected. A crude protein precipitate was made by adding (NH4)2SO4 to 80% saturation. The resulting precipitate was suspended in a minimum volume of 5 mM phosphate buffer and then dialyzed against the same buffer overnight. The enzyme solution was then applied to the Sepharose 4B-tyrosine-p-aminobenzoic acid affinity column (Arslan et al. Citation2004), pre-equilibrated with 5 mM phosphate buffer, pH 5.0. The affinity gel was extensively washed with the same buffer before the banana tyrosinase was eluted with 1M NaCl, 5 mM phosphate, pH 7.0.
Tyrosinase activity
Enzyme activity was determined using catechol by measuring the increase in absorbance at 420 nm (Espin et al. Citation1995), in a Biotek automated recording spectrophotometer. Enzyme activity was calculated from the linear portion of the curve. One unit of tyrosinase activity was defined as the amount of enzyme that causes an increase in absorbance of 0.001 unit's min− 1 for 1 ml of enzyme at 25°C.
Inhibition of tyrosinase activity
The inhibitory effect of diarylureas was determined using Lineweaver–Burk plots of 1/V vs. 1/S at two inhibitor concentrations. The inhibition constant, Ki, was deduced from the points of interception of the plots.
Results and discussion
For evaluating the tyrosinase inhibitory activity, all the synthesized compounds were subjected to tyrosinase inhibition assay with catechol as a substrate. The result showed that all the synthesized compounds (1–11) inhibited the tyrosinase enzyme activity.
The kinetic parameters for the various urea derivatives are presented in . The IC50 values obtained with purified tyrosinase have different values ().
Figure 1. Inhibition graphics of urea derivatives (1–11) on BPPO.
Table I. Inhibition values of urea derivatives (1–11) on BPPO.
The inhibition constants of compounds 1–11 analogues against tyrosinase are summarized in . We have determined the Ki values of 119.22–162.62 μM for the inhibition of banana tyrosinase. Among the compounds synthesized, compound 3 (Ki: 119.22 μM) was found to be the most active one, and the inhibition kinetics analyzed by Lineweaver–Burk double reciprocal plots revealed that compound 3 was a competitive inhibitor ().
Figure 2. Ki graphics of urea derivatives (1–11) on BPPO.
Several compounds reported as tyrosinase inhibitors were also shown to have inhibitory effect on the BPPO. Among the tested anti-browning reagent, the most effective ones were dithiothreithol and sodium metabisulfite (Sayaverde-Soto and Montgomery Citation1986). The action of sulfite in the prevention of enzymatic browning can usually be explained by several processes. One is the action on o-quinones. The formation of quinone–sulfite complexes prevents the quinone polymerization (Embs and Markakis Citation1965). The enzyme also seemed to be sensitive to thiourea since PPO contains copper as a co-factor; the irreversible inactivation of this enzyme can be effected by substances (such as thiol compounds thiourea, hydroxyquinoline), which remove copper from the active site of the enzyme (Schwimmer Citation1981). It was reported that the enzyme inhibited with diarylureas (Demir et al. Citation2012b) and phenylthiourea (Ryazanova et al. Citation2012), propanoic acid (Gheibi et al. Citation2009), alkyldithiocarbonates (Alijanianzadeh et al Citation2007), and coumarin schiff-bases (Gacche et al. Citation2006), n-alkyl dithiocarbamate compounds (Gheibi et al. Citation2005), sodium diethyl dithiocarbamate (Gulcin et al. Citation2005).
Investigating the inhibitory properties, the other groups were changed when isoquinoline part was constant. It was observed that the attached groups didn’t have any effect on inhibitory levels. To investigate the real effect related to inhibition level of the compounds, molecular energy calculations were performed using Gaussian programme (Frisch et al. Citation2009, Dennington et al. Citation2009) ( and ). As a result of the calculations, the HOMO energies of the compounds were detected to increase with inhibition values linearly. In this context, the effect of inhibition was determined to be inversely proportional to the ionization potential.
Figure 3. The calculated HOMO and LUMO for compounds 11, 1, and 4 using HF method with 6-31G basis set.
Figure 4. The calculated HOMO and LUMO for compounds 3, 6, and 2 using HF method with 6-31G basis set.
The HOMO energy level of 1-(3,4-dichlorophenyl)-3-(isoquinolin-5-yl)urea (4) (which has the lowest inhibition values (IC50: 163.50 μM)) was calculated as EHOMO = − 6.3797 eV. The HOMO energy level of 1-(isoquinolin-5-yl)-3-phenylurea (1) which has IC50 = 121.80 μM inhibition value was calculated as EHOMO = − 6.2216, and so 1-isopropyl-3-(isoquinolin-5-yl) urea (11) which has IC50 = 108.00 μM inhibition value was calculated as EHOMO = − 6.1995 ().
As it was expected, thiourea derivatives give similar results as well. The HOMO energy value of 1-(isoquinolin- 5-yl)-3-(4-methoxyphenyl) thiourea (2) which has IC50 = 160.00 μM inhibition value was calculated as EHOMO = − 8.5441 eV and the HOMO energy value of 1-(4-fluorophenyl)-3-(isoquinolin-5-yl)thiourea (6) which has IC50 = 131.00 μM inhibition value was calculated as EHOMO = − 6.024603 eV, and so 1-(4-chlorophenyl)-3-(isoquinolin-5-yl)thiourea (3) which has IC50 = 120.70 μM inhibition value was calculated as EHOMO = − 6.0246 ().
Enzyme activity studies are important issue for drug design and biochemical applications (Aydemir and Kavrayan Citation2009, Demirel and Tarhan Citation2004, Demir et al. Citation2012b, Gencer et al. Citation2012, Sayin et al. Citation2012, Cicek et al. Citation2012, Gokce et al. Citation2012, Sonmez et al. Citation2011, Bytyqi-Damoni et al. Citation2012). The results showed that the synthesized compounds inhibited the hCA I enzyme activity. The compounds have weak inhibitory effects, and they may be taken for further evaluation by means of in vivo studies.
Declaration of interest
The authors report no declarations of interest. The authors alone are responsible for the content and writing of the paper.
The work was financially supported by of Sakarya University Research Project (BAPK 2012-02-04-031).
References
- Alijanianzadeh M, Saboury AA, Mansuri-Torshizi H, Haghbeen K, Moosavi-Movahedi AA. 2007. The inhibitory effect of some new synthesized xanthates on mushroom tyrosinase activities. J Enzyme Inhib Med Chem. 22:239–246.
- Arslan O, Erzengin M, Sinan S, Ozensoy O. 2004. Purification of mulberry (Morus alba L.) polyphenol oxidase by affinity chromatography and investigation of its kinetic and electrophoretic properties. Food Chem. 88:479–484.
- Aydemir T, Kavrayan D. 2009. Purification and characterization of glutathione-S-Transferase from chicken erythrocyte. Artif Cells Blood Substit Immobil Biotechnol. 37:92–100.
- Bytyqi-Damoni A, Genc H, Zengin M, Beyaztas S, Gencer N, Arslan O. 2012. In vitro effect of novel β-lactam compounds on xanthine oxidase enzyme activity. Artif Cells Blood Substit Immobil Biotechnol. 40:369–377.
- Chiarotto I, Feroci M. 2003. Selective and environmentally friendly methodologies based on the use of electrochemistry for fine chemical preparation: an efficient synthesis of N-N’-disubstituted ureas. J Org Chem 68:7137–7139.
- Cicek B, Ergun A, Gencer N. 2012. Synthesis and evaluation in vitro effects of some macro cyclic thiocrown ethers on erythrocyte carbonic anhydrase I and II. Asian J Chem. 24:3729–3731.
- Demir D, Gencer N, Er A. 2012a. Purification and characterization of prophenoloxidase from Galleria mellonella L. Artif Cells Blood Substit Immobil Biotechnol. 40:391–395.
- Demir D, Gencer N, Arslan O, Genc H, Zengin M. 2012b. In vitro inhibition of polyphenol oxidase by some new diarylureas. J Enzyme Inhib Med Chem. 27:125–131.
- Demirel LAB, Tarhan L. 2004. Dismutation properties of purified and GDA modified CuZnSOD from chicken heart. Artif Cells Blood Substit Immobil Biotechnol. 32:609–624.
- Dumas J, Smith RA, Lowinger TB. 2004. Recent developments in the discovery of protein kinase inhibitors from the urea class. Curr Opin Drug Discov Devel 7:600–616.
- Embs RJ, Markakis P. 1965. The mechanism of sulphite inhibition of browning caused by polyphenol oxidase. J Food Sci 30:753–758.
- Espin JC, Morales M, Varon R, Tudela J, Garcia-Canovas F. 1995. A continuous spectrophotometric method for determining the monophenolase and diphenolase activities of apple polyphenol oxidase. Anal Biochem. 43:2807–2812.
- Friedman M. 1996. Food browning and its prevention: an overview. J Agric Food Chem. 44:631–653.
- Frisch MJ, Trucks GW, Schlegel HB, Scuseria GE, Robb MA, Cheeseman JR, et al. 2009. Gaussian 09, Revision A.1; Gaussian, Inc.: Wallingford, CT.
- Dennington R, Keith T, Millam J. 2009. GaussView, Version 5; Semichem. Inc.: Shawnee Mission KS.
- Gabriele B, Salerno G. 2004. Efficient synthesis of ureas by direct palladium-catalyzed oxidative carbonylation of amines. J Org Chem 69:4741–4750.
- Gacche RN, Gond DS, Dhole NA, Dawane BS. 2006. Coumarin Schiff-bases: as antioxidant and possibly anti-inflammatory agents. J Enzyme Inhib Med Chem 21:157–161.
- Gallou I, Eriksson M, Zeng X, Senanayake C, Farina V. 2005. Practical synthesis of unsymmetrical ureas from isopropenyl carbamates. J Org Chem 70:6960–6963.
- Gencer N, Sinan S, Arslan O. 2012. Kinetic properties of polyphenol oxidase obtained from various olives Olea europa L. Asian J Chem. 24:2159–2161.
- Gheibi N, Saboury AA, Mansuri-Torshizi H, Haghbeen K, Moosavi-Movahedi AA. 2005. The inhibition effect of some n-alkyl dithiocarbamates on mushroom tyrosinase. J Enzyme Inhib Med Chem. 20:393–399.
- Gheibi N, Saboury AA, Haghbeen K, Rajaei E, Pahlevan AA. 2009. Dual effects of aliphatic carboxylic acids on cresolase and catecholase reactions of mushroom tyrosinase. J Enzyme Inhib Med Chem. 24:1076–1081.
- Gokce B, Gencer N, Arslan O, Turkoglu SA, Alper M, Kockar F. 2012. Evaluation of in vitro effects of some analgesic drugs on erythrocyte and recombinant carbonic anhydrase I and II. J Enzyme Inhib Med Chem. 27:37–42.
- Gulcin I, Kufrevioglu OI, Oktay M. 2005. Purification and characterization of polyphenol oxidase from nettle (Urtica dioica L.) and inhibitory effects of some chemicals on enzyme activity. J Enzyme Inhib Med Chem. 20:297–302.
- Keating GM, Santoro A. 2009. Sorafenib: a review of its use in advanced hepatocellular carcinoma. Drugs 69;223–240.
- Li HQ, Lv PC, Yan T, Zhu HL. 2009. Urea derivatives as anticancer agents. Anticancer Agents Med Chem. 9:471–480.
- Nakamura K, Taguchi E, Miura T. 2006. KRN951, a highly potent inhibitor of vascular endothelial growth factor receptor tyrosine kinases, has antitumor activities and affects functional vascular properties. Cancer Res. 66:9134–9142.
- Okot-Kotber M, Liavoga A, Yong KJ, Bagorogoza K. 2002. Activation of polyphenol oxidase in extracts of bran from several wheat (Triticum aestivum) cultivars using organic solvents, detergents, and chaotropes. J Agric Food Chem 50:2410–2417.
- Ryazanova AD, Alekseev AA, Slepneva IA. 2012. The phenylthiourea is a competitive inhibitor of the enzymatic oxidation of DOPA by phenoloxidase. J Enzyme Inhib Med Chem. 27:78–83.
- Sayaverde-Soto LA, Montgomery MW. 1986. Inhibition of polyphenol oxidase by sulfite. J Food Sci. 51:1531–1535.
- Sayin D, Cakir DT, Gencer N, Arslan O. 2012. Effects of some metals on paraoxonase activity from shark scyliorhinus canicula. J Enzyme Inhib Med Chem. 27:595–598.
- Schwimmer S. (ed.). 1981. Source Book of Food Enzymology. Westport: AVI Publishing, pp. 267–274.
- Schroeder DC. 1955. Thioureas. Chem Rev. 55:181–228.
- Sonmez F, Sevmezler S, Atahan A, Ceylan M, Demir D, Gencer N, et al. 2011. Evaluation of new chalcone derivatives as polyphenol oxidase inhibitors. Bioorg Med Chem Lett. 21: 7479–7482.
- Vamos-Vigyazo L. 1981. Polyphenoloxidase and peroxidase in fruits and vegetables. Crit Rev Food Sci Nutr. 15:49–127.
- Yan Q, Cao R, Yi W, Yu L, Chen Z, Ma L, Song H. 2009. Synthesis and evaluation of 5-benzylidene (thio) barbiturate-beta-D-glycosides as mushroom tyrosinase inhibitors. Bioorg Med Chem Lett. 19: 4055–4058.
- Wesche-Ebeling P, Montgomery MW. 1990. Strawberry polyphenol oxidase: extraction and partial characterization. J Food Sci. 55: 1320–1325.
- Zheng S, Li F. 2007. A novel and efficient (NHC)CuI (NHC = N- heterocyclic carbene) catalyst for the oxidative carbonylation of amino compounds. Tetrahedron Lett. 48:5883–5886.