Hubert Wojtasek
Institute of Chemistry, Opole University, Ul. Oleska 48, 45-052 Opole, Poland. E-mail: Hubert.Wojtasek@uni.opole.pl; Fax: +48 77 452 7101; Tel: +48 77 452 7122
First published on 14th February 2022
A review article has been published recently (RSC Advances, 2021, 11, 22159–22198) describing flavonoids as inhibitors of tyrosinase. However, many compounds included in this review have been previously shown to act as substrates of this enzymes or antioxidants reducing tyrosinase-generated o-quinones. Products of their oxidation absorb light in a range different than dopachrome, the oxidation product of L-tyrosine or L-dopa, whose concentration is measured spectrophotometrically in the standard enzymatic assay to monitor the activity of this enzyme. This effect is interpreted as enzyme inhibition, which, in fact, is only apparent and results from inadequate methodology.
Many papers cited in this review1 describe flavonoids containing a catechol group in either ring A or B, or a conjugated system of hydroxy groups in position 3 and either 2′ or 4′ as inhibitors of tyrosinase. They should, however, be substrates of this enzyme or should undergo oxidation by tyrosinase-generated o-quinones, as we described for quercetin, kaempferol, morin and catechin.13 Compounds, which should show such properties, or for which they have already been demonstrated, are listed below, with their numbers given in parenthesis:
malvidin (1), peonidin (2), pelargonidin (3), delphinidin (5), cyanidin 3-O-glucoside (7), delphinidin 3-O-glucoside (8), baicalein (57), luteolin (58), 6,7-dihydroxy-2-phenyl-4H-chromen-4-one (63), 7,8,4′-trihydroxyflavone (64), dihydromyricetin (74), taxifolin (75), compound 77 (which is labeled as dihydroxykaempferol, but whose structure actually corresponds to dihydroquercetin), compound 80 (which is labeled as chlorophorin, but whose structure actually corresponds to 2,3,4,5-tetrahydroxycyclohexyl derivative of dihydrofisetin), proanthocyanidins (81), 2-(3,4-dihydroxyphenyl)-5,7-dihydroxy-3-methoxychroman-4-one (84), 2-(3,4-dihydroxyphenyl)-3,5,7-trihydroxychroman-4-one (87), 2-(3,4-dihydroxyphenyl)-3,5-dihydroxy-7-[2,3,4-trihydroxy-5-(hydroxymethyl)cyclohexyl]oxychroman-4-one (89), 7,3′,4′-trihydroxyisoflavone (92), 7,8,4′-trihydroxyisoflavone (93), 3′-hydroxygenistein (95), 6-hydroxydaidzein (96), calycosin (97), 7,8-dihydroxy-3-(4-hydroxyphenyl)-4H-chromen-4-one (106, which is identical with compound 93), 7,8-dihydroxy-3-(3-methoxyphenyl)-4H-chromen-4-one (107), 3-(3,4-dihydroxyphenyl)-8-hydroxy-7-methyl-4H-chromen-4-one (108), 7,8-dihydroxy-3-(3-hydroxyphenyl)-4H-chromen-4-one (109), 7,8-dihydroxy-3-(4-methoxyphenyl)-4H-chromen-4-one (110), 7,8-dihydroxy-3-(2-methoxyphenyl)-4H-chromen-4-one (111), 6,7-dihydroxy-3-(4-methoxyphenyl)-4H-chromen-4-one (112), 6,7-dihydroxy-3-(4-hydroxyphenyl)-4H-chromen-4-one (113, which is identical with compound 96), quercetin (not quercitin, 139), kaempferol (140), rutin (142), 2-(3,4-dihydroxyphenyl)-3,5,7-trihydroxy-4H-chromen-4-one (143, which is identical with compound 139), 2-(3,4-dihydroxyphenyl)-3,5-dihydroxy-7-methoxy-4H-chromen-4-one (144), 6-hydroxykaempferol (145), 6-hydroxygalangin (146).
I don't think it would be necessary to demonstrate that such reactions occur in the case of all these compounds. It should have been done when they were studied as inhibitors of tyrosinase. Cyclohexane rings in flavonols 80 and 89 are surprising. Since the most common natural derivatives of flavonoids are their glycosides, one would expect pyranose rings of sugars in these compounds. Unfortunately, it was not possible to verify these structures, because I could not find compound 80 in ref. 172 and compounds 82–90 in ref. 28.
The Authors state that “The flavonol's structure, 3-hydroxy-4-keto moiety, is essential in copper chelation” (p. 22183). This statement has been repeated in many publications describing flavonols as inhibitors of tyrosinase. However, it was demonstrated more than two decades ago that coordination of Cu2+ ion by the 3-hydroxy-4-keto moiety of flavonol does not occur, in contrast to Fe3+.15 Instead, flavonoids may coordinate Cu2+ ion by a catechol group in ring B, but this binding is rapidly followed by their oxidation.16 This oxidation is greatly enhanced by the presence of the 3-hydroxy group in flavonols. It was first demonstrated for quercetin and kaempferol, which were oxidized much more rapidly than flavonoids without this functional group – rutin (quercetin-3-O-rutinoside, with the 3-OH group blocked by glycosylation) and luteolin (a flavone, not containing the 3-OH group). It was therefore postulated that the 3,4′-dihydroxy system of flavonols was preferentially oxidized by Cu2+ ions.16 Details of the oxidation of quercetin by Cu2+ ions may be found in a subsequent publication.17 Such reactions were demonstrated for other flavonols, such as fisetin, also containing a catechol group in ring B (3′-OH, 4′-OH, as in quercetin), myricetin, containing a triol moiety (3′-OH, 4′-OH, 5′-OH), and morin, containing two phenolic groups not forming a catechol (2′-OH, 4′-OH).15 These results confirmed that the conjugated 3,4′-dihydroxy system of flavonols was oxidized under such conditions. Products of these reactions with quercetin and kaempferol were then identified as the 2-substituted 2,4,6-trihydroxy-3(2H)-benzofuranone derivatives,15 the same as that obtained later by oxidation of quercetin with tyrosinase.12
Unfortunately, many researchers searching for inhibitors of tyrosinase are only familiar with articles reporting this function of compounds they are studying, e.g. flavonoids, but are not aware of subsequent papers demonstrating that they are substrates of this enzyme. This may be exemplified by a statement from p. 22187: “Kubo et al. assumed that the chelation mechanism by flavonols may be attributed to the free 3-OH group”, which is followed by several citations, including one of Kubo's earlier publications.7 However, subsequent papers describing oxidation of quercetin and fisetin by this enzyme, in which this concept was abandoned,12,18 are not mentioned.
The review1 also contains other errors. The bibliography contains only 178 positions, yet in Table 10 ref. 180 appears. Some references appear twice or even three times in the bibliography. Examples that I have found are listed below:
D. Sohretoglu, S. Sari, B. Barut and A. Ozel, Bioorg. Chem., 2018, 81, 168–174 (ref. 15, 79, and 129)
K. Bagherzadeh, F. Shirgahi Talari, A. Sharifi, M. R. Ganjali, A. A. Saboury and M. Amanlou, J. Biomol. Struct. Dyn., 2015, 33, 487–501 (ref. 52(d) and 75)
E. J. Land, C. A. Ramsden and P. A. Riley, Tohoku J. Exp. Med., 2007, 212, 341–348. (ref. 63 and 155)
Y. Kakumu, K. Yamauchi and T. Mitsunaga, Holzforschung, 2019, 73, 637–643. (ref. 72 and 162)
R. R. Arroo, S. Sari, B. Barut, A. Ozel, K. C. Ruparelia and D. Sohretoglu, Phytochem. Anal., 2020, 31, 314–321. (ref. 83 and 128)
N. Guo, C. Wang, C. Shang, X. You, L. Zhang and W. Liu, Int. J. Biol. Macromol., 2018, 118, 57–68. (ref. 93 and 166)
N. Kishore, D. Twilley, A. Blom van Staden, P. Verma, B. Singh, G. Cardinali, D. Kovacs, M. Picardo, V. Kumar and N. Lall, J. Nat. Prod., 2018, 81, 49–56. (ref. 101 and 163)
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