Journal of Enzyme Inhibition and Medicinal Chemistry has recently published an article describing inhibition of tyrosinase by flavonoidsCitation1. The most active compounds were identified as hesperetin, luteolin, kaempferol, quercetin, tiliroside, and robinetin. However, at least three of them (quercetin, kaempferol, and luteolin) were shown before to be substrates of tyrosinase and/or antioxidants reducing tyrosinase-generated o-quinones. These compounds were first described as tyrosinase inhibitors almost three decades agoCitation2, but later oxidation of quercetin by this enzyme was demonstratedCitation3. The reaction pathway was elucidated and the product was identified as a protocatechuate derivative − 2-(3,4-dihydroxybenzoyl)-2,4,6-trihydroxy-3(2H)-benzofuranoneCitation4. We later demonstrated that quercetin, kaempferol, morin, and catechin were not only oxidised by tyrosinase, but also by o-quinones generated from catechols by oxidation with either tyrosinase or sodium periodateCitation5. TheseCitation2–5 and other papers describing oxidation of flavonoids by tyrosinase were not cited in the article published in the Journal of Enzyme Inhibition and Medicinal ChemistryCitation1.
Tyrosinase activity is routinely monitored by measuring the concentration of dopachrome (the oxidation product of L-tyrosine or L-dopa) spectrophotometrically at 475 nm. However, compounds with reducing properties, such as these flavonoids, reduce dopaquinone, an intermediate in this pathway, thus preventing formation of dopachrome. This is interpreted as enzyme inhibition, but it is only apparent. Actual rates of the enzymatic reactions can be easily determined by oxygen consumption measurementsCitation5, but this is very rarely done.
Quercetin 3-O-glucoside (compound 36) and 3-O-rutinoside (compound 37) showed substantially lower inhibitory activity than quercetinCitation1. This difference has already been describedCitation2. Although restricted access to the active siteCitation1 may contribute to lowering the rate of enzymatic oxidation of quercetin glycosides, we explained this effect by differences in reactions following their oxidationCitation5. All three compounds are oxidised to corresponding o-quinones but only quercetin isomerises to p-quinonemethide, which is ultimately converted to the protocatechuate derivative mentioned aboveCitation4, with a maximum of absorption at 335 nmCitation3-Citation5. Therefore smaller increase of absorbance at 475 nm is observed. Modification of the 3-OH group of flavonols by glycosylation or methylation prevents tautomerization of o-quinones to p-quinonemethides. Products of oxidation of such derivatives absorb in a range similar to dopachrome and therefore smaller decrease or even increase of absorbance at 475 nm is observedCitation5.
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References
- Jakimiuk K, Sari S, Milewski R, Supuran CT, Şöhretoğlu D, Tomczyk M. Flavonoids as tyrosinase inhibitors in in silico and in vitro models: basic framework of SAR using a statistical modelling approach. J Enzyme Inhib Med Chem. 2022;37(1):421–430.
- Kubo I, Kinst-Hori I, Ishiguro K, Chaudhuri SK, Sanchez Y, Ogura T. Tyrosinase inhibitory flavonoids from Heterotheca inuloides and their structural functions. Bioorg Med Chem Lett. 1994;4(12):1443–1446.
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