Human tyrosinase is able to oxidize both enantiomers of rhododendrol

Abstract

Racemic RS-4-(4-hydroxyphenyl)-2-butanol (rhododendrol, RD) was used as a topical skin-whitening agent until it was recently reported to induce leukoderma. We then showed that oxidation of RD with mushroom tyrosinase rapidly produces RD-quinone, which is quickly converted to RD-cyclic quinone and RD-hydroxy-p-quinone. In this study, we examined whether either or both of the enantiomers of RD can be oxidized by human tyrosinase. Using a chiral HPLC column, racemic RD was resolved optically to R(-)-RD and S(+)-RD enantiomers. In the presence of a catalytic amount of l-dopa, human tyrosinase, which can oxidize l-tyrosine but not d-tyrosine, was found to oxidize both R(-)- and S(+)-RD to give RD-catechol and its oxidation products. S(+)-RD was more effectively oxidized than l-tyrosine, while R(-)-RD was less effective. These results support the notion that the melanocyte toxicity of RD depends on its tyrosinase-catalyzed conversion to toxic quinones and the concomitant production of reactive oxygen species.

A convenient screening method to differentiate phenolic skin whitening tyrosinase inhibitors from leukoderma-inducing phenols

“…However, the oxidation products were not characterized in that study. In the present study, HPLC analysis confirmed that an initial product of the oxidation is the corresponding ortho-quinone by identifying the NaBH 4 reduction product as rhododendrol-catechol [9,10]. This primary quinone is rapidly converted to secondary products, as shown by spectrophotometry (Fig.…”

“…5). It is interesting that the carbonyl group in raspberry ketone structure may participate in the secondary reactions, perhaps through a tautomeric enol form, like the hydroxy group in rhododendrol [9,10]. The slow oxidation of 4-tert-butylphenol (5) was surprising when we consider its potent depigmenting effect [17].…”

“…A common chemical feature of the latter group of compounds is that they are readily oxidized by tyrosinase to form reactive orthoquinone species. This study used, as a source of tyrosinase, mushroom tyrosinase that exhibits much lower specificity toward various phenols as compared to human tyrosinase [10]. This is not a disadvantage of mushroom tyrosinase, but rather an advantage Fig.…”

“…For example, 4-n-butylresorcinol (10) is a potent inhibitor of tyrosinase with an IC 50 value for human tyrosinase being 21 mM (against the substrate L-dopa) [23]. Rhododendrol (1) competitively inhibits mushroom tyrosinase with a K i value of 24 mM against the substrate L-tyrosine [8] but at the same time serves as a substrate for mushroom and human tyrosinases better than Ltyrosine, leading to the production of toxic ortho-quinone(s) [9,10].…”

“…RD was recently shown to exert melanocyte toxicity via a tyrosinase-dependent mechanism [7,8]. In independent studies [9,10], we reported that the oxidation of 1 with mushroom tyrosinase rapidly produces RD-quinone, which is quickly converted to RD-cyclic quinone through an intramolecular addition of the hydroxy group. These quinones were identified as RD-catechol and RD-cyclic catechol after NaBH 4 reduction.…”