Regioselective Hydroxylation oftrans-ResveratrolviaInhibition of Tyrosinase fromStreptomyces avermitilisMA4680

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Secreted tyrosinase from melanin-forming Streptomyces avermitilis MA4680 was involved in both ortho-hydroxylation and further oxidation of trans-resveratrol, leading to the formation of melanin. This finding was confirmed by constructing deletion mutants of melC(2) and melD(2) encoding extracellular and intracellular tyrosinase, respectively; the melC2 deletion mutant did not produce piceatannol as well as melanin, whereas the melD2 deletion mutant oxidized resveratrol and synthesized melanin with the same yields, suggesting that MelC2 is responsible for ortho-hydroxylation of resveratrol. Extracellular tyrosinase (MelC2) efficiently converted trans-resveratrol into piceatannol in the presence of either tyrosinase inhibitors or reducing agents such as catechol, NADH, and ascorbic acid. Reducing agents slow down the dioxygenase reaction of tyrosinase. In the presence of catechol, the regio-specific hydroxylation of trans-resveratrol was successfully performed by whole cell biotransformation, and further oxidation of trans-resveratrol was efficiently blocked. The yield of this ortho-hydroxylation of trans-resveratrol was dependent upon inhibitor concentration. Using 1.8 mg of wild-type Streptomyces avermitilis cells, the conversion yield of 100 μM trans-resveratrol to piceatannol was 78% in 3 h in the presence of 1 mM catechol, indicating 14 μM piceatannol h(-1) DCW mg(-1) specific productivity, which was a 14-fold increase in conversion yield compared to that without catechol, which is a remarkably higher reaction rate than that of P450 bioconversion. This method could be generally applied to biocatalysis of various dioxygenases.