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Tyrosinase inhibitors identified from phytochemicals and their mechanism of control


The enzyme tyrosinase is responsible for catalyzing the first steps of the melanin formation pathway. Essential oil of the fresh leaves of Polygonumodoratum (Polygonaceae), commonly known as Vietnam coriander, was found to inhibit the oxidation catalyzed by tyrosinase (EC Twenty five scent compounds were characterized in the essential oil by GC-MS analysis. Aldehyde compounds are noted to be the most abundant, followed by alcoholic compounds but to a much lesser extent. Alkanals; dodecanal (55.49%), and decanal (11.57%); were the two most abundant in the essential oil, followed by anisaldehyde (6.35%). Dodecanal and decanal inhibited the oxidation of both L-tyrosine and L-3,4-dihydroxyphenylalanine (L-DOPA) catalyzed by mushroom tyrosinase.

In our search for plant derived tyrosinase inhibitors, benzaldehydes were explored. A series of benzaldehyde structural analogues inhibited the tyrosinase-catalyzed oxidation of L-3,4-dihydroxyphenylalanine (L-DOPA) and a handful suppressed melanogenesis activity. The mechanism behind tyrosinase suppression has not been well understood. In cell free experiments, it is likely that the aldehyde group forms a Schiff base with the primary amino acid group on the enzyme. The hydroxyl and methoxy groups then contribute by stabilizing a newly formed complex that suppresses the enzyme.

The final class of phytochemicals explored for their enzyme inhibition mechanisms were tannins. The hydrolysable tannin gallic acid has been reported as a tyrosinase inhibitor but the mechanism has not been clearly understood. Enzymatic assays found that gallic acid inhibited the oxidation of L-3,4-dihydroxyphenylalanine (L-DOPA) catalyzed by mushroom tyrosinase but the tannin was not oxidized when mixed with the enzyme. The available oxygen in the reaction mixture was used for the oxidation of gallic acid and L-DOPA. The enzymatically generated intermediates produced complex mixtures, and thus gallic acid suppressed the initial rate of pigmented product formation.

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