Tyrosinase is responsible for browning in plants which lowers the color quality of plant-derived foods. In our search for tyrosinase inhibitors from plants, gallic acid (3,4,5-trihydroxybenzoic acid) was characterized from Fuchsia tetradactyla (Onagraceae). Gallic acid was previously reported to inhibit the oxidation of L-3,4-dihydroxyphenylalanine (L-DOPA) catalyzed by mushroom tyrosinase (EC 1.14.18.1). However, gallic acid acted neither as an inhibitor nor as a substrate for this enzymatic oxidation. Dopaquinone, which is the first enzymatic oxidized intermediate of L-DOPA, oxidized gallic acid to the corresponding o-quinone as a redox cycler and was reduced back to L-DOPA. Gallic acid acted as a reducing agent for dopaquinone and itself being oxidized to the corresponding o-quinone, and as a result, dopachrome formation was suppressed. This gave the appearance that gallic acid inhibited the enzymatic formation of dopachrome. Thus, gallic acid does not inhibit the tyrosinase-catalyzed oxidation of L-DOPA but partly suppressed the dopachrome formation.

Effects of gallic acid on the enzyme tyrosinase: A closer look

248th National Meeting (2014)

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