Benzoic acid inhibition of the α, β, and γ Isozymes of Agaricus bisporus tyrosinase

Abstract

Inhibition of the catecholase and cresolase reactions of the α, β, and γ isozymes of Agaricus bisporus tyrosinase by benzoic acid was investigated at 25.0 and 8.0 °C at pH 5.60 in air-saturated solutions. Benzoic acid is a simple competitive inhibitor of the cresolase reaction of all three isozymes. In the catecholase reaction, however, benzoic acid is a partial uncompetitive inhibitor of the α and β isozymes and a simple competitive inhibitor of γ-tyrosinase. Equilibrium dialysis experiments, conducted under identical conditions to the kinetic studies, indicate that benzoic acid can bind to the α and γ isozymes in the absence of organic substrate. The dissociation constants obtained by equilibrium dialysis are in good agreement with the kinetic K i values determined from inhibition studies. Maximum binding of benzoic acid to α and γ tyrosinase, however, is significantly less than one mole per mole of active sites. A scheme in which benzoic acid binds to the oxy-form of tyrosinase is proposed to account for the kinetic and equilibrium results.