BENZOIC-ACID INHIBITION OF THE ALPHA-ISOZYME, BETA-ISOZYME, AND GAMMA-ISOZYME OF AGARICUS-BISPORUS TYROSINASE

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 Ki 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. © 1990.