The action of tyrosinase on monophenols

Abstract

Colorimetric methods for the estimation of o-diphenol in en-zymic reaction mixtures containing ascorbic acid and the corresponding monophenol were described. The results of studies of the course of oxygen uptake, o-diphenol formation and disappearance of ascorbic acid during the oxidation by tyrosinase, in the presence of ascorbic acid, of tyrosine, phenol, p-cresol and 4,5-dimethylphenol (3,4-dimethylphenol) were reported. The primary oxidation of monophenol to o-diphenol by tyrosinase in the presence of ascorbic acid was found to proceed at maximal rate at the moment of addition of enzyme to substrate. There was no induction period such as is observed in the absence of ascorbic acid. This led to the conclusion that o-diphenol played no specific part in facilitating the oxidation of monophenols by tyrosinase. The behavi or of the reacting system containing initially monophenol, tyrosinase and ascorbic acid was adequately explained in terms of (a) specific monophenolase activity of the enzyme prepn.; (b) specific catecholase activity of the enzyme prepn.; (c) progressive inactivation of the enzyme during the reaction; (d) competitive inhibition of catecholase activity by monophenols. The bearing of the results on current views of the nature and action of tyrosinase was discussed. It was suggested that they supported the view that tyrosinase was a single enzyme or enzyme complex having independent centers associated specifically with its monophenolase and with its catecholase functions.