Inhibition of the Tyrosinase Oxidation

The catalytic oxidation of catechol by crude preparations of mushroom tyrosinase was studied by a method yielding data on initial reaction velocities. Graphical analysis of the results suggests that an excess of catechol inhibits its own oxidation by a competitive process, thus accounting for the observed optimum in the substrate concentration. However, added phenol, though itself a substrate, inhibits the enzymatic oxidation of catechol by a process that is neither competitive nor non-competitive, but a mixture of the two types. Mechanisms of this inhibition of the enzyme by a second substrate are discussed in exploring the problem of substrate-substrate inhibition. Tyrosinase catalyzes the oxidation of monohydric phenols, as well as 0-dihydric phenols. Yet phenol inhibits the action of tyrosinase on catechol (1). The nature of this inhibition of one substrate by another of a different class is, therefore, of interest as one facet of the general problem of enzyme function. References to the m a n y reports on inhibitors of tyrosinase action are reviewed by Lerner (2). Tyrosinase, a copper-bearing phenolase, is inhibited by compounds which complex with copper, by analogs which competitively inhibit its action, and by metals which compete with the copper. Few papers have dealt with the mechanism of inhibition using Michaelis-Menton kinetics. Kendal (3), using a manometric method, demonstrated that the addition of phenol competitively inhibits the oxidation of catechol. However, in the presence of catechol, phenol itself is rapidly converted to catechol by the action of tyrosinase and might complicate the interpretation of data from the relatively slow manometric assay he used. Later, Warner (4) showed that m-hydroxybenzoic acid acts as a competitive inhibitor in the tyrosinase-catalyzed oxidation of catechol. Hackney (5) studied the inhibitory effect of resorcinol on tyrosinase, manometrically. However, her interpretation was questioned by Warner. Recently Schneider and Schmidt (6) published data concerning the competitive inhibition by resorcinol, orcinol, and phloroglucinol of the tyrosinasecatalyzed oxidation of p-cresol. These compounds are analogs, but not sub-