Differential effect of phenylhydrazine on the catecholase and cresolase activities of Vitis catechol oxidase

The simultaneous addition of phenylhydrazine and p-cresol to grape catechol oxidase resulted in enhanced oxidation of p-cresol. Carbonyl reagents such as hydrazine, borohydride and semicarbazide also enhanced cresolase activity but had no effect on catecholase activity. Pretreatment of the enzyme with periodate abolished cresolase activity. The effects of periodate and ascorbate or semicarbazide on cresolase activity were mutually reversible. The simultaneous addition of phenylhydrazine and 4-methylcatechol to the enzyme did not result in inhibition of the initial rate of oxidation of the phenolic substrate. It is concluded that phenylhydrazine does not react with a carbonyl group on the enzyme. The possible involvement of conformational changes in the enzyme, determining phenylhydrazine inhibition is discussed.