Kahn, V., Department of Biological Chemistry, University of Maryland, School of Medicine, Baltimore, MA 21201, United States
Pomerantz, S.H., Department of Biological Chemistry, University of Maryland, School of Medicine, Baltimore, MA 21201, United States

Polyphenol oxidase of avocado mesocarp catalyses (a) the orthohydroxylation of monophenols like l-tyrosine, d-tyrosine, tyramine and p-cresol, and (b) the oxidation of the corresponding o-dihydroxyphenols to quinones. The rate of step b is much greater than that of step a. The hydroxylation of monophenols occurs after a lag period. DOPA or ascorbate effectively eliminate the lag but not dl-6-methyltetrahydropteridine or tetrahydrofolic acid. At 1.66 × 10-4 M, α,α-dipyridyl has no effect, while diethyldithiocarbamate at this concentration inhibits the hydroxylation reaction by 90%. The tyrosinase activity of avocado polyphenol oxidase is inactivated in the course of the reaction; this inactivation occurs faster and is more pronounced in the presence of exogenously added DOPA. This inactivation is partially prevented by a large excess of ascorbate. The Km values indicate that tyramine, dopamine, p-cresol and 4-methyl catechol are better substrates for avocado polyphenol oxidase than tyrosine or DOPA. © 1980.