Inhibition of potato polyphenol oxidase by anions and activity in various carboxylate buffers (pH 4.8) at constant ionic strength

The activity of potato polyphenol oxidase (tyrosinase) toward DL-3,4-dihydr oxyphenylalanine (K-M 5.39 mM) was studied using a variety of carboxylate b uffers at a common pH and ionic strength. Enzyme activity, greatest in citr ate and least in oxalate, correlated with increasing carboxyl concentration and molecular mass. The lower activity in oxalate was attributed to more e ffective chelation of a copper(II) form of the enzyme by the oxalate dianio n. Sodium halide salts inhibited the enzyme. Although there was little diff erence in inhibition between sodium and potassium salts, the degree and typ e of inhibition was anion dependent; K-is, values for NaCl and KCl, (compet itive inhibitors) were 1.82 and 1.62 mM, whereas Na2SO4 and K2SO4 (mixed in hibitors) had K-is and K-ii values in the 250 to 450 mM range.