The activity of potato polyphenol oxidase (tyrosinase) toward DL-3,4-dihydroxyphenylalanine (K(M) 5.39 mM) was studied using a variety of carboxylate buffers at a common pH and ionic strength. Enzyme activity, greatest in citrate and least in oxalate, correlated with increasing carboxyl concentration and molecular mass. The lower activity in oxalate was attributed to more effective chelation of a copper(II) form of the enzyme by the oxalate dianion. Sodium halide salts inhibited the enzyme. Although there was little difference in inhibition between sodium and potassium salts, the degree and type of inhibition was anion dependent; K(is), values for NaCl and KCl, (competitive inhibitors) were 1.82 and 1.62 mM, whereas Na(2) SO(4) and K(2) SO(4) (mixed inhibitors) had K(is) and K(ii) values in the 250 to 450 mM range.