Purification and Characterization of Polyphenol Oxidase From Potato: Ii. Inhibition and Catalytic Mechanism

KCN and ascorbic acid showed competitive inhibition patterns with Kis values of 0.032 and 0.27 mM, respectively. Uncompetitive inhibition patterns were obtained with sodium azide, L-cysteine and NaCl with Kii values of 3.3 mM, 0.12 mM and 0.3 M, respectively. A noncompetitive inhibition pattern was obtained for thiourea with 0.067 mM for Kis and 0.59 mM for Kii. Cu2+ increased the activity about 2.5 fold at or above 40 μM and K+ decreased the enzyme activity about 33% at 0.4 M. Other metal ions did not have any effects on the activity. Two pK values of 5.8 and 8.0 were obtained from Vmax profile and two pK values of 5.9 and 8.1 from Vmax/Km profile. The data suggest that cysteine is likely to be involved in catalysis and histidine in binding. Data from chemical modification show that cysteine was completely inactivated at 1.74 mM o-methylisourea, and histidine and tryptophan were modified at much higher concentrations of diethylpyrocarbonate and N-bromosuccinimide, respectively. It is suggested that the protonated cysteine works as a general base, tryptophan as a substrate binding residue and histidine as a oxygen binding residue.