pH-induced kinetic co-operativity of a thylakoid-bound polyphenol oxidase

A study of the catecholase activity of a latent plant polyphenol oxidase, extracted and purified from the chloroplast membranes of grapes (Vitis vinifera cv. Airen), revealed for the first time a lag phase above pH 5.0, whereas a steady-state rate was reached immediately when pH values were lower, thus suggesting the hysteretic nature of the enzyme. During steady state, the enzyme showed negative co-operativity concomitant with the presence of the lag period, and followed classical Michaelis-Menten kinetics under more acid pH conditions. Statistical analysis of these data showed a minimal value for the extreme Hill coefficient of 0.54 at pH 6.0. This kinetic behaviour of polyphenol oxidase has been interpreted in terms of the pH-induced 'slow' transition mechanism reported by Ricard, Noat & Nari [(1984) Eur. J. Biochem. 145, 311-317] in which the conformational change does not affect the active site of the enzyme.