PH-DEPENDENT EFFECT OF SODIUM-CHLORIDE ON LATENT GRAPE POLYPHENOL OXIDASE

This paper presents a kinetic study of the effect of sodium chloride o n the catecholase activity of latent grape polyphenol oxidase. The mod ifier showed a strongly pH dependent inhibitory effect at pH values <5 but acted as an activator at higher values. Other salts such as potas sium nitrate and sulfate also activated the enzyme, indicating an unsp ecific activation effect by ionic strength. Furthermore, at pH values <5, at which the enzyme followed Michaelis-Menten behavior, the presen ce of sodium chloride gave rise to a lag period in the product accumul ation curves, generating positive kinetic cooperativity in the steady- state kinetics. In contrast, at higher pH values, at which the enzyme showed a lag phase and exhibited negative cooperativity, the presence of a salt decreased cooperativity. The inhibition data are consistent with a mechanism by which chloride ions bind to both protonated forms of the enzyme, free enzyme and the enzyme-substrate complex, the latte r species undergoing a conformational change. This mechanism was simul ated by computer, and good agreement was obtained with the experimenta l results.