This paper presents a kinetic study of the effect of sodium chloride on the catecholase activity of latent grape polyphenol oxidase. The modifier showed a strongly pH dependent inhibitory effect at pH values <5 but acted as an activator at higher values. Other salts such as potassium nitrate and sulfate also activated the enzyme, indicating an unspecific activation effect by ionic strength. Furthermore, at pH values <5, at which the enzyme followed Michaelis-Menten behavior, the presence of sodium chloride gave rise to a lag period in the product accumulation 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 latter species undergoing a conformational change. This mechanism was simulated by computer, and good agreement was obtained with the experimental results. [References: 32]
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