Cathepsins muscle physiology

In all the replacement reactions catalyzed by cathepsin and papain studied by Fruton and his collaborators the optimum pH of the replacement reaction was at 7-8, whereas that of hydrolysis is near to 5. The authors suggest that at normal physiological pH values the catalysis of replacement may represent a major intracellular function of cathepsins and papainases. But what measurements there are of intracellular pH in mammalian kidney, liver, and muscle indicate it to be nearer 6 than 7 (49,51,98). The pH dependence in these transfer reactions is related to the pK of the cation of the replacement agent. 

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