1. The proteolitic activities of both crystalline native papain [EO 3. 4. 4. 10] and mercuripapain were remarkably inhibited with dimedone, diketohydrindene and N, N'-dimethyl burbituric acid, respectively, at final concentrations of 5×10^-3 M.
2. The inhibitory effects of dimedone and diketohydrindene were always much greater than that of N, N'-dimethyl barbituric acid.
3.Papain inhibited with diketohydrindene was shown to be red.
4. Mercuripapain was more rapidly effected by dimedone and diketohydrindene than native papain.
5. Ninhydrin and N, N'-dimethyl alloxaninhibited native papain greatly, but scarcely inhibited mercuripapain.
6. The cyanide-activated papain was completely inhibited by Nethylmaleimide whereas the native papain was hardly affected by this reagent.
7. The sulfhydryl content of the native papain signiflcantly increased when the native papain was treated with some carbonyl reagents.
8. A thiohemiacetal in equilibrium with its dissociation form was proposed as a model of the active site of papain, and various reactions of papain were discussed on the basis of this model.
9. Hippuric acid phenylhydrazide affected the activityof the mercaptoethanol-activated papain.This mechanism of inhibition indicated that the aldehyde group involved in the active site plays the role of amino-acceptor in the enzymatic reaction.