HEAVY-METAL IONS AND p-GLUCURONIDASE 509 2. Mercuric chloride and silver nitrate have a qualitatively similar inhibitory action on the enzyme. This appears primarily to be competitive, but an effect on KS, the dissociation constant of the inactive ES2 enzyme-substrate complex, can also be distinguished, thereby increasing substrate inhibition. A third effect develops only in the presence of substrate, increasing the overall inhibition. Phenylmercuric hydroxide exhibits a novel form of inhibition in that it only decreases Ks2. p-Chloromercuribenzoate is a purely competitive inhibitor and appears not to affect K.. Cupric sulphate probably inhibits competitivefy and also decreases KS, so combining the two independent actions of tNhe organic mercurials. 3. The inactivating action of copper is potentiated by various reducing agents. Ascorbate is shown to act by reducing cupric copper to the much more toxic cuprous form. In the presence of bisulphite, copper catalyses a substrate-protected decomposition of the enzyme. 4. It is concluded that both SH and SS groups are involved in the active centre of f-glucuronidase. 5. Cuprous ion is shown to cause a non-enzymic hydrolysis of several aryl glucosides, including phenolphthalein P-glucuronide.
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