The Interaction of Cystine and Cysteine with Beef Liver Catalase.

Abstract

A study has been made of the interaction of crystalline beef liver catalase with cystine and with the thiols, cysteine, cysteamine, and penicillamine. Native beef liver catalase contains 8 SH-groups readily titratable with p-chloromercuribenzoate. The catalase SH-groups interact with S34-cystine to form mixed disulphide groups. A maximum of 4 moles of cysteine residues are bound to the enzyme while concurrently 4 enzyme SH-groups are oxidized to SS-groups. Incubation of catalase with thiols is associated with the formation of the inactive catalase hydrogen peroxide complex II, confirming the report of Keilin and Nicholls. The inhibition of catalase by thiols is spontaneously reversible. The degree of inhibition obtained under different experimental conditions is proportional to the amount of complex II formed. The data provide evidence that the thiol inhibition of catalase can be quantitatively accounted for by formation of the inactive catalase complex II formed from hydrogen peroxide generated in the oxidation of the thiols. The inhibition of catalase by cystine is prevented by prior blocking of the enzyme SH-groups with p-chloromercuribenzoate. It is spontaneously reversible and, like the cysteine inhibition, it is prevented by ethylenediaminetetraacetate or ethanol. The data indicate that the inhibiting effect of cystine is due to its interaction with catalase SH-groups with the consequent liberation of cysteine, which in turn inhibits the enzyme according to the mechanism outlined above.