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The effect of low-molecular-mass biological disulfides and their related reduced compounds on the activity of two calcium-dependent neutral proteinases (calpains) from rat liver has been investigated. L-Cystine and L-cystamine bring about the inactivation of both enzymes, while the related reduced compounds L-cysteine and L-cysteamine are without effect. Calpain II is more sensitive to the inactivating effect of glutathione disulfide in comparison with calpain I. The inactivation rates of both calpains depend on the concentration of glutathione disulfide. Reduced glutathione, added at physiological concentration (5 mM), neither affects the proteinase activities nor protects the enzymes from the inactivating effect of glutathione disulfide. The enzymes inactivated by biological disulfides cannot be restored by a large excess of a reducing thiolic compound (dithiothreitol). It is suggested that calcium-dependent proteinases might be inactivated also in vivo by enhanced level of glutathione disulfide.

1. Tyrosine aminotransferase from guinea-pig liver is inactivated at neutral pH by a factor localized in the microsomal fraction. The inactivation, independent of exogenous L-cysteine, is rapidly reversed by addition of dithiothreitol. 2. The effects of physiological reducing agents on the enzyme inactivation were investigated. L-Cysteine and L-cysteamine enhance the inactivation rate of the enzyme in the presence of microsomal membranes, and also they are able to bring about the loss in enzyme activity independently of microsomal action. Reduced glutathione, at physiological concentration, and NADPH decrease the inactivation rate. Other physiological reducing compounds, as well as oxidized glutathione and NADP+, are without effect. 3. Neither reduced glutathione nor NADPH, unlike dithiothreitol and mercaptoethanol, is able to restore the activity of partially inactivated tyrosine aminotransferase. 4. It is proposed that the intracellular concentration of reduced glutathione might modulate the rate of inactivation of the enzyme in vivo.