On the role of thiol groups in the inhibition of liver microsomal ca²⁺ sequestration by toxic agents

ATP-dependent Ca²⁺ sequestration by rat liver microsomes was assayed using three different methods, and characterized with regard to the effect of various inhibitors. When glucose and hexokinase were added in combination to deplete ATP in the incubation, Ca²⁺ uptake was followed by rapid release of Ca²⁺ from the microsomes. Ca²⁺ sequestration was inhibited by reagents that cause alkylation (e.g. p-chloromercuribenzoate) or oxidation (e.g. diamide) of protein sulfhydryl groups. Moreover, pretreatment of the microsomes with cystamine, which causes formation of mixed disulfides with protein thiols, also resulted in the inhibition of Ca²⁺ sequestration. It is concluded that microsomal Ca²⁺ sequestration is critically dependent on protsin sulfhydryl groups, and that modification of protein thiols may be an important mechanism for the inhibition of microsomal Ca¹²⁺ sequestration by a variety of toxic agents.