The effects of sulphydryl reagents on the binding and mixed function oxidation of hexobarbital in rat hepatic microsomes.

1. The effects of the sulphydryl reagents p-chloromercuribenzoate, N-ethylmaleimide and iodoacetamide on the binding spectrum, oxygen consumption and formation of a suspected substrate-cytochrome P-450-oxygen complex for hexobarbital in rat liver microsomes were investigated. 2. The oxygen consumption caused by hexobarbital oxidation was inhibited non-competitively by all three agents, with 50% inhibition at 4 times 10(-5) M for p-chloromercuribenzoate, 3-7 times 10(-4) M for N-ethylmaleimide and 1-9 times 10(-3) M for iodoacetamide. Cysteamine protected and at least partially reversed this inhibition. 3. p-chloromercuribenzoate inhibited the formation of the cytochrome P-450-substrate-oxygen complex, while N-ethylmaleimide and iodoacetamide also inhibited the formation of this complex but to a lesser extent. The p-chloromercuribenzoate inhibition was protected against and reversed by cysteamine. 4. p-Chloromercuribenzoate and N-ethylmaleimide caused a 50% reduction in the magnitude of the hexobarbital-induced binding spectrum, and this was paralleled by the conversion of cytochrome P-450 to cytochrome P-420. Cysteamine protected against this effect but could not reverse it. Iodoacetamide had no effect on the binding spectrum of hexobarbital and failed to convert cytochrome P-450 to cytochrome P-420. 5. Points of attack within the reaction sequence of drug oxidation are tentatively ascribed to the sulphydryl reagents used in this study.