Selective inactivation of four rat liver microsomal androstenedione hydroxylases by chloramphenicol analogs.

The steroid androstenedione has been shown to be a valuable tool for the study of the selective inactivation of cytochrome P-450 isozymes in intact rat liver microsomes. The validity of this approach was investigated using microsomes, purified cytochrome P-450 isozymes, antibodies to particular cytochromes P-450, and the known mechanism-based inactivator chloramphenicol. Enzyme inactivation and antibody inhibition studies show that microsomes from both phenobarbital- and non-phenobarbital-treated rats are needed to accurately monitor the inactivation of the major phenobarbital-inducible cytochrome P-450 isozyme (PB-B) and of the major constitutive androstenedione 16 alpha-hydroxylase (UT-A). Similar experiments indicate that, although isozyme P-450g does catalyze the 6 beta-hydroxylation of androstenedione in a reconstituted system, this cytochrome appears to make only a minimal contribution to microsomal 6 beta-hydroxylase activity, which reflects instead the activity of pregnenolone-16 alpha-carbonitrile-induced isozymes. With these parameters investigated, initial enzyme inactivation studies showed that the antibiotic chloramphenicol caused different rates of NADPH-dependent enzyme inactivation among the four androstenedione hydroxylases monitored (16 beta greater than 6 beta greater than 16 alpha greater than 7 alpha). Based on these data, 12 chloramphenicol analogs were examined, and the results with these compounds show that their selectivity as cytochrome P-450 inactivators is a function of at least three structural features: 1) the number of halogen atoms, 2) the presence of a para-nitro group on the phenyl ring, and 3) substitutions on the ethyl side chain. For example, the compound N-(2-phenethyl)dichloroacetamide was shown to reversibly inhibit but not inactivate the cytochrome(s) P-450 responsible for androstenedione 6 beta-hydroxylase activity, whereas N-(2-p-nitrophenethyl) and N-(1,2-diphenethyl)dichloroacetamide rapidly inactivated the 6 beta-hydroxylase. The ability to monitor the activity of multiple isozymes with a single substrate should allow the development of a systematic approach to the design of selective inactivators of rat liver cytochromes P-450.