Accessibility of cytochrome P450 in microsomal membranes: inhibition of metabolism by antibodies to cytochrome P450.

Specific antibodies against highly purified cytochrome P450 from phenobarbital (PB)-treated rats and cytochrome P448 from 3-methycholanthrene (MC)-treated rats were produced in rabbits. These antibodies are inhibitors of drug metabolism catalyzed by liver microsomes from untreated rats or from rats treated with PB, MC, or pregnenolone-16α-carbonitrile (PCN). Microsomal metabolism of five substrates was examined; the extent of antibody inhibition was dependent not only on the source of the antibody and microsomes but also on the specific substrate and the reactions catalyzed. A comparison of the antibody inhibition patterns of the various substrates examined indicates a marked difference in the proportions of the different forms of cytochrome P450 present in liver microsomes from untreated and PB-, MC-, and PCN-treated rats. Antibody inhibition of microsomal metabolism indicates that the membrane-bound terminal oxidase, cytochrome P450 or P448, is at least partially exposed to the hydrophilic environment on the exterior of microsomal membranes.