The drug and carcinogen metabolism system of rat colon microsomes

Abstract

The hydroxylation of N- and O-methyl drugs and polycyclic hydrocarbons has been demonstrated in microsomes prepared from colon mucosal cells. The hydroxylation of the drugs benzphetamine, ethylmorphine, p-nitroanisole, and p-nitrophenetole by colon microsomes is inducible two- to fourfold by pretreatment with phenobarbital/hydrocortisone. Colon microsomal benzo[α]pyrene hydroxylation is inducible 35-fold by pretreatment with β-naphthoflavone. Phenobarbital/hydrocortisone pretreatment also induces a fourfold increase in the specific content of colon microsomal cytochrome P-450, while β-naphthoflavone pretreatment causes a shift in the reduced CO difference spectrum peak to 448 nm and an eightfold increase in the specific content of this cytochrome. SKF 525-A inhibits the hydroxylation of the drug benzphetamine by colon microsomes or liver microsomes by 77% at a concentration of 2.0 m m. 7,8-Benzoflavone, on the other hand, inhibits the hydroxylation of the polycyclic hydrocarbon benzo[α]pyrene by colon microsomes by 76% and by liver microsomes by 44% at a concentration of 10 μ m. Carbon monoxide, an inhibitor of oxygen interaction with cytochromes P-450 and P-448, inhibits benzphetamine hydroxylation and benzpyrene hydroxylation by colon microsomes 30 and 51%, respectively, at an oxygen to carbon monoxide ratio of 1:10. The K m values of colon microsomal cytochrome P-450 reductase for the artificial electron acceptors cytochrome c, dichloroindophenol, and ferricyanide (10–77 μ m) are in agreement with those for purified rat liver cytochrome P-450 reductase. These data support the conclusions that hydroxylation of drugs and polycyclic hydrocarbons is catalyzed by colon mucosal microsomes and that the hydroxylation activity is attributable to a cytochrome P-450-dependent drug metabolism system similar to that found in liver microsomes.