Tissue-specific induction of the carcinogen inducible cytochrome P450 isoform, P450IAI, in colonic epithelium.

The epithelial cells of the gastrointestinal tract have the capacity to engage in biotransformation of ingested chemicals. A principal component of phase I metabolism of xenobiotics is the family of hemeproteins referred to as cytochrome(s) P450. The presence of cytochrome P450 isoforms was examined by Western blot analysis in the epithelial cells of the colon and proximal small intestine of male rats following oral administration with either beta-naphthoflavone or phenobarbital. The appearance of beta-naphthoflavone-inducible cytochrome P450IAI was observed in the colon and small intestine. The appearance of this cytochrome P450 isoform was concurrent with increases (up to 150-fold) in cytochrome P450-related O-deethylation of 7-ethoxycoumarin and 7-ethoxyresorufin in both tissues. Following administration of phenobarbital, cytochrome P450IIBI was identified immunochemically in the small intestine. However, this isozyme could not be detected in colon. These data suggest that the epithelial cells of the proximal small intestine respond to beta-naphthoflavone and phenobarbital in a manner similar to the liver, whereas colonic epithelial cells may have a greater capacity to respond to P450IAI-type inducers such as beta-naphthoflavone. Evidence exists that differences in cytochrome P450 isozyme composition can affect the ultimate metabolic fate of ingested chemicals, including carcinogens, and thus a role for colonic P450-dependent monooxygenase activity in the biogenesis of cancer in this tumor-susceptible tissue is suggested.