Identification of an inducible form of cytochrome P-450 in human liver

It has not yet been determined whether human liver contains inducible cytochromes P-450 similar to those that catalyze the oxidative metabolism of foreign substances in animals. We carried out immunoblot analyses of liver microsomes isolated from eight patients and found that each contained a cytochrome P-450, termed HLp, that reacted with antibodies directed against P-450p, a rat liver cytochrome that is inducible by the anti-glucocorticoid pregnenolone-16 alpha-carbonitrile, by glucocorticoids, by anti-seizure drugs, and by such macrolide antibiotics as triacetyloleandomycin. In the two patients who received dexamethasone and anti-seizure medications and in the one patient who was given triacetyloleandomycin, the concentrations of immunoreactive HLp and the ability to demethylate erythromycin and/or to convert triacetyloleandomycin to a metabolite that forms a spectral complex with cytochrome P-450 heme (catalytic properties unique to P-450p in rat liver) were significantly higher as compared to the values for patients who received no inducing drugs. We purified HLp to homogeneity and found that it was immunochemically related to P-450p and to its homologue in the rabbit (LM3c), actively demethylated erythromycin in a reconstituted system, exhibited electrophoretic mobility identical to that of P-450p, and shared 57% homology in its NH2-terminal amino acid sequence with that of a pregnenolone-16 alpha-carbonitrile-inducible rat cytochrome P-450. We conclude that HLp is a human representative of the multigene family of the glucocorticoid-inducible cytochromes P-450.