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

Abstract

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-16a-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 pregnenolone16a-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.The cytochromes P-450 are a family of hemoproteins, abundant in the endoplasmic reticulum of the hepatocyte, that catalyze the oxidative metabolism of many drugs, environmental chemicals, and endogenous compounds (1). An important characteristic of some of the forms of cytochrome P-450 is that they are inducible. For example, different forms of liver cytochrome P-450 accumulate in rats treated by a member of one of three "classes" of inducers (as reviewed in ref.2) typified, respectively, by phenobarbital (P-450b, P-450e), 3-methylcholanthrene (P-450c, P-450d), and pregnenolone-16a-carbonitrile (PCN) (P-450p). Since the amounts and types of cytochromes P-450 in the liver may be rate-limiting for metabolism of foreign chemicals, enzyme induction may play an important role in such clinically relevant phenomena as interactions among therapeutic drugs (3), metabolic "idiosyncrasy" in hepatotoxic drug reactions (4), and interindividual differences in susceptibility to toxic effects of environmental chemicals (5).There is abundant, albeit indirect, evidence that human liver also contains cytochromes P-450 that are inducible. For example, exposure of humans to inducers of animal cytochromes P-450 including such drugs as phenobarbital (6), macrolide antibiotics (7), or diphenylhydantoin (8), or environmental chemicals such as organochlorine pesticides (9) or polychlorinated biphenyls (10), accelerates the disappearance of administered substrates for the cytochr...

Expression of human liver cytochrome P450 IIIA4 in yeast

“…The very high affinity of dihydroergotamine towards human liver P45Q NF25 shown in this study suggests that this drug could be oxidized in human liver by P450 NF25. Since this cytochrome also appears to bind and demethylate erythromycin or troleandomycin and since cytochromes of the IIIA subfamily are involved in the formation of inhibitory P450-iron-metabolite complexes during macrolide antibiotic metabolism (Sartori et al, 1985;Watkins et al, 1985;Wrighton et al, 1985;Mansuy, 1987;Sartori and Delaforge, 1990;, it is likely that inhibition of dihydroergotamine metabolism by erythromycin and troleandomycin is at the origin of the severe secondary effects observed during association of erythromycin (or troleandomycin) and dihydroergotamine. This illustrates the potential interest of such human liver P45Os expressed in yeast for understanding or predicting drug/drug interactions.…”

A malleable catalyst dominates the metabolism of drugs

“…P450 3A4 is the most abundant P450 in the liver and small intestine and plays a role in the metabolism of one-half of the drugs on the market and in development (2-4). P450 3A4 was discovered two decades ago in early studies on the purification of P450s from human liver (5,6) and was soon shown to have a wide repertoire of substrates, ranging in size from acetaminophen (M r 151) to cyclosporin A (M r 1201). The enzyme is highly inducible by barbiturates and numerous other compounds (7) and is also prone to both competitive and mechanism-based inhibition by drugs, leading to important drug-drug interactions.…”

High affinity of ergopeptides for cytochromes P450 3A

“…CYP 3A4, a P450 subfamily highly expressed in human liver, is important from a pharmacological and toxicological point of view, not only because of its relative abundance in human liver (Guengerich and Turvy, 1991) but also because it is involved in the metabolism of many widely used drugs including nifedipine (Guengerich et al, 1986a), quinidine (Guengerich et al, 1986b), erythromycin and troleandomycin (Pessayre et a]., 1982;Watkins et al, 1985;Combalbert et al, 1989;Brian et al, 1990;Renaud et al, 1990), cyclosporin A (Kronbach et al, 1988;Aoyama et al, 1989;Combalbert et al, 1989), 17 a-ethynylestradiol (Guengerich, 1988), midazolam (Kronbach et al, 1989), lidocaine (Bar- (1982,1983). getzi et al.…”