title: Differential inhibition of individual human liver cytochromes P-450 by cimetidine.
abstract: Cimetidine binds to cytochrome P-450 and inhibits hepatic metabolism of various drugs in humans. However, cytochrome P-450 is a family of enzymes rather than a single protein, and effects of cimetidine on individual human liver cytochromes P-450 have not been previously characterized. Metabolism of selected substrates and cimetidine-binding assays have been performed using human liver microsomes, purified human liver cytochromes P-450, and cytochrome P-450 complementary DNA-expressed yeast proteins to probe interaction of cimetidine with these individual enzymes. Cimetidine (3.0 mmol/L) in incubations reduced bufuralol hydroxylase activity by 80% and strongly inhibited microsomal nifedipine oxidation (23% +/- 13% of control activity). The same concentration of cimetidine produced intermediate inhibition of cytochrome enzymes responsible for ethoxyresorufin deethylation and aniline hydroxylation (77% +/- 6% and 68% +/- 17% of activity in control microsomal incubations, respectively), but little effect on tolbutamide hydroxylation was observed. Concordantly, the calculated binding constant for the binding of cimetidine to a purified cytochrome P-450 with high tolbutamide hydroxylase activity was 4.4 mmol/L, whereas the calculated binding concentration constant for a purified cytochrome P-450-metabolizing nifedipine was 0.7 mmol/L. These studies show a high variability in the effect of cimetidine on drug metabolism by individual human liver cytochromes P-450. In vitro studies using human liver microsomes and genetically engineered human cytochromes P-450 can be very useful in exploring important clinical questions of hepatic drug metabolism.
authors.affiliation: Gastroenterology Division, University of Maryland School of Medicine, Baltimore.
authors.forename: R G
authors.initials: RG
authors.lastname: Knodell
authors.forename: D G
authors.initials: DG
authors.lastname: Browne
authors.forename: G P
authors.initials: GP
authors.lastname: Gwozdz
authors.forename: W R
authors.initials: WR
authors.lastname: Brian
authors.forename: F P
authors.initials: FP
authors.lastname: Guengerich
mesh_terms: D000328:Adult; D002927:Cimetidine / Q000378:metabolism / Q000494:pharmacology*; D065607:Cytochrome P-450 Enzyme Inhibitors; D003577:Cytochrome P-450 Enzyme System / Q000378:metabolism; D005260:Female; D006801:Humans; D066298:In Vitro Techniques; D008099:Liver / Q000187:drug effects / Q000201:enzymology*; D008297:Male; D008862:Microsomes, Liver / Q000187:drug effects / Q000201:enzymology; D011485:Protein Binding
chemical_list: D065607:Cytochrome P-450 Enzyme Inhibitors; D002927:Cimetidine; D003577:Cytochrome P-450 Enzyme System