title: Structure-function analysis of human cytochrome P450 3A4 using 7-alkoxycoumarins as active-site probes.
abstract: The oxidation of a series of seven alkyl ethers of 7-hydroxycoumarin by cytochrome P450 3A4 (CYP3A4) has been studied to probe the active site of the enzyme. TLC of the reaction mixture showed formation of metabolites other than 7-hydroxycoumarin. The separation and characterization of the different metabolites of the C4 to C7 compounds were achieved using a combination of TLC, HPLC, and gas chromatography-electron impact mass spectra. Among the 7-alkoxycoumarins, 7-hexoxycoumarin was found to be the most suitable candidate for investigating the active site of cytochrome CYP3A4, due to the well-separated metabolite peaks on TLC and HPLC. 7-hexoxycoumarin was found to produce three side-chain hydroxylated products besides 7-hydroxycoumarin: 7-(5-hydroxyhexoxy)coumarin, 7-(4-hydroxyhexoxy)coumarin, and 7-(3-hydroxycoumarin). The substitution of residues from substrate recognition sites -1, -4, -5, and -6 of CYP3A4 showed a strong influence on the product profile of 7-hexoxycoumarin, the most prominent effects observed with mutants at residues 119, 301, 305, 370, 373, and 479. The docking of 7-hexoxycoumarin into a molecular model of CYP3A4 also confirmed the presence of these residues within 5 A of the substrate. A comparative study of cytochrome P450 2B1 showed that the active-site mutants F206L, T302V, V363A, and S478G but not V363L exhibited a dramatic decrease in total 7-hexoxycoumarin hydroxylation. The study suggests that although the electronic nature of the substrate is important, enzymatic constraints significantly contribute to CYP3A4 selectivity.
authors.affiliation: Department of Pharmacology and Toxicology, The University of Texas Medical Branch, Route 1031, 301 University Boulevard, Galveston, Texas 77555-1031, USA. kkkhan@utmb.edu
authors.forename: K K
authors.initials: KK
authors.lastname: Khan
authors.forename: J R
authors.initials: JR
authors.lastname: Halpert
mesh_terms: D001665:Binding Sites; D002851:Chromatography, High Pressure Liquid; D002855:Chromatography, Thin Layer; D003374:Coumarins / Q000737:chemistry*; D019362:Cytochrome P-450 CYP2B1 / Q000235:genetics / Q000378:metabolism; D051544:Cytochrome P-450 CYP3A; D003577:Cytochrome P-450 Enzyme System / Q000737:chemistry* / Q000235:genetics; D005456:Fluorescent Dyes; D008401:Gas Chromatography-Mass Spectrometry; D006801:Humans; D006899:Mixed Function Oxygenases / Q000737:chemistry* / Q000235:genetics; D008958:Models, Molecular; D009154:Mutation; D011994:Recombinant Proteins / Q000737:chemistry / Q000235:genetics; D013329:Structure-Activity Relationship
chemical_list: D003374:Coumarins; D005456:Fluorescent Dyes; D011994:Recombinant Proteins; D003577:Cytochrome P-450 Enzyme System; D006899:Mixed Function Oxygenases; C104464:CYP3A protein, human; D019362:Cytochrome P-450 CYP2B1; D051544:Cytochrome P-450 CYP3A; C510163:CYP3A4 protein, human