title: Separation of human liver microsomal tolbutamide hydroxylase and (S)-mephenytoin 4'-hydroxylase cytochrome P-450 enzymes.
abstract: Purification and immunoinhibition studies have suggested that the hydroxylations of (S)-mephenytoin and tolbutamide are catalyzed by rather similar forms of human liver cytochrome P-450 (P-450). However, the two activities are not well correlated in vivo; sulfaphenzaole is a selective inhibitor of tolbutamide hydroxylation, and expression of P-450 2C10 cDNA in yeast yields a protein that hydroxylates tolbutamide but not (S)-mephenytoin. The P-450 2C8, 2C9, and 2C10 cDNAs have all been isolated, and their sequences are known to be closely related (greater than 80%). Highly sensitive radiochromatographic assays were set up, and tolbutamide and (S)-mephenytoin hydroxylation activities were monitored during chromatography of human liver microsomal fractions. The two activities could be separated by chromatography, and proteins were purified to near-homogeneity that catalyzed either tolbutamide hydroxylation (P-450TB) or (S)-mephenytoin 4'-hydroxylation (P-450MP) but not both. Approximately 16 and 45% of the primary sequences of P-450TB and P-450MP, respectively, were determined by analysis of the tryptic peptides. The sequences of the P-450TB peptides matched those predicted by the P-450 2C9 and 2C10 cDNAs exactly; the P-450MP peptides showed two mismatches (of 219 residues) with the P-450 2C10 sequence. Proteins with the P-450 2C10 and P-450 2C9 sequences were expressed in Saccharomyces cerevisiae grown under different nutritional conditions, and both were found to be proficient in the hydroxylation of tolbutamide but not (S)-mephenytoin. We conclude, on the basis of this and previous work, that 1) P-450s 2C8, 2C9, and 2C10 all catalyze the hydroxylation of tolbutamide and 2) the protein involved in polymorphic (S)-mephenytoin 4'-hydroxylation is closely related to but distinct from P-450 2C8, 2C9, and 2C10.
authors.affiliation: Department of Biochemistry, Vanderbilt University School of Medicine, Nashville, Tennessee 37232-0146.
authors.forename: P K
authors.initials: PK
authors.lastname: Srivastava
authors.forename: C H
authors.initials: CH
authors.lastname: Yun
authors.forename: P H
authors.initials: PH
authors.lastname: Beaune
authors.forename: C
authors.initials: C
authors.lastname: Ged
authors.forename: F P
authors.initials: FP
authors.lastname: Guengerich
mesh_terms: D000595:Amino Acid Sequence; D001189:Aryl Hydrocarbon Hydroxylases; D002851:Chromatography, High Pressure Liquid; D065731:Cytochrome P-450 CYP2C19; D003577:Cytochrome P-450 Enzyme System / Q000302:isolation & purification* / Q000378:metabolism; D004247:DNA / Q000235:genetics; D006801:Humans; D008617:Mephenytoin / Q000378:metabolism; D008862:Microsomes, Liver / Q000737:chemistry / Q000201:enzymology*; D006899:Mixed Function Oxygenases / Q000302:isolation & purification* / Q000378:metabolism; D008969:Molecular Sequence Data; D012441:Saccharomyces cerevisiae / Q000201:enzymology; D012689:Sequence Homology, Nucleic Acid; D014044:Tolbutamide / Q000378:metabolism; D014316:Tritium
chemical_list: D014316:Tritium; D004247:DNA; D003577:Cytochrome P-450 Enzyme System; D014044:Tolbutamide; D006899:Mixed Function Oxygenases; D001189:Aryl Hydrocarbon Hydroxylases; C045793:CYP2C19 protein, human; D065731:Cytochrome P-450 CYP2C19; C053553:tolbutamide 4-hydroxylase; D008617:Mephenytoin