Tolbutamide and phenytoin hydroxylations by cDNA-expressed human liver cytochrome P4502C9

Maurice E. Veronese, Peter I. Mackenzie, Christopher J. Doecke, Michael E. McManus, John O. Miners, Donald J. Birkett

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    227 Citations (Scopus)


    A human cytochrome P4502C9 cDNA clone has been isolated from a human liver bacteriophage Lambda gt11 library using oligonucleotide probes. Expression of the 1762 base pair cDNA in COS cells demonstrated that the encoded enzyme has a molecular mass of 55 kDa as determined by sodium dodecyl sulphate-polyacrylamide gel electrophoresis. The expressed enzyme catalysed the methylhydroxylation of tolbutamide with an apparent Km of 131.7 μM, similar to that observed in human liver microsomes. P4502C9 also catalysed the 4-hydroxylation of phenytoin, and inhibition experiments demonstrated that phenytoin was a competitive inhibitor of tolbutamide hydroxylation with an apparent Ki of 19.1 μM. Sulphaphenazole was a potent inhibitor of the expressed enzyme with respect to both tolbutamide and phenytoin hydroxylations. These data demonstrate that a single isozyme can catalyse the hydroxylations of both tolbutamide and phenytoin, and suggest that both reactions are mediated by the same isozyme(s) of cytochrome P450 in human liver.

    Original languageEnglish
    Pages (from-to)1112-1118
    Number of pages7
    JournalBiochemical and Biophysical Research Communications
    Issue number3
    Publication statusPublished - 29 Mar 1991


    • human liver
    • isozymes
    • tolbutamide


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