title: Tolbutamide 4-hydroxylase activity of human liver microsomes: effect of inhibitors.
abstract: Eight samples of human liver have been characterised for microsomal protein content, cytochrome P-450 content, tolbutamide 4-hydroxylase and ethinyloestradiol 2-hydroxylase activities. Cytochrome P-450 content correlated significantly with ethinyloestradiol 2-hydroxylase activity but not with tolbutamide 4-hydroxylase activity. There was no significant correlation between ethinyloestradiol 2-hydroxylase and tolbutamide 4-hydroxylase activities. The maximum tolbutamide 4-hydroxylase activity was 0.45 nmol min-1 mg-1 microsomal protein, with a Km value of 74 microM. A number of compounds were tested for their ability to inhibit tolbutamide metabolism. All the compounds showing inhibition were either non-competitive or mixed non-competitive inhibitors of tolbutamide 4-hydroxylation. These studies suggest that tolbutamide is metabolised by an isozyme of cytochrome P-450 which appears to be distinct from those isozymes metabolising many other drugs.
mesh_terms: D000293:Adolescent; D000328:Adult; D000368:Aged; D002648:Child; D002675:Child, Preschool; D019363:Cytochrome P-450 CYP1A1; D065607:Cytochrome P-450 Enzyme Inhibitors; D003577:Cytochrome P-450 Enzyme System / Q000378:metabolism*; D005260:Female; D006801:Humans; D066298:In Vitro Techniques; D007527:Isoenzymes / Q000378:metabolism; D007700:Kinetics; D008297:Male; D008862:Microsomes, Liver / Q000201:enzymology*; D008875:Middle Aged; D006899:Mixed Function Oxygenases / Q000037:antagonists & inhibitors / Q000378:metabolism*; D011506:Proteins / Q000378:metabolism; D013250:Steroid Hydroxylases / Q000037:antagonists & inhibitors / Q000378:metabolism; D014044:Tolbutamide / Q000378:metabolism*
chemical_list: D065607:Cytochrome P-450 Enzyme Inhibitors; D007527:Isoenzymes; D011506:Proteins; D003577:Cytochrome P-450 Enzyme System; D014044:Tolbutamide; D006899:Mixed Function Oxygenases; D013250:Steroid Hydroxylases; D019363:Cytochrome P-450 CYP1A1; C020120:estrogen 2-hydroxylase; C053553:tolbutamide 4-hydroxylase