HEPATIC-MICROSOMAL TOLBUTAMIDE HYDROXYLATION IN JAPANESE - INVITRO EVIDENCE FOR RAPID AND SLOW METABOLIZERS

Microsomal hydroxylation of tolbutamide in Japanese livers was studied in vitro to ascertain the enzyme catalysing this reaction. Rates of t olbutamide hydroxylation differed individually 33-fold and 42-fold at 0.1 mm and 2.4 mm tolbutamide concentrations, respectively, and were s egregated into two groups, rapid and slow metabolizers. An antibody ra ised against P450 human-2 (a form of CYP2C9) strongly inhibited the hy droxylation in livers of rapid metabolizers but only weakly inhibited in the slow metabolizer. Kinetic experiments further demonstrated a cl ear distinction in tolbutamide hydroxylation between two groups; the m ean of apparent Km values for tolbutamide was 0.25 mm (n = 3) in the r apid group and 2.58 mm (n = 2) in the slow, respectively. These data s uggest that different enzymes are involved in the hydroxylation in bot h metabolizer groups. Furthermore, CYP2C9 produced by cDNA expression in yeasts, catalysed tolbutamide hydroxylation at rates similar to the rapid metabolizer group at both the 0.1 mm and 2.4 mm concentrations. The apparent Km value of the expressed protein for tolbutamide, 0.26 mm, was similar to that determined for the rapid group of microsomal s amples. Clear correlations were observed between the rate of microsoma l tolbutamide hydroxylation at 0.1 mm and CYP2C9 protein content or th e rate of S-mephenytoin 4'-hydroxylation in human liver. These results indicate that considerable portions of microsomal tolbutamide hydroxy lation are catalysed by CYP2C9 or the closely related form in the rapi d metabolizers.