Cytochrome P-4502B6 is responsible for interindividual variability of propofol hydroxylation by human liver microsomes

Background: Oxidation of propofol to 4-hydroxypropofol represents a signifi cant pathway in the metabolism of this anesthetic agent in humans. The aim of this study was to identify the principal cytochrome P-450 (CYP) isoforms mediating this biotransformation. Methods: Propofol hydroxylation activities and enzyme kinetics mere determi ned using human liver microsomes and cDNA-expressed CYPs. CYP-specific mark er activities and CYP2B6 protein content were also quantified in hepatic mi crosomes for correlational analyses. Finally, inhibitory antibodies were us ed to ascertain the relative contribution of CYPs to propofol hydroxylation by hepatic microsomes. Results: Propofol hydroxylation by hepatic microsomes showed more than 19-f old variability and was most closely correlated to CYP2B6 protein content ( r = 0.904), and the CYP2B6 marker activities, S-mephenytoin N-demethylation (r = 0.919) and bupropion hydroxylation (r = 0.854). High- and intermediat e-activity livers demonstrated high-affinity enzyme kinetics (K-m < 8 <mu>M ), whereas low-activity livers displayed low-affinity kinetics (K-m > 80 mu M). All of the CYPs evaluated were capable of hydroxylating propofol; howev er, CYP2B6 and CYP2C9 were most active. Kinetic analysis indicated that CYP 2B6 is a high-affinity (K-m = 10 +/- 2 muM; mean +/- SE of the estimate), h igh-capacity enzyme, whereas CYP2C9 is a low-affinity (K-m = 41 +/- 8 muM), high-capacity enzyme. Furthermore, immunoinhibition showed a greater contr ibution of CYP2B6 (56 +/- 22% inhibition; mean +/- SD) compared with CYP2C isoforms (16 +/- 7% inhibition) to hepatic microsomal activity. Conclusions: Cytochrome P-450 2B6, and to a lesser extent CYP2C9, contribut e to the oxidative metabolism of propofol. However, CYP2B6 is the principal determinant of interindividual variability in the hydroxylation of this dr ug by human liver microsomes.