title: Cytochrome P-450 2B6 is responsible for interindividual variability of propofol hydroxylation by human liver microsomes.
abstract: BACKGROUND
  Oxidation of propofol to 4-hydroxypropofol represents a significant 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 were determined using human liver microsomes and cDNA-expressed CYPs. CYP-specific marker activities and CYP2B6 protein content were also quantified in hepatic microsomes for correlational analyses. Finally, inhibitory antibodies were used to ascertain the relative contribution of CYPs to propofol hydroxylation by hepatic microsomes.
  
  
  RESULTS
  Propofol hydroxylation by hepatic microsomes showed more than 19-fold 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 intermediate-activity livers demonstrated high-affinity enzyme kinetics (K(m) < 8 microm), whereas low-activity livers displayed low-affinity kinetics (K(m) > 80 microm). All of the CYPs evaluated were capable of hydroxylating propofol; however, CYP2B6 and CYP2C9 were most active. Kinetic analysis indicated that CYP2B6 is a high-affinity (K(m) = 10 +/- 2 microm; mean +/- SE of the estimate), high-capacity enzyme, whereas CYP2C9 is a low-affinity (K(m) = 41 +/- 8 microm), high-capacity enzyme. Furthermore, immunoinhibition showed a greater contribution 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, contribute to the oxidative metabolism of propofol. However, CYP2B6 is the principal determinant of interindividual variability in the hydroxylation of this drug by human liver microsomes.
authors.affiliation: Department of Pharmacology and Experimental Therapeutics, Tufts University, Boston, Massachusetts 02111, USA. mcourt01@emerald.tufts.edu
authors.forename: M H
authors.initials: MH
authors.lastname: Court
authors.forename: S X
authors.initials: SX
authors.lastname: Duan
authors.forename: L M
authors.initials: LM
authors.lastname: Hesse
authors.forename: K
authors.initials: K
authors.lastname: Venkatakrishnan
authors.forename: D J
authors.initials: DJ
authors.lastname: Greenblatt
mesh_terms: D001189:Aryl Hydrocarbon Hydroxylases; D001711:Biotransformation; D065702:Cytochrome P-450 CYP2B6; D065729:Cytochrome P-450 CYP2C9; D003577:Cytochrome P-450 Enzyme System / Q000378:metabolism / Q000494:pharmacology / Q000502:physiology*; D006801:Humans; D006900:Hydroxylation; D008862:Microsomes, Liver / Q000187:drug effects / Q000201:enzymology / Q000378:metabolism*; D010089:Oxidoreductases, N-Demethylating / Q000378:metabolism / Q000502:physiology*; D015742:Propofol / Q000378:metabolism* / Q000493:pharmacokinetics; D039181:Steroid 16-alpha-Hydroxylase; D013250:Steroid Hydroxylases / Q000378:metabolism / Q000494:pharmacology
chemical_list: D003577:Cytochrome P-450 Enzyme System; D013250:Steroid Hydroxylases; C450260:CYP2C9 protein, human; D065729:Cytochrome P-450 CYP2C9; D001189:Aryl Hydrocarbon Hydroxylases; C585599:CYP2B6 protein, human; D065702:Cytochrome P-450 CYP2B6; D039181:Steroid 16-alpha-Hydroxylase; D010089:Oxidoreductases, N-Demethylating; D015742:Propofol