Cytochrome P4502C9 is the principal catalyst of racemic acenocoumarol hydroxylation reactions in human liver microsomes.
The oral anticoagulant acenocoumarol is given as a racemic mixture. The (S)-enantiomer is rapidly cleared and is the reason why only (R)-acenocoumarol contributes to the pharmacological effect. The objective of the study was to establish the cytochrome P450 (CYP) enzymes catalyzing the hydroxylations of the acenocoumarol enantiomers. Of various cDNA-expressed human CYPs, only CYP2C9 hydroxylated (S)-acenocoumarol. Hydroxylation occurred at the 6-, 7-, and 8-position with equal K(m) values and a ratio of 0.9:1:0.1 for V(max). CYP2C9 also mediated the 6-, 7-, and 8-hydroxylations of (R)-acenocoumarol with K(m) values three to four times and V(max) values one-sixth times those of (S)-acenocoumarol. (R)-Acenocoumarol was also metabolized by CYP1A2 (6-hydroxylation) and CYP2C19 (6-, 7-, and 8-hydroxylation). In human liver microsomes one enzyme only catalyzed (S)-acenocoumarol hydroxylations with K(m) values < 1 microM. In most of the samples tested the 7-hydroxylation of (R)-acenocoumarol was also catalyzed by one enzyme only. The 6-hydroxylation was catalyzed by at least two enzymes. Sulfaphenazole could completely inhibit in a competitive way the hydroxylations of (S)-acenocoumarol and the 7-hydroxylation of (R)-acenocoumarol. The 6-hydroxylation of (R)-acenocoumarol could be partially inhibited by sulfaphenazole, 40 to 50%, and by furafylline, 20 to 30%. Significant mutual correlations were obtained between the hydroxylations of (S)-acenocoumarol, the 7-hydroxylation of (R)-acenocoumarol, the 7-hydroxylation of (S)-warfarin, and the methylhydroxylation of tolbutamide. The results demonstrate that (S)-acenocoumarol is hydroxylated by a single enzyme, namely CYP2C9. CYP2C9 is also the main enzyme in the 7-hydroxylation of (R)-acenocoumarol. Other enzymes involved in (R)-acenocoumarol hydroxylation reactions are CYP1A2 and CYP2C19. Drug interactions must be expected, particularly for drugs interfering with CYP2C9. Also, drugs interfering with CYP1A2 and CYP2C19 may potentiate acenocoumarol anticoagulant therapy.
Thijssen
H H
HH
Department of Pharmacology, University of Maastricht, the Netherlands. h.thijssen@farmaco.unimaas.nl
Flinois
J P
JP
Beaune
P H
PH
D000074:Acenocoumarol / Q000037:antagonists & inhibitors / Q000493:pharmacokinetics*; D001189:Aryl Hydrocarbon Hydroxylases; D002384:Catalysis; D019388:Cytochrome P-450 CYP1A2; D065729:Cytochrome P-450 CYP2C9; D003577:Cytochrome P-450 Enzyme System / Q000378:metabolism*; D006801:Humans; D006900:Hydroxylation; D008862:Microsomes, Liver / Q000201:enzymology* / Q000378:metabolism; D011994:Recombinant Proteins / Q000378:metabolism; D039181:Steroid 16-alpha-Hydroxylase; D013250:Steroid Hydroxylases / Q000378:metabolism*; D014859:Warfarin / Q000037:antagonists & inhibitors / Q000493:pharmacokinetics
D011994:Recombinant Proteins; D014859:Warfarin; D003577:Cytochrome P-450 Enzyme System; D013250:Steroid Hydroxylases; C450260:CYP2C9 protein, human; D065729:Cytochrome P-450 CYP2C9; D001189:Aryl Hydrocarbon Hydroxylases; C504573:CYP1A2 protein, human; D019388:Cytochrome P-450 CYP1A2; D039181:Steroid 16-alpha-Hydroxylase; D000074:Acenocoumarol