Arachidonic acid is oxidized by cytochromes P450 2C (CYP2C) to epoxyeicosatrienoic acids (EETs), possessing vasoactive properties, with 11,12-EET as the endothelium derived hyperpolarization factor. Genetic variants of CYP2C enzymes have altered drug metabolizing capacity. Our primary aim was to determine whether EET biosynthesis differed in human liver microsomes with known CYP2C genotypes. Human liver microsomes (n = 25) of different CYP2C genotypes or yeast-expressed CYP2C enzymes were used. Analysis of metabolites was performed by liquid chromatography/mass spectrometry. Samples genotyped as CYP2C8*3/*3/CYP2C9*2/*2 exhibited a 34% (p < 0.05) decreased EET biosynthesis, compared to other CYP2C8/CYP2C9 haplotypes. Inhibition experiments suggested CYP2C8 and CYP2C9 to be the predominant catalysts of EETs. We found no differences between the three recombinantly expressed CYP2C9 variants, but CYP2C8.1 had lower K-m than these isoforms. In conclusion, there are genetic differences in the CYP2C-dependent oxidation of arachidonic acid to vasoactive metabolites, of which the relevance to cardiovascular pathophysiology is still unclear. (C) 2004 Elsevier Inc. All rights reserved.