Omeprazole is one of the most widely used proton pump inhibitors for the treatment of gastric acid-related disorders. The major metabolic pathway of omeprazole is 5-hydroxylation, which is catalysed by CYP2C19. In this study, the effect of CYP2C19*18 and CYP2C19*19 alleles on omeprazole 5-hydroxylation was studied using recombinant CYP2C19 enzymes of wild-type (CYP2C19.1B having Ile331Val) and variants (CYP2C19.18 having Arg329His/Ile331Val and CYP2C19.19 Ser51Gly/Ile331Val) expressed in yeast cells. The K(m) value for omeprazole 5-hydroxylation of CYP2C19.1B was 1.46 microM. The K(m) value of CYP2C19.19 was significantly higher (1.5-fold) than that of CYP2C19.1B. V(max) and V(max)/K(m) values for omeprazole 5-hydroxylation of CYP2C19.1B on the basis of cytochrome P450 protein level were 8.09 pmol/min./pmol CYP and 5.45 microl/min./pmol CYP, respectively. The V(max) value of CYP2C19.19 was significantly higher (1.8-fold) than that of CYP2C19.1B, whereas the V(max)/K(m) value was comparable to that of CYP2C19.1B. In contrast, K(m), V(max) and V(max)/K(m) values of CYP2C19.18 were similar to those of CYP2C19.1B. These results suggest that CYP2C19*19 allele decreases the affinity between CYP2C19 enzyme and the substrate in omeprazole metabolism.