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Abstract
Cytochrome P450 2C19 (CYP2C19) plays an important role in the metabolism of a wide range of therapeutic drugs and exhibits genetic polymorphism with interindividual differences in metabolic activity. We have previously described two CYP2C19 allelic variants, namely CYP2C19*18 and CYP2C19*19 with Arg329His/Ile331Val and Ser51Gly/Ile331Val substitutions, respectively. In order to investigate precisely the effect of amino acid substitutions on CYP2C19 function, CYP2C19 proteins of the wild-type (CYP2C19.1B having Ile331Val) and variants (CYP2C19.18 and CYP2C19.19) were heterologously expressed in yeast cells, and their S-mephenytoin 4′-hydroxylation activities were determined. The Km value of CYP2C19.19 for S-mephenytoin 4′-hydroxylation was significantly higher (3.0-fold) than that of CYP2C19.1B. Although no significant differences in Vmax values on the basis of microsomal and functional CYP protein levels were observed between CYP2C19.1B and CYP2C19.19, the Vmax/Km values of CYP2C19.19 were significantly reduced to 29–47% of CYP2C19.1B. By contrast, the Km, Vmax or Vmax/Km values of CYP2C19.18 were similar to those of CYP2C19.1B. These results suggest that Ser51Gly substitution in CYP2C19.19 decreases the affinity toward S-mephenytoin of CYP2C19 enzyme, and imply that the genetic polymorphism of CYP2C19*19 also causes variations in the clinical response to drugs metabolized by CYP2C19.