Cytochrome P450 2C8 (CYP2C8) plays important roles in the metabolism of various drugs, including the anti-cancer drug, paclitaxel. We recently identified two novel CYP2C8 alleles (CYP2C8*13 and CYP2C8*14; wild-type, CYP2C8*1A) with non-synonymous single nucleotide polymorphisms in a Japanese population. To precisely investigate the effect of amino acid substitutions (CYP2C8*13, Ile223Met; CYP2C8*14, Ala238Pro) on CYP2C8 function, CYP2C8 proteins of the wild-type (CYP2C8.1) and variants (CYP2C8.13 and CYP2C8.14) were heterologously expressed in yeast cells, and their paclitaxel 6alpha-hydroxylation activities were determined. The K(m), V(max) and CL(int) values for paclitaxel 6alpha-hydroxylation of CYP2C8.1 were 2.3 microM, 4.1 pmol/min./pmol CYP and 1.7 microl/min./pmol CYP, respectively. The K(m) value of CYP2C8.14 was significantly higher (2.9-fold) than that of CYP2C8.1. The V(max) value of CYP2C8.14 was comparable to that of CYP2C8.1 and the CL(int) value was reduced to 46% of CYP2C8.1. In contrast, the K(m), V(max) and CL(int) values of CYP2C8.13 were similar to those of CYP2C8.1. These results suggest that Ala238Pro substitution in CYP2C8.14 decreases the affinity toward paclitaxel of the CYP2C8 enzyme, and that the genetic polymorphism of the CYP2C8*14 allele may influence the clinical response to drugs metabolized mainly by CYP2C8.