Significant variations in the metabolism of various drugs and environmental chemicals which are metabolized via cytochrome P450 (CYP) enzymes exist between humans. Many of these interindividual variations are attributed to polymorphisms in the CYP2C subfamily of enzymes. In the current work, two new CYP2C8 polymorphisms were identified which contain coding changes. One allele termed CYP2C8*2 had an lle269Phe substitution in exon 5. The second allele termed CYP2C8*3 contained Arg139Lys and Lys399Arg amino acid substitution in exons 3 and 5. An HPLC method was developed to assess the ability of these enzymes to metabolize paclitaxol was developed. When CYP2C8*1 (wild type), CYP2C8*2, and CYP2C8*3 cDNAs were expressed in an E. Coli expression system, the ability of CYP2C8*3 to metabolize paclitaxol was markedly impaired. The CYP2C8*2 allele exhibited a 2 fold higher Km and 1 2-fold lower intrinsic clearance than that of CYP2C8*1. The turnover number for CYP2C8*3 for paclitaxol was only 16% of CYP2C8*1. Thus, the CYP2C8*3 allele is defective in metabolism of CYP2C8 substrates such as paclitaxol. This polymorphism may have important clinical and physiological implications in individuals homozygous for this allele.
