The role of the CYP2C9-Leu(359) allelic variant in the tolbutamide polymorphism

Tolbutamide undergoes hydroxylation in humans via a cytochrome P450-mediated pathway, The primary P450 isozyme responsible for this metabolism is thought to be CYP2C9, Population studies have indicated the existence of slow metabolizers of tolbutamide (similar to 1 in 500) suggesting a rare polymorphism associated with 2C9, Several allelic variants of 2C9 have been identified; however, the effect of these allelic variations on metabolism in vivo is not established, In the present study, the coding regions, intron-exon junctions, and upstream region of CYP2C9 were amplified by PCR and sequenced in two slow metabolizers. One individual was homozygous for Leu(359)/Leu(359) and the other individual was heterozygous for Arg(144)/Cys(144) and for Ile(359)/Leu(359), No other genetic variations in 2C9 were detected in these individuals, PCR-RFLP tests showed that Arg(144)Tyr(358)Ile(359)Gly(417) is the principle CYP2C9 allele, Frequencies of the rarer Leu(359) and Cys(144) alleles were 0.06 and 0.08, respectively, in a Caucasian-American population and 0.005 and 0.01 respectively in African-Americans. The freqnency of the Leu(359) allele was 0.026 in Chinese-Taiwanese, but the Cys(144) allele was not detected in this population, Studies in a recombinant yeast expression system showed that the Leu(359) variant had the highest K-m and the lowest V-mac for hydroxylation of tolbutamide of all the CYP2C9 allelic variants, This allelic variant also had the highest K-m for the 7-hydroxylation of S-warfarin, The present data suggest that the incidence of the Leu(359) allelic variant of CYP2C9 may account for the occurrence of poor metabolizers of tolbutamide.