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Genomic DNA was isolated from livers of 39 Japanese and 45 Caucasians and the genotypes of CYP2C9 and 2C19 genes were determined with PCR methods using synthetic oligonucleotide primers. Liver microsomes were also prepared from these human samples and activities for tolbutamide methyl hydroxylation and S-mephenytoin 4'-hydroxylation were determined. The single base mutation of C416T (Argl44Cys) in CYP2C9 was detected in 22% of Caucasians but not in Japanese samples. Another single base mutation at A1061C (IIe359Leu) in the CYP2C9 gene was found with frequencies of about 8% in both races. We did not detect any individuals who have either homozygous Cysl44/Cysl44 or Leu359/Leu359 CYP2C9 variant nor both heterozygous Cysl44.I1e359 and Argl44.Leu359 CYP2C9 variant in the human samples examined. The CYP2C19tn2 genetic polymorphism was found only in Japanese people, while CYP2C19ml type was determined in both races, with higher incidence in Japanese than in Caucasian population. Immunoblotting analysis of human liver microsomes suggested that CYP2C9 is a major component of the human CYP2C enzyme pool; it accounted for ˜20% of total P450 in liver microsomes of both human populations. The levels of CYP2C19 protein were determined to be about 0.8% and 1.4% of total P450 (mean) in Japanese and Caucasians, respectively. We did not detect CYP2C19 protein in liver microsomes of humans who were genotyped for CYP2C19 gene as ml/ml, ml/m2, and m2/m2 variants but detected CYP2C9 protein in all of the samples examined. Good correlations were found between levels of CYP2C9 and activities of tolbutamide methyl hydroxylation (r=0.77) and between levels of CYP2C19 and activities of S-mephenytoin 4'-hydroxylation (r=0.86) in liver microsomes of the human samples examined. Tolbutamide methyl hydroxylation activities were lower in human samples with the Leu359 allele of CYP2C9 than those with the Cysl44 allele and wild-type (Argl44.IIe359); the former type showed slightly higher Km values. When calculated on P4 5 0 basis, liver microsomes of individuals having m 1 /m 1, ml/m2, and ni2/m2 types of CYP2C19 had very low catalytic activities for S-mephenytoin 4'- hydroxylation. These results provide useful comparisons for pharmacokinetic and toxicokinetic models of some of the clinically used drugs that are oxidized by CYP2C proteins in humans.