ABSTRACT:Cytochrome P450 2C8 is one of the primary enzymes responsible for the metabolism of a wide range of drugs such as paclitaxel, cerivastatin, and amiodarone. We have sequenced the CYP2C8 gene from 201 Japanese subjects and found five novel nonsynonymous single nucleotide polymorphisms (SNPs): 511G>A (G171S), 556C>T (R186X; X represents the translational stop codon), 556C>G (R186G), 740A>G (K247R), and 1149G>T (K383N), with the allele frequency of 0.0025. The CYP2C8 variants were heterologously expressed in COS-1 cells and functionally characterized in terms of expression level, paclitaxel 6␣-hydroxylase activity, and intracellular localization. The prematurely terminated R186X variant was undetectable by Western blotting and inactive toward paclitaxel 6␣-hydroxylation. The G171S, K247R, and K383N variants exhibited properties similar to those of the wild-type CYP2C8. Paclitaxel 6␣-hydroxylase activity of the R186G transfectant was only 10 to 20% that of wild-type CYP2C8. Furthermore, the R186G variant displayed a lower level of protein expression in comparison to the wild type, which was restored by the addition of a proteasome inhibitor (MG-132; Z-Leu-Leu-Leu-aldehyde). The reduced CO-difference spectral analysis using recombinant proteins from an insect cell/baculovirus system revealed that the R186G variant has a minor peak at 420 nm in addition to the characteristic Soret peak at 450 nm, suggesting the existence of improperly folded protein. These results indicate that the novel CYP2C8 SNPs, 556C>T (R186X) and 556C>G (R186G), could influence the metabolism of CYP2C8 substrates such as paclitaxel and cerivastatin.

Twenty genetic variations, including seven novel ones, were found in the human SLC22A1 gene, which encodes organic cation transporter 1, from 116 Japanese individuals. The novel variations were as follows: -94C>A in the 5'-untranslated region (A of the translation start codon is numbered +1 in the cDNA sequence; MPJ6_OC1001), 350C>T (MPJ6_OC1004), IVS1-35T>C (MPJ6_OC1006), 561G>A (MPJ6_OC1010), IVS6+75C>G (MPJ6_OC1014), IVS8+108A>G (MPJ6_OC1017), and 1671_1673delATG (MPJ6_OC1020). The frequencies were 0.082 for IVS1-35T>C, 0.022 for IVS6+75C>G, 0.009 for 561G>A, and 0.004 for the other 4 variations. Among them, 350C>T resulted in the amino acid substitution Pro117Leu, which is located in the large extracellular loop between transmembrane domains 1 and 2. Also, we detected the four previously reported nonsynonymous variations, 123C>G (Phe41Leu), 480C>G (Phe160Leu), 1022C>T (Pro341Leu), and 1222A>G (Met408Val) with frequencies of 0.004, 0.086, 0.168, and 0.810, respectively.

Bufuralol (BF), a nonselective beta-adrenoceptor blocking agent, has a chiral center in its molecule, yielding the enantiomers 1'R-BF and 1'S-BF. beta-Adrenoceptor blocking potency is much higher in 1'S-BF than in 1'R-BF. One of the metabolic pathways of BF is 1"-hydroxylation of an ethyl group attached at the aromatic 7-position forming a carbinol metabolite (1"-hydroxybufuralol, 1"-OH-BF), and further oxidation (or dehydrogenation) produces a ketone metabolite (1-oxobufuralol, 1"-Oxo-BF). Both 1"-OH-BF and 1"-Oxo-BF are known to have beta-adrenoceptor blocking activities comparable to or higher than those of the parent drug. The 1"-hydroxylation introduces another chiral center into the BF molecule and four 1"-OH-BF diastereomers are formed from BF racemate in mammals, including humans, making elucidation of the metabolic profiles complicated. HPLC methods employing derivatization, reversed phase, or chiral columns have been developed to efficiently separate the four 1"-OH-BF diastereomers formed from BF enantiomers or racemate. Accumulated in vitro experimental results revealed that 1'R-BF is a much more preferential substrate than 1'S-BR for BF 1"-hydroxylation in human liver microsomes. Kinetic studies using recombinant human cytochrome P450 (CYP) enzymes indicate that CYP2D6 serves as a major BF 1"-hydroxylase and that CYP1A2 and CYP2C19 also contribute to BF 1"-hydroxylation in human livers. This mini-review summarizes the knowledge reported so far on the pharmacology of BF and its metabolites and the profiles of BF metabolism, especially focusing on the stereoselectivity in the oxidation of BF mainly in human livers and recombinant CYP enzymes.

The functional roles of phenylalanine at position 120 in drug oxidation by cytochrome P450 2D6 (CYP2D6) were examined using a yeast cell expression system and bufuralol (BF) enantiomers as a chiral substrate. Two mutated cDNAs, one encoding a CYP2D6 mutant having alanine instead of Phe-120 (F120A) and another encoding a mutant having alanine instead of Glu-222 (E222A), were prepared by site-directed mutagenesis and transformed into yeast cells via pGYRI vectors. The enantiomeric BF 1''-hydroxylase activities of the mutants were compared with those of the wild type. When enantiomeric BF 1''-hydroxylase activities at a substrate concentration of 100 microM were compared, the CYP2D6 wild type showed substrate enantioselectivity of (R-BF >> S-BF) and the F120A mutant exhibited substrate enantioselectivity of (R-BF < or = S-BF), whereas the product diastereoselectivity of (1''R-OH-BF << 1''-S-OH-BF) was similar between the wild type and the mutant. The activities of the other mutant (E222A) were much lower than those of the wild type and the F120A mutant, while its substrate enantioselectivity and product diastereoselectivity were the same as those of the wild type. The kinetics demonstrated that apparent K(m) values were similar among the recombinant enzymes, and V(max) values clearly reflected the selectivity described above. These results indicate that Phe-120 has a key role in the enantioselective BF 1''-hydroxylation by CYP2D6.

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