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CYP2C9 amino acid residues influencing phenytoin turnover and metabolite regio- and stereochemistry.

ABSTRACT: Phenytoin has been an effective anticonvulsant agent for over 60 years, although its clinical use is complicated by nonlinear pharmacokinetics, a narrow therapeutic index, and metabolically based drug-drug interactions. Although it is well established that CYP2C9 is the major cytochrome P450 enzyme controlling metabolic elimination of phenytoin through its oxidative conversion to (S)-5-(4-hydroxyphenyl)-5-phenylhydantoin (p-HPPH), nothing is known about the amino acid binding determinants within the CYP2C9 active site that promote metabolism and maintain the tight stereocontrol of hydroxy metabolite formation. This knowledge gap was addressed here through the construction of nine active site mutants at amino acid positions Phe100, Arg108, Phe114, Leu208, and Phe476 and in vitro analysis of the steady-state kinetics and stereochemistry of p-HPPH formation. The F100L and F114W mutants exhibited 4- to 5-fold increases in catalytic efficiency, whereas the F100W, F114L, F476L, and F476W mutants lost >90% of their phenytoin hydroxylation capacity. This pattern of effects differs substantially from that found previously for (S)-warfarin and (S)-flurbiprofen metabolism, suggesting that these three ligands bind within discrete locations in the CYP2C9 active site. Only the F114L, F476L, and L208V mutants altered phenytoin's orientation during catalytic turnover. The L208V mutant also uniquely demonstrated enhanced 6-hydroxylation of (S)-warfarin. These latter data provide the first experimental evidence for a role of the F-G loop region in dictating the catalytic orientation of substrates within the CYP2C9 active site.

SUBMITTER: Mosher CM

PROVIDER: S-EPMC2683772 | biostudies-literature | 2009 Jun

REPOSITORIES: biostudies-literature

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CYP2C9 amino acid residues influencing phenytoin turnover and metabolite regio- and stereochemistry.

Mosher Carrie M CM Tai Guoying G Rettie Allan E AE

The Journal of pharmacology and experimental therapeutics 20090303 3

Phenytoin has been an effective anticonvulsant agent for over 60 years, although its clinical use is complicated by nonlinear pharmacokinetics, a narrow therapeutic index, and metabolically based drug-drug interactions. Although it is well established that CYP2C9 is the major cytochrome P450 enzyme controlling metabolic elimination of phenytoin through its oxidative conversion to (S)-5-(4-hydroxyphenyl)-5-phenylhydantoin (p-HPPH), nothing is known about the amino acid binding determinants within ...[more]

PMID: 19258521

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Project description:The enzyme AHAS (acetohydroxy acid synthase), which is involved in the biosynthesis of valine, leucine and isoleucine, is the target of several classes of herbicides. A model of tobacco AHAS was generated based on the X-ray structure of yeast AHAS. Well conserved residues at the herbicide-binding site were identified, and the roles of three of these residues (Phe-205, Val-570 and Phe-577) were determined by site-directed mutagenesis. The Phe-205 mutants F205A, F205H, F205W and F205Y showed markedly decreased levels of catalytic efficiency, and cross-resistance to two or three classes of herbicides, i.e. Londax (a sulphonylurea herbicide), Cadre (an imidazolinone herbicide) and TP (a triazolopyrimidine derivative). None of the mutations caused significant changes in the secondary or tertiary structure of the enzyme. Four mutants of Phe-577, i.e. F577D, F577E, F577K and F577R, showed unaltered V(max) values, but substantially decreased catalytic efficiency. However, these mutants were highly resistant to two or three of the tested herbicides. The three mutants F577D, F577E and F577R had a similar secondary structure to that of wild-type AHAS. Conservative mutations of Phe-577, i.e. F577W and F577Y, did not affect the kinetic properties of the enzyme or its inhibition by herbicides. The mutation Val-570 to Asn abolished the binding affinity of the enzyme for FAD as well as its activity, and also caused a change in the tertiary structure of AHAS. However, the mutant V570Q was active, but resistant to two classes of herbicides, i.e. Londax and TP. The conservative mutant V570I was substantially reduced in catalytic efficiency and moderately resistant to the three herbicides. The results of this study suggest that residues Phe-205, Val-570 and Phe-577 in tobacco AHAS are located at or near the binding site that is common for the three classes of herbicides. In addition, Phe-205 and Val-570 are probably located at the herbicide-binding site that may overlap partially with the active site. Selected mutants of Phe-577 are expected to be utilized to construct herbicide-resistant transgenic plants.

Dataset Information

CYP2C9 amino acid residues influencing phenytoin turnover and metabolite regio- and stereochemistry.

Publications

CYP2C9 amino acid residues influencing phenytoin turnover and metabolite regio- and stereochemistry.

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