Covalent linkage of prosthetic heme to CYP4 family P450 enzymes

An extensive body of research on the structural properties of cytochrome P450 enzymes has established that these proteins possess a b-type heme prosthetic group which is noncovalently bound at the active site. Coordinate, electrostatic, and hydrogen bond interactions between the protein backbone and heme functional groups are readily overcome upon mild acid treatment of the enzyme, which releases free heme from the protein. In the present study, we have used a combination of HPLC, LC/ESI-MS, and SDS-PAGE techniques to demonstrate that members of the mammalian CYP4B, CYP4F, and CYP4A subfamilies bind their heme in an unusually tight manner. HPLC chromatography of CYP4B1 on a POROS R2 column under mild acidic conditions caused dissociation of less than one-third of the heme from the protein. Moreover, heme was not substantially removed from CYP4B1 under electrospray or electrophoresis conditions that readily release the prosthetic group from other non-CYP4 P450 isoforms. This was evidenced by an intact protein mass value of 59,217 +/- 3 amu for CYP4B1 (i.e., apoprotein plus heme) and extensive staining of this approximately 60 kDa protein with tetramethylbenzidine/H(2)O(2) following SDS-PAGE. In addition, treatment of CYP4B1, CYP4F3, and CYP4A5/7 with strong base generated a new, chromatographically distinct, polar heme species with a mass of 632.3 amu rather than 616.2 amu. This mass shift is indicative of the incorporation of an oxygen atom into the heme nucleus and is consistent with the presence of a novel covalent ester linkage between the protein backbone of the CYP4 family of mammalian P450s and their heme catalytic center.

Active site characteristics of CYP4B1 probed with aromatic ligands

The active site topography of rabbit CYP4B1 has been studied relative to CYP2B1 and CYP102 using a variety of aromatic probe substrates. Oxidation of the prochiral substrate cumene by CYP4B1, but not CYP2B1 or CYP102, resulted in the formation of the thermodynamically disfavored omega-hydroxy metabolite, 2-phenyl-1-propanol, with product stereoselectivity for the (S)-enantiomer. Reaction of CYP4B1, CYP2B1, and CYP102 with phenyldiazene produced spectroscopically observable sigma-complexes for each enzyme. Subsequent oxidation of the CYP2B1 and CYP102 complexes followed by LC/ESI--MS analysis yielded heme pyrrole migration patterns similar to those in previous literature reports. Upon identical treatment, no migration products were detected for CYP4B1. Intramolecular deuterium isotope effects for the benzylic hydroxylation of o-xylene-alpha-(2)H(3), p-xylene-alpha-(2)H(3), 2-(2)H(3),6-dimethylnaphthalene, and 4-(2)H(3),4'-dimethylbiphenyl were determined for CYP4B1 and CYP2B1 to further map their active site dimensions. These probes permit assessment of the ease of equilibration, within P450 active sites, of oxidizable methyl groups located between 3 and 10 A apart [Iyer et al. (1997) Biochemistry 36, 7136--7143]. Isotope effects for the CYP4B1-mediated benzylic hydroxylation of o- and p-xylenes were fully expressed (k(H)/k(D) = 9.7 and 6.8, respectively), whereas deuterium isotope effects for the naphthyl and biphenyl derivatives were both substantially masked (k(H)/k(D) approximately equal to 1). In contrast, significant suppression of the deuterium isotope effects for CYP2B1 occurred only with the biphenyl substrate. Therefore, rapid equilibration between two methyl groups more than 6 A apart is impeded within the active site of CYP4B1, whereas for CYP2B1, equilibration is facile for methyl groups distanced by more than 8 A. Collectively, all data are consistent with the conclusion that the active site of CYP4B1 is considerably restricted relative to CYP2B1.

Enzymatic determinants of the substrate specificity of CYP2C9: role of B'-C loop residues in providing the pi-stacking anchor site for warfarin binding

Previous modeling efforts have suggested that coumarin ligand binding to CYP2C9 is dictated by electrostatic and pi-stacking interactions with complementary amino acids of the protein. In this study, analysis of a combined CoMFA-homology model for the enzyme identified F110 and F114 as potential hydrophobic, aromatic active-site residues which could pi-stack with the nonmetabolized C-9 phenyl ring of the warfarin enantiomers. To test this hypothesis, we introduced mutations at key residues located in the putative loop region between the B' and C helices of CYP2C9. The F110L, F110Y, V113L, and F114L mutants, but not the F114Y mutant, expressed readily, and the purified proteins were each active in the metabolism of lauric acid. The V113L mutant metabolized neither (R)- nor (S)-warfarin, and the F114L mutant alone displayed altered metabolite profiles for the warfarin enantiomers. Therefore, the effect of the F110L and F114L mutants on the interaction of CYP2C9 with several of its substrates as well as the potent inhibitor sulfaphenazole was chosen for examination in further detail. For each substrate examined, the F110L mutant exhibited modest changes in its kinetic parameters and product profiles. However, the F114L mutant altered the metabolite ratios for the warfarin enantiomers such that significant metabolism occurred for the first time on the putative C-9 phenyl anchor, at the 4'-position of (R)- and (S)-warfarin. In addition, the Vmax for (S)-warfarin 7-hydroxylation decreased 4-fold and the Km was increased 13-fold by the F114L mutation, whereas kinetic parameters for lauric acid metabolism, a substrate which cannot interact with the enzyme by a pi-stacking mechanism, were not markedly affected by this mutation. Finally, the F114L mutant effected a greater than 100-fold increase in the Ki for inhibition of CYP2C9 activity by sulfaphenazole. These data support a role for B'-C helix loop residues F114 and V113 in the hydrophobic binding of warfarin to CYP2C9, and are consistent with pi-stacking to F114 for certain aromatic ligands.

Chiral phase analysis of warfarin enantiomers in patient plasma in relation to CYP2C9 genotype

A direct chiral-phase high-performance liquid chromatographic method for measuring the ratio of S-warfarin/R-warfarin in patient plasma is described. Plasma samples are first extracted using solid-phase C18 extraction columns, and the concentrated extracts analyzed using an (R,R) Whelk-O 1 column with a mobile phase of 0.5% glacial acetic acid in acetonitrile. The resulting chromatography provides baseline resolution of the warfarin enantiomers and internal standard (racemic ethylwarfarin), and is free from interference from other plasma components. Calibration curves were linear (mean r2 of 0.999 for both enantiomers) over the concentration range 0.25-1.5 microg/ml. The intra-day and inter-day coefficients of variation for analysis of plasma spiked with 0.33 microg/ml S-warfarin and 0.67 microg/ml R-warfarin (S/R=0.5:1) was less than 7% for each enantiomer, with an accuracy of more than 93%. Plasma extracts from thirty-one patients homozygous for wild-type CYP2C9*1 provided an S/R ratio of 0.51+/-0.15. Two warfarin patients homozygous for the mutant CYP2C9*2 and CYP2C9*3 alleles exhibited elevated S/R ratios relative to the mean for individuals homozygous for the wild-type CYP2C9*1 allele. This method is suitable for population studies aimed at establishing the effect of polymorphic expression of CYP2C9 alleles on S-warfarin elimination in humans.

Genetic association between sensitivity to warfarin and expression of CYP2C9*3

Cytochrome P4502C9 (CYP2C9) is largely responsible for terminating the anticoagulant effect of racemic warfarin via hydroxylation of the pharmacologically more potent S-enantiomer to inactive metabolites. Mutations in the CYP2C9 gene result in the expression of three allelic variants, CYP2C9*1, CYP2C9*2 and CYP2C9*3. Both CYP2C9*2 and CYP2C9*3 exhibit altered catalytic properties in vitro relative to the wild-type enzyme. In the present study, a patient was genotyped who had proven unusually sensitive to warfarin therapy and could tolerate no more than 0.5 mg of the racemic drug/day. PCR-amplification of exons 3 and 7 of the CYP2C9 gene, followed by restriction digest or sequence analysis, showed that this individual was homozygous for CYP2C9*3. In addition, patient plasma warfarin enantiomer ratios and urinary 7-hydroxywarfarin enantiomer ratios were determined by chiral-phase high performance liquid chromotography in order to investigate whether either parameter might be of diagnostic value in place of a genotypic test. Control patients receiving 4-8 mg warfarin/day exhibited plasma S:R ratios of 0.50 +/- 0.25:1, whereas the patient on very low-dose warfarin exhibited an S:R ratio of 3.9:1. In contrast, the urinary 7-hydroxywarfarin S:R ratio of 4:1 showed the same stereoselectivity as that reported for control patients. Therefore, expression of CYP2C9*3 is associated with diminished clearance of S-warfarin and a dangerously exacerbated therapeutic response to normal doses of the racemic drug. Analysis of the plasma S:R warfarin ratio may serve as a useful alternative test to genotyping for this genetic defect.