Human microsomal epoxide hydrolase: Genetic poloymorphism and functional expression in vitro of amino acid variants

Human mlcrosomal epoxide hydrolase (mEH) is a blotransformation enzyme that metabolizes reactive epoxide Intermediates to more water-soluble trans-dlhydrodiol derivatives. We compared protein-coding sequences from six full-length human mEH DNA clones and assessed potential amlno acid variation at seven positions. The prevalence of these variants was assessed In at least 37 unrelated individuals using polymerase chain reaction experiments. Only Tyr/Hls 113 (exon 3) and His/Arg 139 (exon 4) variants were observed. The genotype frequencies determined for residue 113 alleles indicate that this locus may not be In Hardy-Weinberg equilibrium, whereas frequencies observed for residue 139 alleles were similar to expected values. Nucleotide sequences coding for the variant amino acids were constructed In an mEH cDNA using site-directed mutagenesls, and each was expressed In vitro by transient transfection of COS-1 cells. Epoxide hydrolase mRNA level, catalytic activity, and immunoreactive protein were evaluated for each construct. The results of these analyses demonstrated relatively uniform levels of mEH RNA expression between the constructs. mEH enzymatic activity and immunoreactive protein were strongly correlated, indicating that mEH specific activity was similar for each variant. However, marked differences were noted in the relative amounts of immunoreactive protein and enzymatic activity resulting from the amino acid substitutions. These data suggest that common human mEH amlno acid polymorphisms may alter enzymatic function, possibly by modifying protein stability.