HUMAN MICROSOMAL EPOXIDE HYDROLASE - GENETIC-POLYMORPHISM AND FUNCTIONAL EXPRESSION IN-VITRO OF AMINO-ACID VARIANTS

Human microsomal epoxide hydrolase (mEH) is a biotransformation enzyme that metabolizes reactive epoxide intermediates to more water-soluble trans-dihydrodiol derivatives. We compared protein-coding sequences f rom six full-length human mEH DNA clones and assessed potential amino acid variation at seven positions. The prevalence of these variants wa s assessed in at least 37 unrelated individuals using polymerase chain reaction experiments. Only Tyr/His 113 (exon 3) and His/Arg 139 (exon 4) variants were observed. The genotype frequencies determined for re sidue 113 alleles indicate that this locus may not be in Hardy - Weinb erg equilibrium, whereas frequencies observed for residue 139 alleles were similar to expected values. Nucleotide sequences coding for the v ariant amino acids were constructed in an mEH cDNA using site-directed mutagenesis, and each was expressed in vitro by transient transfectio n of COS-1 cells. Epoxide hydrolase mRNA level, catalytic activity, an d immunoreactive protein were evaluated for each construct. The result s of these analyses demonstrated relatively uniform levels of mEH RNA expression between the constructs. mEH enzymatic activity and immunore active protein were strongly correlated, indicating that mEH specific activity was similar for each variant. However, marked differences wer e noted in the relative amounts of immunoreactive protein and enzymati c activity resulting from the amino acid substitutions. These data sug gest that common human mEH amino acid polymorphisms may alter enzymati c function, possibly by modifying protein stability.