Identification and functional characterization of human soluble epoxide hydrolase genetic polymorphisms

Human soluble epoxide hydrolase (sEH), an enzyme directing the functional d isposition of a variety of endogenous and xenobiotic-derived chemical epoxi des, was characterized at the genomic level for interindividual variation c apable of impacting function. RNA was isolated from 25 human liver samples and used to generate full-length copies of soluble epoxide hydrolase cDNA. The resulting cDNAs were polymerase chain reaction amplified, sequenced, an d eight variant loci were identified. The coding region contained five sile nt single nucleotide polymorphisms (SNPs) and two variant loci resulting in altered protein sequence. An amino acid substitution was identified at res idue 287 in exon 8, where the more common arginine was replaced by glutamin e. A second variant locus was identified in exon 13 where an arginine resid ue was inserted following serine 402 resulting in the sequence, arginine 40 3-404, instead of the more common, arginine 403. This amino acid insertion was confirmed by analyzing genomic DNA from individuals harboring the polym orphic allele. Slot blot hybridization analyses of the liver samples indica ted that sEH mRNA steady-state expression varied approximately 10-fold. Tra nsient transfection experiments with CHO and COS-7 cells were used to demon strate that the two new alleles possess catalytic activity using trans-stil bene oxide as a model substrate. Although the activity of the glutamine 287 variant was similar to the sEH wild type allele, proteins containing the a rginine insertion exhibited strikingly lower activity. Allelic forms of hum an sEH, with markedly different enzymatic profiles, may have important phys iological implications with respect to the disposition of epoxides formed f rom the oxidation of fatty acids, such as arachidonic acid-derived intermed iates, as well in the regulation of toxicity due to xenobiotic epoxide expo sures.