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The evolution of the C‐terminal domain of soluble epoxide hydrolase

The mammalian soluble epoxide hydrolase is a multidomain enzyme composed of N‐ and C‐terminal regions that contain active sites for epoxide hydrolase and phosphatase activities, respectively. The spatial separation and differing homologies of the N‐ and C‐terminal domains have led to the hypothesis that the mammalian enzyme is the result of a gene fusion event between two ancestral genes. Although several plant and mammalian enzymes have been identified, little is known about sEH epoxide hydrolase activity in non‐mammalian animal systems. Analysis of genomic databases identified chicken, nematode and urchin sEH homologs which were subsequently cloned and expressed. The chicken and urchin enzymes contained both the N‐ and C‐terminal domains, while two nematode sEH homologs contained only the C‐terminal domain. All enzymes displayed sEH‐like epoxide hydrolase activity, which was described in terms of the ability to hydrolyze trans‐1,3‐diphenylpropene oxide and fatty acid epoxides such as the epoxyeicosatrienoic acids, and susceptibility to inhibition by urea‐based sEH inhibitors. The identification of sEH homologs with similar substrate selectivity as the mammalian enzyme suggests a conserved role for sEH epoxide hydrolase activity, perhaps involving the hydrolysis of epoxide lipid signaling molecules. This work has been supported by NIEHS Grant R37 ES02710 and NIEHS Grant P42 ES004699.