The Soluble Epoxide Hydrolase Encoded by EPXH2 Is a Bifunctional Enzyme with Novel Lipid Phosphate Phosphatase Activity

The gene EPXH2 encodes for the soluble epoxide hydrolase (sEH), an enzyme involved in the regulation of cardiovascular and renal physiology containing two distinct domains connected via a proline-rich linker. The C-terminal domain containing the EH catalytic activity has been well studied. In contrast, a function for the N-terminal domain, which has high homology to the haloacid dehalogenase family of phosphatases, has not been definitively reported. In this study we describe the N-terminal domain as a functional phosphatase unaffected by a number of classic phosphatase inhibitors. Assuming a functional association between these catalytic activities, dihydroxy lipid phosphates were rationalized as potential endogenous substrates. A series of phosphorylated hydroxy lipids were therefore synthesized and found to be excellent substrates for the human sEH. The best substrate tested was the monophosphate of dihydroxy stearic acid (threo-9/10-phosphonoxy-hydroxy-octadecanoic acid) with$K_m = 21 \pm 0.3\>\mu M,\>V_{Max} = 338 \pm 12\>nmol\!\cdot\!min^{-1}$, and$k_{cat} = 0.35 \pm 0.01\>s^{-1}$. Therefore dihydroxy lipid phosphates are possible candidates for the endogenous substrates of the sEH N-terminal domain, which would represent a novel branch of fatty acid metabolism with potential signaling functions.