Characterization of the human sEH phosphatase by site directed mutagenesis and LC‐MS/MS analysis

Abstract

The bifunctional human soluble epoxide hydrolase (sEH) is implicated as a regulator of diverse physiological processes due to the brakedown of arachidonic acid derived signalling molecules like epoxyeicosatrienoic acids (EETs) by its epoxide hydrolase domain. Recently, we discovered that the sEH N‐terminal domain displays a novel phosphatase activity. sEH accepts the generic substrate 4‐NPP as well as lipid phosphates, but the physiological role remains uncertain.The phosphatase domain contains three conserved sequence motifs, including the potential catalytic nucleophile Asp9, and several residues implicated in substrate turnover and/or Mg2+ binding. To enlighten the proposed catalytic mechanism we constructed active site mutants by site directed mutagenesis, which were recombinantly expressed as soluble proteins, purified and analysed for their kinetic properties. An exchange of Asp9, Lys160, Asp184 or Asn189 results in a complete loss of phosphatase activity, emphasising the requirement of these amino acids for catalysis, whereas a substitution of Asp11, Thr123, Asn124 and Asp185 leads to sEH mutant proteins with residual phosphatase activity. To confirm the role of Asp9 as catalytic nucleophile we presently analyze the presumed phosphoester intermediate by mass spectrometry. The dual phosphatase and epoxide hydrolase activities give new insights into the physiological role of human sEH.