MOLECULAR AND BIOCHEMICAL-EVIDENCE FOR THE INVOLVEMENT OF THE ASP-333-HIS-523 PAIR IN CATALYTIC MECHANISM OF SOLUBLE EPOXIDE HYDROLASE

In order to investigate the involvement of amino acids in the catalyti c mechanism of the soluble epoxide hydrolase, different mutants of the murine enzyme were produced using the baculovirus expression system. Our results are consistent with the involvement of Asp-333 and His-523 in a catalytic mechanism similar to that of other alpha/beta hydrolas e fold enzymes. Mutation of His-263 to asparagine led to the loss of a pproximately half the specific activity compared to wild-type enzyme. When His-332 was replaced by asparagine, 96.7% of the specific activit y was lost and mutation of the conserved His-523 to glutamine led to a more dramatic loss of 99.9% of the specific activity. No activity was detectable after the replacement of Asp-333 by serine. However, more than 20% of the wild-type activity was retained in an Asp-333 --> Asn mutant produce din Spodoptera frugiperda cells. We purified, by affini ty chromatography, the wild-type and the Asp-333 --> Asn mutant enzyme s produced in Trichoplusia ni cells. We labeled these enzymes by incub ating them with the epoxide containing radiolabeled substrate juvenile hormone III (JH III). The purified Asp-333 --> Asn mutant bound 6% of the substrate compared to the wild-type soluble epoxide hydrolase. Th e mutant also showed 8% of the specific activity of the wild-type. Pre incubation of the purified Asp-333 --> Asn mutant at 37 degrees C (pH 8), however, led to a complete recovery of activity and to a change of isoelectric point (pI), both of which are consistent with hydrolysis of Asn-333 to aspartic acid. This intramolecular hydrolysis of asparag ine to aspartic acid may explain the activity observed in this mutant. Wild-type enzyme that had been radiolabeled with the substrate was di gested with trypsin. Using reverse phase-high pressure liquid chromato graphy, we isolated four radiolabeled peptides of similar polarity. Th ese peptides were not radiolabeled if the enzyme was preincubated with a selective competitive inhibitor of soluble epoxide hydrolase 4-fluo rochalcone oxide. This strongly suggested that these peptides containe d a catalytic amino acid. Each peptide was characterized with N-termin al amino acid sequencing and electrospray mass spectrometry. All four radiolabeled peptides contained overlapping sequences. The only aspart ic acid present in all four peptides and conserved in all epoxide hydr olases was Asp-333. These peptides resulted from cleavage at different trypsin sites and the mass of each was consistent with the covalent l inkage of Asp-333 to the substrate.