Visualization of a covalent intermediate between microsomal epoxide hydrolase, but not cholesterol epoxide hydrolase, and their substrates

Frank Müller, Michael Arand, Heinz Frank, Albrecht Seidel, Willy Hinz, Lars Winkler, Karen Hänel, Elizabeth Blée, Jeffrey K. Beetham, Bruce D. Hammock, Franz Oesch

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Abstract

Mammalian soluble and microsomal epoxide hydrolases have been proposed to belong to the family of α/β-hydrolase-fold enzymes. These enzymes hydrolyse their substrates by a catalytic triad, with the first step of the enzymatic reaction being the formation of a covalent enzyme-substrate ester. In the present paper, we describe the direct visualization of the ester formation between rat microsomal epoxide hydrolase and its substrate. Microsomal epoxide hydrolase was precipitated with acetone after brief incubation with [1-14C]epoxystearic acid. After denaturing SDS gel electrophoresis the protein-bound radioactivity was detected by fluorography. Pure epoxide hydrolase and crude microsomes showed a single radioactive signal of the expected molecular mass that could be suppressed by inclusion of the competitive inhibitor 1,1,1-trichloropropene oxide in the incubation mixture. In a similar manner, 4-fluorochalcone-oxide-sensitive binding of epoxystearic acid to rat soluble epoxide hydrolase could be demonstrated in rat liver cytosol. Under similar conditions, no covalent binding of [26-14C]cholesterol-5α,6α-epoxide to microsomal proteins or solubilized fractions tenfold enriched in cholesterol epoxide hydrolase activity could be observed. Our data provide definitive proof for the formation of an enzyme-substrate-ester intermediate formed in the course of epoxide hydrolysis by microsomal epoxide hydrolase, show no formation of a covalent intermediate between cholesterol epoxide hydrolase and its substrate under the same conditions as those under which an intermediate was shown for both microsomal and soluble epoxide hydrolases and therefore indicate that the cholesterol epoxide hydrolase apparently does not act by a similar mechanism and is probably not structurally related to microsomal and soluble epoxide hydrolases.

Original languageEnglish (US)
Pages (from-to)490-496
Number of pages7
JournalEuropean Journal of Biochemistry
Volume245
Issue number2
StatePublished - 1997

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Keywords

  • α/β hydrolase fold
  • Cholesterol
  • Epoxide hydroxylase
  • Fatty acid metabolism
  • Mechanism

ASJC Scopus subject areas

  • Biochemistry

Cite this

Müller, F., Arand, M., Frank, H., Seidel, A., Hinz, W., Winkler, L., Hänel, K., Blée, E., Beetham, J. K., Hammock, B. D., & Oesch, F. (1997). Visualization of a covalent intermediate between microsomal epoxide hydrolase, but not cholesterol epoxide hydrolase, and their substrates. European Journal of Biochemistry, 245(2), 490-496.