Chalcone oxides-potent selective inhibitors of cytosolic epoxide hydrolase

The systematic screening of over 150 compounds for inhibitory activity on mammalian cytosolic epoxide hydrolase led to identification of chalcone oxide (trans-1-benzoyl-2-phenyloxirane) as an optimal inhibitory structure. Important structural features for inhibition include two hydrophobic moieties preferably orientating in a trans manner from an electrophilic center such as an activated olefin or epoxide, with the epoxide giving maximal activity. Synthesis of chalcone oxide derivatives bearing a single p-substituent on either phenyl ring has led to very potent inhibitors of the enzyme, the best being 4-phenylchalcone oxide (50% inhibition at 6.4 × 10^-8 m). Multiple factorial analysis on the inhibition data for the two series of chalcone oxides prepared (phenyl or benzoyl substituted) revealed both the essentialness of hydrophobic interactions and the apparent nonequivalence of the two hydrophobic sites involved in the inhibitory process. Steric factors were considerably less crucial while electronic effects were unimportant in the compounds examined. The chalcone oxides were either inactive or only weak inhibitors of the other major epoxide-metabolizing enzymes in mouse liver, cytosolic glutathione S-transferase, and microsomal epoxide hydrolase. The nature of the inhibition of cytosolic epoxide hydrolase by chalcone oxides was further investigated through steady-state kinetic analysis and the use of amino acid modifiers. Chalcone oxides give a slowly reversible mixed-noncompetitive inhibition. They may interact covalently with a cysteine residue possibly essential to the catalytic action of cytosolic epoxide hydrolase, and may indeed be alternative substrates with very low turnover. The cytosolic and microsomal epoxide hydrolases can be clearly distinguished by these inhibitors, further indicating different catalytic mechanisms.