stilbenes and chalcone-epoxide

A nuclear envelope-associated epoxide hydrolase in mouse liver that hydrates trans-stilbene oxide has been identified and characterized. This epoxide hydrolase is distinct from the enzyme in nuclear envelopes that hydrates benzo[a]pyrene 4,5-oxide and other arene oxides. This distinction was demonstrated by the criteria of pH optima, response to specific inhibitors in vitro, and precipitation by specific antibodies. The new epoxide hydrolase had a pH optimum of 6.8, was poorly inhibited by trichloropropene oxide, was potently inhibited by 4-phenylchalcone oxide, and did not bind to antiserum against benzo[a]pyrene 4,5-oxide hydrolase. This nuclear enzyme is similar in many of its properties to cytosolic and microsomal trans-stilbene oxide hydrolases and may be nuclear envelope-bound form of these other epoxide hydrolases. It differed from these other trans-stilbene oxide hydrolases in that its affinities for both trans-stilbene oxide (measured as apparent Km) and 4-phenylchalcone oxide (measured as I50) were 4- to 20-fold lower than those of either the cytosolic or microsomal forms.

Topics: Animals; Benzopyrenes; Cell Nucleus; Chalcone; Chalcones; Epoxide Hydrolases; Hydrogen-Ion Concentration; Kinetics; Liver; Male; Mice; Mice, Inbred C57BL; Stilbenes; Trichloroepoxypropane

Chalcone oxides and several isosteric compounds have been prepared to examine the importance of the alpha,beta-epoxyketone moiety in the inhibition of the hydrolysis of [3H]-trans-stilbene oxide to its meso-diol by mouse liver cytosolic epoxide hydrolase (cEH). Inhibition of microsomal EH and glutathione S-transferase were also examined. For cEH, replacement of the carbonyl by methylidene reduces inhibitor potency by a factor of 44, while replacement of the epoxide ring with a cyclopropyl ring reduces inhibition by a factor of 450. A 2'-hydroxyl also reduces cEH inhibition by 100 times. These observations are consistent with a model of the active site in which the carbonyl is hy-hydrogen-bonded to an acidic site presumed to be involved in initiating epoxide hydrolysis. The chalcone oxides thus bind tightly but are not readily turned over as substrates.

Topics: Animals; Binding Sites; Chalcone; Chalcones; Cytosol; Epoxide Hydrolases; Epoxy Compounds; Ethers, Cyclic; Glutathione Transferase; Liver; Mice; Microsomes, Liver; Stilbenes; Structure-Activity Relationship