The interaction of cytosolic epoxide hydrolase with chiral epoxides

1. The kinetic parameters of the cytosolic epoxide hydrolase were examined with two sets of spectrophotometric substrates. The (2S,3S)- and (2R,3R)-enantiomers of 4-nitrophenyl trans-2,3-epoxy-3-phenylpropyl carbonate had a KM of 33 and 68 microns and a Vmax of 16 and 27 mumol/min/mg, respectively. With the (2S,3S)- and (2R,3R)-enantiomers of 4-nitrophenyl trans-2,3-epoxy-3-(4-nitrophenyl)propyl carbonate, cytosolic epoxide hydrolase had a KM of 8.0 and 15 microM and a Vmax of 7.8 and 5.0 mumol/min/mg, respectively. 2. Glycidyl 4-nitrobenzoate had the lowest I50 of the compounds tested in the glycidyl 4-nitrobenzoate series (I50 = 140 microM). The I50 of the (2R)-enantiomer was 3.7-fold higher. The inhibitor with the lowest I50 in the glycidol series, and the lowest I50 of any compound tested, was (2S,3S)-3-(4-nitrophenyl)glycidol (I50 = 13.0 microM). It also showed the greatest difference in I50 between the enantiomers (330-fold). 3. All enantiomers of glycidyl 4-nitrobenzoates and trans-3-phenylglycidols gave differential inhibition of cytosolic epoxide hydrolase. However, neither the (S,S)-/(S)- or (R,R)-/(R)-enantiomer always had the lower I50. 4. Addition of one or more methyl groups to either enantiomer of glycidyl 4-nitrobenzoate resulted in increased I50. However, addition of a methyl group to C2 of either enantiomer of 3-phenylglycidol resulted in a decreased I50. Finally, when the hydroxyl group of trans-3-(4-nitrophenyl)glycidol was esterified the I50 of the (2S,3S)- but not the (2R,3R)-enantiomer increased.