Interaction of valproic acid and some analogues with microsomal epoxide hydrolase

Valproic acid (VPA) and its analogues valpromide (VPM), valproyl-Coenzyme A (VP-CoA) and valproyl-ethylester (VPE) were examined as potential inhibitors of microsomal epoxide hydrolase (mEHb) using styrene-7,8-oxide (STO) and benzo(a)pyrene-4,5-oxide (BPO) as enzyme substrates. The effect of each potential inhibitor was examined using mEHb from rat liver, human livers (from a child, woman and man) and from human placenta. Of the compounds tested, only VPM (2 mM) expressed significant inhibition of mEHb activity with a maximum inhibition of 49%, 48%, 35% and 33% for liver microsomes from the child, woman, man and rat, respectively, using STO (2 mM) as substrate. Human placenta mEHb was inhibited 59% under the same conditions. The inhibition was found to be competitive, with closely related KI values of 0.11, 0.16, 0.28, 0.27 and 0.31 mM for mEHb obtained from rat liver, human placenta, child, female and male liver, respectively. VPA demonstrated only a slight inhibition (maximum 16%) of mEHb at high concentrations (10 mM), and VP-CoA was found to activate STO hydrolysis slightly at concentrations between 1 and 5 mM. VPE caused a moderate concentration-dependent activation of mEHb in all microsomal preparations examined. The inhibitory or activating properties of each compound were independent of the substrate and influenced slightly by the pH used in the incubation medium. The lack of inhibition of mEHb by VPA and its analogues other than VPM shows that neither masking of the carboxyl function of VPA nor the introduction of higher lipophilicity are sufficient to account for the inhibitory properties of VPM for mEHb. A molecular mechanism for the inhibition of mEHb by VPM is discussed.