INTERACTION OF VALPROIC ACID AND SOME ANALOGS 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 (mEH(b)) 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 mEH(b) 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 mEH(b) 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 mEH(b) was inhibited 59% under the same conditions. The inhibition was found to be competitive, with closely related K(I) values of 0.11, 0.16, 0.28, 0.27 and 0.31 mM for mEH(b) obtained from rat liver, human placenta, child, female and male liver, respectively. VPA demonstrated only a slight inhibition (maximum 16%) of mEH(b) 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 mEH(b) 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 mEH(b) 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 mEH(b). A molecular mechanism for the inhibition of mEH(b) by VPM is discussed.