INHIBITION OF PARAOXONASE ACTIVITY IN HUMAN LIVER-MICROSOMES BY EXPOSURE TO EDTA, METALS AND MERCURIALS

Inhibition of paraoxon hydrolase (paraoxonase) activity by 'in vitro' exposure to EDTA, Mg2+, Co2+, Ba2+, La3+, Zn2+, Cu2+, Hg2+, p-hydroxym ercuribenzoate (p-OH-MB) and phenyl mercuric acetate (PMA) was investi gated in human liver microsomes. Enzyme activity was totally inhibited by 1 mM EDTA in a time-dependent manner, in contrast to previous data obtained in rat liver where an EDTA-resistant fraction was detected. The possible influence of postmortem changes in these results was chec ked in a parallel experiment using rat livers with different postmorte m intervals. From our results the existence in human liver of an EDTA- resistant fraction cannot be discarded. Ba, La and PMA showed immediat e inhibition. By contrast the other compounds tested were time-depende nt inhibitors. Ba and Zn showed the highest IC50 values. Cu and mercur ials (Hg, p-OH-MB, PMA) were the most potent inhibitors of human liver paraoxonase. Kinetic analysis (Lineweaver-Burk and Dixon plots) indic ated that different inhibitors exhibit different inhibition patterns: competitive (EDTA, Ba, La, Cu, p-OH-MB and PMA), non competitive (Zn) and mixed (Hg). The pretreatment of sample with dithiothreitol (DTT) p rotects against the inhibitory effect of mercurials. Furthermore after inhibition by mercurials the activity was restored by DTT. These resu lts confirmed the essential role of the -SH groups to maintain the cat alytic activity of paraoxonase and suggest the existence of two types of -SH groups that could differ in their localization. (C) 1997 Elsevi er Science Ireland Ltd.