EFFECT OF SOME METALLIC CATIONS AND ORGANIC-COMPOUNDS ON THE O-HEXYL O-2,5-DICHLOROPHENYL PHOSPHORAMIDATE HYDROLYZING ACTIVITY IN HEN PLASMA

One of the main detoxification mechanisms of organophosphorus (OP) com pounds is hydrolysis by Op hydrolysing enzymes (OP-hydrolases) or phos phoric triester hydrolases. We previously reported an OP-hydrolase fro m hen plasma which hydrolyses O-hexyl O-2,5-dichlorophenyl phosphorami date (HDCP). In this study, a total of 18 cations, as well as several thiol blocking reagents, ethylenediaminetetraacetic acid (EDTA) and mi pafox (NN-diisopropyl phosphorodiamidofluoridate) were assayed as acti vators or inhibitors of the HDCP hydrolysing activity of hen plasma in vitro. Of the 18 inorganic cations only 1 M Na+ caused any inhibition . Most of the cations, including Ca2+, exerted no detectable effect; h owever, 1 mM Cu2+ was found to produce an activation of up to 263%, wi th a lesser activation of up to 168% for 1 mM Zn2+. The thiol blocking reagents methyl vinyl ketone (MVK) and N-ethylmaleimide (NEM) inhibit ed the enzyme in a time-dependent manner, the maximum effect depending upon concentration in the case of NEM, but not in the case of MVK; ho wever, 5,5'-dithiobis (2-nitrobenzoic acid) caused inhibition that was concentration dependent but which was independent of time. Other thio l blocking reagents such as p-hydroxymercuribenzoic acid (sodium salt) , phenylmercuric acetate, iodoacetic acid (sodium salt) and iodoacetam ide produced only slight inhibition, as did EDTA. Finally, the OP comp ound mipafox exerted no detectable effect.