NON-CALCIUM DEPENDENT ACTIVITY HYDROLYZING ORGANOPHOSPHORUS COMPOUNDSIN HEN PLASMA

O-Hexyl O-2,5, dichlorophenyl phosphoramidate (HDCP) is a chiral compo und that induces delayed neuropathy in hens. The chicken has very low activity of Ca-dependent organophosphorus-hydrolases (OP-hydrolases) s uch as paraoxonase. HDCP is degraded at a similar rate in rat and hen plasma (16 and 21 nmol/min/mu l plasma, respectively) when measured by the loss of its anti-cholinesterase potency (Diaz-Alejo et al., 1990) . The time course of the HDCP hydrolysis was not significantly affecte d by the following treatments: (a) 0.5-1 mM Ca2+ or 1-10 mM EDTA added at 30 min before starting the reaction at 37 degrees C; (b) preincuba tion with a carboxylesterase inhibitor 100 mu M diisopropyl phosphoros -fluoridate (DFP) for 60 min at 37 degrees C; (c) preincubation with 1 00 mu M HDCP for 60 min at 37 degrees C; and (d) the presence of 50 mu M DCP. However, the hydrolysis of HDCP was slightly modified by the o ther product of its hydrolysis. There is no contribution to the HDCP h ydrolysis by covalent binding to carboxylesterase proteins. The course of the hydrolysis of HDCP was similar when measured by either the los s of anti-cholinesterase potency or the DCP liberated. HDCP is hydroly sed by an OP-hydrolase which is not Ca-dependent and is present in hen in contrast to the best known OP-hydrolases which are Ca-dependent an d are undetectable in birds.