Non-calcium dependent activity hydrolysing organophosphorus compounds in hen plasma

O-Hexyl O-2,5, dichlorophenyl phosphoramidate (HDCP) is a chiral compound that induces delayed neuropathy in hens. The chicken has very low activity of Ca-dependent organophosphorus-hydrolases (OP-hydrolases) such as paraoxonase. HDCP is degraded at a similar rate in rat and hen plasma (16 and 21 nmol/min/microliters plasma, respectively) when measured by the loss of its anti-cholinesterase potency (Díaz-Alejo et al., 1990). The time course of the HDCP hydrolysis was not significantly affected 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) preincubation with a carboxylesterase inhibitor 100 microM diisopropyl phosphorosfluoridated (DFP) for 60 min at 37 degrees C; (c) preincubation with 100 microM HDCP for 60 min at 37 degrees C; and (d) the presence of 50 microM DCP. However, the hydrolysis of HDCP was slightly modified by the other product of its hydrolysis. There is no contribution to the HDCP hydrolysis by covalent binding to carboxylesterase proteins. The course of the hydrolysis of HDCP was similar when measured by either the loss of anti-cholinesterase potency or the DCP liberated. HDCP is hydrolysed 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 and are undetectable in birds.