INTERACTION OF PHOSPHAMIDON WITH NEUROPATHY TARGET ESTERASE AND ACETYLCHOLINESTERASE OF HEN BRAIN

Phosphamidon (PSM) is an organophosphorus insecticide widely used in a griculture. This study was undertaken to examine the interaction of PS M with acetylcholinesterase (AChE) and neuropathy target esterase (NTE ) of hen brain in vitro and in vivo. PSM was a potent inhibitor of ACh E, with an I-50 of 2.9 mu M and second-order rate constant (k(a)) of 1 .2 x 10(4) M(-1) min(-1) at 37 degrees C. PSM-inhibited AChE aged rapi dly (t(1/2) = 1.9 h). Pyridinium oximes pralidoxime, trimedoxime, obid oxime and HI-6 were effective reactivators of PSM-inhibited AChE, prov iding up to 75% reactivation. PSM was one of the weakest inhibitors of NTE among organophosphorus compounds, with an I-50 of 19 mM and k(a) of 1.8 M(-1) min(-1) at 37 degrees C. Inhibited NTE did not reactivate spontaneously and KF-induced reactivation was not obtained even at th e earliest tested moments, so it was not clear whether aging of PSM-in hibited NTE occurred very quickly or the KF molecule could not affect the stability of phosphoryl-NTE bond. From the ratio of k(a)s for NTE and AChE (0.00015) it was predicted that delayed neuropathic effects o f PSM in vivo would appear only at doses far above the acute LD(50). T he LD(50) value of PSM p.o. for hens was 9 mg/kg. Hens were treated wi th a single oral dose of PSM, combined with standard antidotal treatme nt which included atropine, physostigmine, pralidoxime and anticonvuls ant midazolam. Doses of 90 and 250 mg/kg caused up to 27% and 45% NTE inhibition 48 h after poisoning, respectively. Clinical signs of neuro pathy were not seen up to 25 days after treatment, which could be expe cted, since the proposed level (> 70%) of NTE inhibition necessary for the occurrence of delayed neuropathy was not achieved. The results su ggest that PSM, at doses tested, has no ability to cause delayed neuro pathy in hens without showing signs of severe cholinergic intoxication .