Interaction of organophosphorus compounds with carboxylesterases in the rat

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Abstract
Carboxylesterases (CarbE) are involved in detoxication of organophosphorus compounds (OPC) through two mechanisms: hydrolysis of ester bonds in OPC which contain them and binding of OPC at the active site of CarbE which reduces the amount of OPC available for acetylcholinesterase inhibition. This study of the interaction of rat plasma and liver CarbE with dichlorvos, soman and sarin in vitro and in vivo was undertaken in order to contribute to better understanding of the role of CarbE in detoxication of OPC. The results obtained have shown that inhibitory potency (I50) of dichlorvos, sarin and soman towards rat liver CarbE was 0.2 μM, 0.5 μM and 4.5 μM, respectively, for 20-min incubation at 25°C. Second-order rate constants (ka) for liver CarbE inhibition were 2.3×105 M-1 min-1, 6.9×104 M-1 min-1 and 1.1× 104 M-1 min-1 for dichlorvos, sarin and soman, respectively. The corresponding values for plasma CarbE could not be calculated because of dominant spontaneous reactivation of inhibited CarbE. CarbE inhibited with these OPC in vitro spontaneously reactivate with half-times of 18, 143 and 497 min for sarin, dichlorvos and soman in plasma and 111, 163 and 297 min for sarin, soman and dichlorvos in liver, respectively. These results were also confirmed in experiments in vivo in which rats were subcutaneously treated with 0.5 LD50 of these agents. The half-times of spontaneous reactivation of rat plasma CarbE in vivo were 1.2, 2.0 and 2.7 h for dichlorvos, sarin and soman, respectively. These findings have changed current understanding of the mechanism of interaction of CarbE with OPC and involvement of the enzymes in detoxication of OPC, suggesting an active and important role of the enzymes in metabolic conversions of OPC to their less toxic metabolites.