Organophosphate Inhibition of Nematode Esterases and Interaction with Chlorinated Hydrocarbon Insecticides

The discovery and subsequent development of a vast array of synthetic organic pesticides has provided nematologists with a rich source of potential nematicides. Certain types of insecticides, notably the organophosphates and more recently the carbamates, have been of significant value in nematode control, whereas, the chlorinated hydrocarbon insecticides are generally ineffective. The wide discrepancy in the chemical structure-toxicity relationship between these two phytogenetically separate animals serves to emphasize the need for greater understanding of nematode chemistry and physiology. Development of a sophisticated nematicide technology requires greater knowledge of nematode response to toxic and non-toxic chemicals. In this study, investigations were carried out into nematode esterase inhibition by a nematicidal organophosphate and the interactions with chlorinated hydrocarbon insecticides. Phorate suppression of esterase activity and the interference by chlorinated hydrocarbon insecticides were studied in whole homogenates of the free-living nematodes Panagrellus redivivus and the plant parasitic species Ditylenchus dipsaci and on their electrohporetically isolated enzymes. The NE enzymes of P. redivivus were somewhat more sensitive than cholinesterases (ChE) to phorate with pI50 values (negative log of the molar concentration inhibiting activity 50%) were 5.4 and 3.7 respectively. Greater resistance and more rapid recovery from NE inhibition by the organophosphate was demonstrated in homogenates of D. dipsaci. A comparison of the inhibition curves of both species indicates phorate to be moderately toxic to NE activity in vitro, approximately one-tenth that of the standard organophosphate paraoxon and slightly more inhibitory than the anti-ChE carbamate eserine. Seven of the eight esterases of P. redivivus and one of the three isolated from homogenate of D. dipsaci were inhibited to varying degrees by phorate and the two standard inhibitors. The pattern of relative degree of sensitivity among the esterases was similar for the two organophosphates but differed with the carbamate eserine. Homogenates pretreated with one of four chlorinated hydrocarbon insecticides, chlordane, DDT, dieldrin, or lindane, reduced the antiesterase toxicity of 5 x 10-7M concentration of phorate 26, 18, 20, and 12% ( P. redivivus), respectively. Reduction of inhibition by 10 -3M concentration of chlordane increased inversely with phorate concentration in homogenates of both species. A similar reduction, but to a higher degree, was produced by pretreatment of homogenates with the microsomal stimulant phenobarbitol. Both chlordane and phenobarbitol reduced esterase inhibition by the carbamate nematicide aldicarb but had no effect on eserine toxicity. Electrophoretically isolated esterases of P. redivivus were not protected from phorate inhibition by pretreatment with chlordane. In an in vivo assay no reduction of phorate toxicity was found in P. redivivus cultured in insecticide treated oatmeal or presoaked in an aqueous solution of the insecticide. Chlorinated hydrocarbon insecticides appear to stimulate microsomal release of aliesterases capable of hydrolyzing organophosphates in a manner similar to the interaction phenomena occurring in rats and mice.