A‐esterase activities in relation to the differential toxicity of pirimiphos‐methyl to birds and mammals

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Abstract
Pirimiphos‐methyloxon (2‐diethylamino‐6‐methylpyrimidin‐4‐yl dimethyl phosphate) the phosphate analogue of pirimiphos‐methyl, and paraoxon (diethyl 4‐nitrophenyl phosphate) the phosphate analogue of parathion were used as substrates to determine the esterase activity in plasma. Aryl groups released were measured by high‐performance liquid chromatography or with a recording spectrophotometer. In a survey of 14 species of birds representing six different avian orders, the plasma esterase activities (expressed as nmol min−1 ml−1 of plasma) were always low, ranging from 0‐71 for pirimiphos‐methyloxon and from 0‐0.63 for paraoxon. By contrast, mammalian activities were very much higher than these, and in no case was a sample of mammalian plasma less than 13 times more active than any sample of avian plasma using the same assay procedure. It is concluded that birds are deficient in A‐esterase activity towards pirimiphos‐methyloxon and paraoxon. The importance of this deficiency in determining the relatively high susceptibility of birds to these and other organophosphorus insecticides is discussed.
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