Metabolism of parathion and brain cholinesterase inhibition in four species of wild birds

Rates of activation and degradation of parathion by the microsomal fraction and the 12,000g supernatant of cell-free preparations were measured in four species of wild birds. Cholinesterase activity and K_i values for cholinesterase inhibition by paraoxon were also determined. Cytochrome P-450 content was lowest in the African bulbul and house sparrow, intermediate in the barn owl, and highest in the blackbird. Microsomal fractions of the bulbul and the sparrow metabolized parathion to paraoxon and p-nitrophenol at higher rates than those of the blackbird and the barn owl, but arylesterase activity was lower in the two former species than in the latter two. Cholinesterase activity was lowest in the barn owl which also exhibited the lowest K_i value toward cholinesterase inhibition by paraoxon. These manifestations coupled with the high rate of arylesterase activity might afford the barn owl and the blackbird with a potential defense mechanism against the toxic action of parathion and related compounds.