Selective toxicity of chlorpyrifos to several species of fish during an environmental exposure: Biochemical mechanisms

The organophosphorus insecticide chlorpyrifos is frequently used for termite control. A small pond near a construction site was contaminated with chlorpyrifos via runoff resulting in the death of several species of fish including largemouth bass, bluegill sunfish, and golden shiners, but not mosquitofish. Inhibition of acetylcholinesterase (AChE) and aliesterases (AliE) was determined in the tissues of exposed fish and the presence of chlorpyrifos was confirmed in fish livers by analytical chemistry. Brain AChE was inhibited 87 to 93% in bass, bluegill, and shiners but only 73% in mosquitofish. Skeletal muscle AChE was inhibited 91 to 97% in bass, shiners, and mosquitofish but only 77% in bluegill. Liver AChE and AliE were inhibited greater than 90% in bass, shiners, and mosquitofish but in bluegill, the activities were inhibited only 86% and 77%, respectively. After 60 d, brain AChE activity in exposed mosquitofish was fully recovered, and liver AliE activity was recovered to about 50% of control values, but there was little recovery of skeletal muscle and liver AChE activities. The sensitivity of these enzymes to inhibition by chlorpyrifos-oxon was assessed by determination of 50% inhibitory concentrations (IC50s). Mosquitofish brain and liver AChE and liver AliE were less sensitive to inhibition than the enzymes from the other species. In contrast, mosquitofish skeletal muscle AChE was more sensitive to inhibition compared to the other species. The lower inhibition of mosquitofish brain AChE and the lower in vitro sensitivity of mosquitofish brain AChE to inhibition by chlorpyrifos-oxon compared to the brain AChE of the other species suggest that the species difference in toxicity exhibited in this exposure resulted primarily from species differences in the sensitivity of brain AChE to inhibition by chlorpyrifos-oxon.