This study was initiated to explain the acute toxicity difference (14 times in LC50) of killifish (Oryzias latipes) and loach (Misgurnus anguilicaudatus), two dominant fish species in Korean rice paddies, to diazinon, one of the widely used organophosphorus insecticides in Korea. Enzymatic activity of acetylcholinesterase (AchE) and microsomal fraction was used for the detection of inhibitory action of diazinon and Phase I metabolic activity of fish homogenate in vitro. Ethoxy-14C-labelled diazinon was used for tracing the absorption difference, as well as confirmation of the enzymatic assay result.
The results showed that: (1) inhibition of AchE by diazoxon in loach was 22-fold more potent than in killifish; (2) total radioactivity exposed to ethoxy-14C labeled diazinon was decreasing in killifish with daily analysis, while increasing in loach; (3) initial absorption ratio was 4.5:1 for killifish:loach; and (4) approximately 10 times more polar metabolites were formed in killifish within a 15-h period. It was concluded that the three factors, AchE inhibition, detoxification, and absorption were all significant for the selective toxicity of diazinon. Further study is under way to reveal the identity of the metabolites.