A study was performed on the metabolism of fenitrothion (FS) in tiger shrimp Penaeus japonicus at different larval stages to elucidate the cause of the remarkable increase in the toxicity of organophosphorothionates to the shrimp larvae with the progress of the stages. The shrimp larvae (zoea, mysis and postlarvae) were exposed to 0.5 ppb [14C]FS. After 1.5-12 h exposure, [14C]FS and its metabolites accumulated in the larvae were extracted with benzene and then with ethyl ether. The benzene-extracts were determined by TLC and the ethyl ether-extracts were subjected to an enzymatic analysis. Zoea and mysis showed very low FS metabolic activity, and most of the FS absorbed by the larvae was present as FS itself in their bodies. However, their FS metabolic activity increased abruptly with their growth and resulted in an increase in the amount of fenitrooxon which displays extremely high toxicity as compared with FS, accompanying the simultaneous increase in the FS detoxication activity such as demethylation, hydrolysis and conjugation. The occurrence of the remarkable increase in the toxicity of organophosphorothionates to tiger shrimp larvae with the progress of the stages is probably due to the increase in the oxidative desulfuration activity, beyond the effect of the detoxication activity.
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