CORRELATION BETWEEN BIOCHEMICAL PARAMETERS AND SUSCEPTIBILITY OF FRESH-WATER FISH TO MALATHION

Acute toxicity (96-h LC50) of malathion was tested using five species of freshwater fish, namely, topmouth gudgeon (Pseudorasbora parva), go ldfish (Carassius auratus), nile tilapia (Tilapia nilotica), mosquitof ish (Gambusia affinis), and rainbow trout (Salmo gairdneri). Correlati on was found between susceptibility and biochemical parameters such as activity of brain acetylcholinesterase (AChE) and in vitro resistance of the enzyme to inhibition (IC50) of malaoxon (a major metabolite of malathion). The in vitro study also showed that malaoxon instead of m alathion war the main inhibitor of AChE. Susceptibility to malathion w as considerably changed as the fish was pretreated with piperonyl buto xide (PB, a P-450 inhibitor) and triphenyl phosphate (TPP, an inhibito r of carboxylesterase), respectively. Toxicity of malathion was signif icantly increased by TPP, but the responses of fish to PB were quite d ifferent among species. This suggested that both carboxylesterase and monooxygenase played an important role in susceptibility determination , and great variations existed among species in activity of monooxygen ase.