Expression of branchial flavin-containing monooxygenase is directly correlated with salinity-induced aldicarb toxicity in the euryhaline fish (Oryzias latipes)

Earlier studies in our laboratory have demonstrated a reduction of flavin-containing monooxygenase (FMO) activity when salt-water adapted euryhaline fish were transferred to water of less salinity. Since FMOs have been shown to be responsible for the bioactivation of the carbamate insecticide, aldicarb, to a more potent cholinesterase inhibitor, it would be predicted that euryhaline fish exposed to aldicarb at higher salinity should have greater levels of FMO and, hence, be more susceptible to aldicarb toxicity. It was not surprising, therefore, that mature female Japanese medaka (Oryzias latipes) were significantly more susceptible to aldicarb toxicity at higher salinity. A direct correlation was observed between salinity and FMO expression, but salinity had no significant effect on the uptake of aldicarb. These data suggest that FMOs are upregulated by salinity and may be responsible for the enhanced toxicity of aldicarb at higher salinities.