Correction of salinity with flavin-containing monooxygenase activity but not cytochrome P450 activity in the euryhaline fish (Platichthys flesus)

To test the association between flavin-containing monooxygenases (FMOs) and osmoregulation, saltwater-adapted euryhaline flounder (Platichthys flesus) were statically exposed to 34 (ambient), 25, and 15 parts per thousand (parts per thousand) salinity for 1 or 2 weeks. FMO activity (thiourea S-oxidase) was assayed in gill and liver microsomes in P. flesus. Branchial FMO activity was reduced dramatically (98%) in fish exposed to a salinity of 15 parts per thousand as compared with control, while hepatic FMO activity was reduced by 60%. Reduction of FMO activity in response to reduced salinity (15 parts per thousand) appeared to occur within 1 week or less in both liver and gill of the flounder. Although hepatic FMO activity continued to fall and was not detected after 2 weeks at 15 parts per thousand, branchial FMO activity was still present. A dose-response relationship in FMO reduction was present in liver, but there was no difference observed between 25 and 15 parts per thousand salinity in FMO activity of flounder gill. Serum osmolality and hepatic cytochrome P450 content were unchanged by salinity. In an attempt to determine whether trimethylamine (TMA) plays a role in piscine FMO, the effect of TMA on hepatic and branchial FMO activity was examined. Intraperitoneal injections of TMA failed to induce activity. Thus, an association between osmoregulatory function and FMO expression was observed in a species of euryhaline fish, indicating that alterations by salinity may affect xenobiotic biotransformation in euryhaline animals.