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 (Platichth ys flesus) were statically exposed to 34 (ambient), 25, and 15 parts p er thousand (parts per thousand) salinity for 1 or 2 weeks. FMO activi ty (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 contr ol, while hepatic FMO activity was reduced by 60%. Reduction of FMO ac tivity in response to reduced salinity (15 parts per thousand) appeare d to occur within 1 week or less in both liver and gill of the flounde r. Although hepatic FMO activity continued to fall and was not detecte d after 2 weeks at 15 parts per thousand, branchial FMO activity was s till present. A dose-response relationship in FMO reduction was presen t in liver, but there was no difference observed between 25 and 15 par ts per thousand salinity in FMO activity of flounder gill. Serum osmol ality and hepatic cytochrome P450 content were unchanged by salinity. In an attempt to determine whether trimethylamine (TMA) plays a role i n piscine FMO, the effect of TMA on hepatic and branchial FMO activity was examined. Intraperitoneal injections of TMA failed to induce acti vity. Thus, an association between osmoregulatory function and FMO exp ression was observed in a species of euryhaline fish, indicating that alterations by salinity may affect xenobiotic biotransformation in eur yhaline animals.