Down-regulation of piscine flavin-containing mono-oxygenase activity by decreased salinity in euryhaline flounder (Platichthys flesus)

Flavin-containing mono-oxygenases (FMOs) play a significant role in xenobiotic biotransformation in fish. However, endogenous functions of the enzyme have not been documented. Earlier studies in our laboratory indicated a relationship between FMO expression and osmoregulation in euryhaline fish. Consequently, in the present study, euryhaline flounder (Platichthys flesus) were exposed to three separate salinity regimes for two weeks. Hepatic and branchial FMO activity was examined in individual animals from each exposure and was found to be directly related to the salinity in which the fish were kept. Thus, an association between osmoregulatory function and FMO expression was observed in euryhaline fish indicating possible divergent or simultaneous evolution of the enzyme system to a xenobiotic metabolizing function.