Abstract
Expression of flavin-containing monooxygenase (FMO) activity has been shown to be directly correlated with salinity in euryhaline fish. Sex steroids play a significant role in osmoregulation in euryhaline fish. The effects of 17β-estradiol and testosterone on hepatic FMO activity ( N, N-dimethylaniline N-oxygenase; thiourea S-oxygenase) and expression was examined in 6-month-old and 1-year-old rainbow trout ( Oncorhynchus mykiss). Fish were treated with 3 mg/kg of each compound on days 1, 4, 7, 10, 13, and 15 with euthanasia on day 19. Treatment of 6-month-old and 1-year-old female rainbow trout with testosterone or estradiol significantly reduced FMO-catalyzed DMA N-oxygenase activity in liver. However, testosterone treatment failed to significantly alter thiourea S-oxidase in 1-year-old males, although 17β-estradiol significantly reduced FMO activity. These results parallel those of studies showing repressive effects of sex steroids on branchial chloride cells and Na +K +ATPase of salmonids and provide further evidence of a role for FMO in teleost osmoregulation.
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