Random expression of branchial flavin-containing monooxygenase (FMO) activity in the Atlantic flounder (Platichthys flesus)

In an attempt to better understand the regulatory basis explaining relationships between FMO and osmoregulation, sexually immature Atlantic flounder were treated for 18 days with abdominal implants containing the osmoregulatory modulators cortisol (CO) and 17 beta-estradiol (E2). NaKATPase mRNA expression was significantly repressed in E2 animals, and tended to be elevated in CO-treated animals. FMO mRNA expression was extremely variable with a slight reduction in E2-treated animals, but no significant differences were observed between any treatment groups. TMA oxygenase activity was unchanged by CO or E2 treatment, and detected in only 40% of the animals (8 out of 20). To assess individual variability, 11 male and nine female flounder were examined for branchial FMO expression and activity. Although FMO mRNA was observed in all animals, FMO activity was only observed in 63% (7 out of 11) of males and 33% of females (3 out of 9). No significant correlations were observed between FMO mRNA expression, FMO activity, serum CO, or E2. Results of this study indicate variable expression of FMO mRNA and sporadic deletion of catalytic activity in Atlantic flounder.