Effect of urea and low temperature on the expression and activity of flavin-containing monooxygenase in the liver and gill of rainbow trout (Oncorhynchus mykiss)

Little is known about the physiological role of flavin containing monooxygenases (FMOs) in teleost fish. Recent studies have indicated induction during saltwater adaptation in several telost species including rainbow trout. A physiological product of FMOs, trimethylamine N-oxide (TMAO) and urea have also been shown to increase in rainbow trout muscle during saltwater adapatation.

TMAO counteracts the denaturing effects of urea. In order to evaluate urea as a possible inducer of FMO expression and activity, adult rainbow trout were infused for 48 h with urea/saline solutions at a loading rate of 8 mmol urea/kg/day. To determine whether low temperature had any effect on FMO expression and activity, one group of animals was infused with urea and exposed to low temperature (2–3 °C) for 48 h.

Gill FMO-catalyzed thiourea oxygenase activity was significantly induced by low temperature, with twice the activity observed at low temperature (1.116±0.356 nmol/min/mg) compared to urea infusion at 10 °C (0.585±0.282 nmol/min/mg). Low temperature without urea treatment caused a 50% increase in gill FMO activity.

In the liver, urea infusion caused an increase in liver FMO activity (from 0.144±0.053 nmol/min/mg to 0.464±0.237 nmol/min/mg), but was unaffected by co-exposure to low temperature (0.523±219 nmol/min/mg). FMO expression and activity correlated with elevated tissue urea levels, but TMAO concentrations were not related.

The interactions between urea, temperature and the tissue-specific induction of FMOs indicate FMOs may contribute to other physiological and cellular processes besides osmoregulation.