Submitted on March 3, 2006
Accepted on July 16, 2006
Maintenance and accumulation of trimethylamine oxide by winter skate (Leucoraja ocellata): reliance on low whole animal losses rather than synthesis
Jason R Treberg1* and William R Driedzic1
1 Ocean Sciences Centre, Memorial University of Newfoundland, St. John's, Canada
* To whom correspondence should be addressed. E-mail: jtreberg{at}mun.ca.
Trimethylamine oxide (TMAO) is typically accumulated as an organic osmolyte in marine elasmobranchs to levels second only to urea (which can reach > 400 mM); however, little is known about the whole animal regulation of TMAO in elasmobranchs. In the present study on the winter skate (Leucoraja ocellata) we determine if this species can maintain levels of TMAO in the absence of feeding and if so, is this due to endogenous synthesis or low whole animal losses. Winter skates maintain plasma TMAO levels for up to 45 days without feeding. The liver displays methimazole oxidation, which is consistent with the presence of flavin-containing monooxygenase (E.C. 1.14.13.8) activity - the class of enzymes responsible for the physiological oxygenation of trimethylamine (TMA) to TMAO in mammals. However, no evidence for TMA oxygenation by winter skates was found using in vivo or in vitro techniques, indicating no significant capacity for endogenous TMAO synthesis. Fed skates displayed low, but measurable (~ 4-13 µmol kg-1 hr-1), efflux of TMAO (plus TMA) whereas fasted skates did not. Using the loss of injected [14C] TMAO, it was determined that whole animal TMAO losses are likely < 1% of whole body TMAO day-1. These results demonstrate that winter skates utilise low whole animal TMAO losses, rather than endogenous synthesis, to maintain TMAO levels when not feeding.
