Elevated concentrations and synthetic pathways of trimethylamine oxide and urea in some teleost fishes of McMurdo Sound, Antarctica

Abstract

Levels of trimethylamine oxide (TMAO) and urea in various tissues of several teleost fishes of McMurdo Sound, Antarctica were determined. TMAO levels in muscle tended to be high, with levels in several species exceeding 140 mM kg-1 wet weight. The high levels appear to be necessary to osmotically balance high levels of sodium and chloride in the blood of these species. In two species (Dissostichus mawsoni and Gymnodraco acuticeps), significant levels of TMAO (> 80 mM l-1) were also found in the blood, while virtually no TMAO was found in the blood of a related, temperate water species. In other Antarctic species, serum TMAO levels varied from moderate to low levels. Urea levels in several species were in the range 15–25 mM l-1, which is higher than those in temperate water fishes. The high TMAO and urea levels make important contributions to the fishes' high osmolarities, and thus help to lower freezing points. These data are consistent with previous data obtained from some northern fishes. Fishes with high serum TMAO levels had liver TMAase activity whereas those with no serum TMAO did not. Activity levels of uricolytic enzymes in D. mawsoni and G. acuticeps were not noticeably different from those of temperate water fishes. These species showed moderate to high activities of three ornithine urea cycle enzymes (CPS, OCT and arginase) but low activity of the argininosuccinate synthetase/argininosuccinyl lyase system. These data suggest that a dietary arginine/arginase system is an important source of urea in these species.