The accumulation and metabolism of methylamine organic osmolytes in elasmobranch fishes

Treberg, Jason R. (2006) The accumulation and metabolism of methylamine organic osmolytes in elasmobranch fishes. Doctoral (PhD) thesis, Memorial University of Newfoundland.

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Abstract

It is widely accepted that marine elasmobranch fishes accumulate the methylamine compounds trimethylamine oxide (TMAO), glycine-betaine (betaine) and sarcosine as osmolytes. The metabolism and accumulation of these compounds has received relatively little study in elasmobranchs, and as such the purpose of this thesis was to investigate aspects of methylamine metabolism, retention and accumulation. Experimental approaches range from multispecies comparisons to directed study of a single species (the winter skate, Leucoraja ocellata ) at different levels of organization from the whole animal down to subcellular components. -- Marine elasmobranchs and a euryhaline species in freshwater accumulate methylamines as the predominant intracellular non-urea organic osmolytes in muscle, whereas, freshwater species preferentially accumulated the β-amino acid taurine. All elasmobranch species examined in this thesis had measurable enzymatic capacity for betaine synthesis in the liver and strong correlation between hepatic betaine synthesizing enzymes and muscle betaine content was found. Only one species of the seven examined had measurable TMAO synthetic capacity and a phylogenetic explanation is proposed for the distribution of TMAO synthesis in elasmobranch fishes. -- In a detailed study of TMAO metabolism in the winter skate, it was concluded that the presence of flavin-containing monoxygenase activity does not indicate the capacity for the synthesis of TMAO. Winter skates lack measurable endogenous TMAO synthesis, apparently obtaining this compound in the diet, and maintain levels without feeding as a result of very low whole animal TMAO losses (<1% day -1 ). Betaine synthesis was also examined in detail in the winter skate, where the liver and kidney are the likely sites of synthesis. The enzymes of betaine synthesis from choline, choline dehydrogenase and betaine aldehyde dehydrogenase, are localized in the mitochondria, as is the case in other animals. Winter skates do not activate betaine synthetic capacity in response to food deprivation or accumulate betaine in the muscle when exposed to a hyperosmotic challenge; however, muscle betaine content increases when they are fed a high betaine diet, suggesting exogenous methylamine supply may be a key determinant in muscle methylamine accumulation in elasmobranchs.

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