期刊
JOURNAL OF EXPERIMENTAL BIOLOGY
卷 209, 期 5, 页码 860-870出版社
COMPANY OF BIOLOGISTS LTD
DOI: 10.1242/jeb.02055
关键词
trimethylamine oxide (TMAO); betaine; trimethylamine; oxidase; choline dehydrogenase; betaine aldehyde dehydrogenase; organic osmolyte; elasmobranch
类别
We compared levels of the major organic osmolytes in the muscle of elasmobranchs, including the methylamines trimethylamine oxide (TMAO), betaine and sarcosine as well as the beta-amino acids taurine and beta-alanine, and the activities of enzymes of methylamine synthesis (betaine and TMAO) in species with a wide range of urea contents. Four marine, a euryhaline in freshwater (Dasyatis sabina), and two freshwater species, one that accumulates urea (Himantura signifer) and one that does not (Potamotrygon motoro), were analyzed. Urea contents in muscle ranged from 229-352 mu mol g(-1) in marine species to 2.0 mu mol g(-1) in P. motoro. Marine elasmobranchs preferentially accumulate methylamines, possibly to counteract urea effects on macromolecules, whereas the freshwater species with lower urea levels accumulate the P-amino acid taurine as the major non-urea osmolyte. A strong correlation (r(2)=0.84, P < 0.001) with a slope of 0.40 was found between muscle urea content and the combined total methylamines plus total P-amino acids, supporting the hypothesis that 'non-urea' osmolytes are specifically maintained at an approximately 2:1 ratio with urea in the muscle of elasmobranchs. All species examined had measurable synthetic capacity for betaine in the liver but only one species had detectable TMAO synthetic capacity. We propose a phylogenetic explanation for the distribution of TMAO synthesis in elasmobranchs and suggest that activation of liver betaine aldehyde dehydrogenase, relative to choline dehydrogenase, coincides with betaine accumulation in elasmobranchs. The latter relationship may be important in maintaining methylamine levels during periods of low dietary TMAO intake for species lacking TMAO synthesis.
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