Raymond J A
Department of Biological Sciences, University of Nevada, Las Vegas 89154, USA.
Physiol Zool. 1998 Sep-Oct;71(5):515-23. doi: 10.1086/515967.
Trimethylamine oxide (TMAO) and urea levels in the blood of rainbow smelt, Osmerus mordax, were previously shown to increase dramatically in winter, but the means by which these osmolytes are acquired has remained unclear. In this study, I show that the smelt can synthesize TMAO via liver trimethylamine oxidase activity and thus are not completely dependent on a dietary source of TMAO. Cold-acclimatized Pacific herring, Clupea harengus, were also found to have high levels of TMAO in the blood, while individuals from a temperate-water population of herring did not. Herring also had liver TMA oxidase activity, which appeared to be due to a flavin-containing monooxygenase. In both species, TMA oxidase activity did not appear to be strongly affected by temperature. TMAO data were obtained for three other northern species (Macrozoarces americanus, Eleginus gracilis, and Platichthys stellatus), and these results, together with previously reported data, suggest that TMA oxidase activity is a necessary condition for high levels of TMAO in the blood. In the smelt, urea appears to be synthesized via uricolysis and also through the action of arginase on dietary arginine, while the ornithine urea cycle appears to be nonfunctional. There was no relation among several species of northern fishes between levels of urea in the blood and levels of uricase or arginase activity. Together, these results provide further evidence of the importance of TMAO and urea in some cold water fishes, demonstrate that the synthetic machinery for these osmolytes is present in the liver, and suggest that the elevated levels in response to cold may be due to conservation rather than to increased production.
先前研究表明，彩虹胡瓜鱼（Osmerus mordax）血液中的氧化三甲胺（TMAO）和尿素水平在冬季会急剧上升，但这些渗透溶质的获取方式仍不明确。在本研究中，我发现胡瓜鱼能够通过肝脏中的三甲胺氧化酶活性合成TMAO，因此并非完全依赖于从食物中获取TMAO。研究还发现，经过冷驯化的太平洋鲱（Clupea harengus）血液中TMAO水平也很高，而来自温带水域的鲱鱼个体则没有。鲱鱼肝脏也具有TMA氧化酶活性，这似乎是由一种含黄素的单加氧酶引起的。在这两个物种中，TMA氧化酶活性似乎不受温度的强烈影响。我们还获得了其他三种北方物种（美洲大绵鳚Macrozoarces americanus、细鳞壮鳕Eleginus gracilis和星斑川鲽Platichthys stellatus）的TMAO数据，这些结果与先前报道的数据一起表明，TMA氧化酶活性是血液中高水平TMAO的必要条件。在胡瓜鱼中，尿素似乎是通过尿酸分解以及精氨酸酶对食物中精氨酸的作用合成的，而鸟氨酸尿素循环似乎不起作用。在几种北方鱼类中，血液中的尿素水平与尿酸酶或精氨酸酶活性水平之间没有关联。这些结果共同提供了进一步的证据，证明TMAO和尿素在一些冷水鱼类中的重要性，表明这些渗透溶质的合成机制存在于肝脏中，并表明对寒冷的反应中水平升高可能是由于保存而非产量增加。
