The Physiology and Biochemistry of Organic Osmolytes in Arctic Fishes

极地鱼类的抗冻性通常归因于非聚合肽抗冻。1991年，人们发现一些北极鱼类在冬季通过产生高浓度的甘油来提高其体液的冰点。对这一现象的后续研究使人们认识到，其他无机离子和有机渗透物在许多极地鱼类的抗冻性中起着重要作用，可能还有其他作用。这些溶质中最突出的是氧化三甲胺（TMAO）和尿素，它们比甘油更广泛地分布在极地鱼类中。这些研究在这些渗透物的合成、作用、保存和分布方面有一些不同寻常的发现。因此，本研究的总体目的是更好地了解北极鱼类甘油、氧化三甲胺和尿素的生化和生理。先前的研究表明，冶炼厂发生了显著的甘油损失，这些损失通过合成途径（通过丙酮酸）得到补偿，这似乎与其他耐寒的甘油生产动物不同。新的研究将集中于可能参与这一途径的各种氨基酸底物，以及三种被认为也参与其中的关键酶活性的季节性变化。在许多北极鱼类中意外出现高血清氧化三甲胺水平也提出了几个问题。氧化三甲胺通常由硬骨鱼通过饮食获得，但在嗅觉科中，它似乎是在对寒冷的反应中合成的。TMAO的合成将通过TMAO氧化酶测定在温暖和寒冷适应的鱼类。极地鱼类中的无机离子浓度高到足以干扰细胞内过程，如酶活性。氧化三甲胺可能部分抵消这种盐抑制作用的可能性——对此有一些初步证据——将进行调查。TMAO在降低上皮膜对低分子量渗透物的渗透性中的可能作用也将被研究。初步证据表明，氧化三甲胺本身是保存的，而不是像大多数鱼类那样被排出体外，这些证据将在胡梅鱼和其他北极物种中进行进一步测试。氧化三甲胺的保存将进一步证明它具有生理作用。硬鱼体内的尿素合成通常是通过嘌呤途径完成的，但在尿素水平高的鱼类（包括一些硬鱼）中，它可以通过鸟氨酸循环来完成。这两种途径都将在冶炼厂中进行分析，以确定它们的高尿素水平是否也是鸟氨酸循环的结果。

9423920 Raymond Freezing resistance in polar fishes is usually attributed to noncolligative peptide antifreezes. In 1991, it was found that some arctic fishes increase the freezing point depression of their body fluids by producing high concentrations of glycerol in winter. Subsequent studies of this phenomenon have led to a realization that other inorganic ions and organic osmolytes have important roles in freezing resistance in many polar fishes, and possibly other roles as well. Prominent among these solutes are trimethylamine oxide (TMAO) and urea, which are more widely distributed in polar fishes than glycerol. These studies have lead to several unusual findings concerning the synthesis, roles, conservation and distribution of these osmolytes. Therefope, the overall objective of this study is to better understand the biochemistry and physiology of glycerol, TMAO and urea in arctic fishes. Previous studies have shown that significant glycerol losses occur in smelt and that these losses are compensated for by a synthetic pathway (via pyruvate) that appears to differ from that in other cold-hardy, glycerol-producing animals. New studies will focus on various amino acid substrates that may be involved in this pathway, and the seasonal variation in activity of three key enzymes that are also thought to be involved. The unexpected occurrence of high serum TMAO levels in many arctic fishes also raises several questions. TMAO is usually acquired by teleosts through the diet, but in the smelt family, it appears to be synthesized in response to cold. TMAO synthesis will be tested by an assay for TMAO oxidase in warm and cold-acclimated fishes. Inorganic ion concentrations in polar fishes are high enough to interfere with intracellular processes, such as enzyme activities. The possibility that TMAO may partially counteract this salt inhibition--for which there is some preliminary evidence--will be investigated. A possible role of TMAO in reducing epithelial membrane permeabilities to low molecular weight osmolytes will also be investigated. Preliminary evidence that TMAO itself is conserved, rather than excreted as it is in most fishes, will be further tested in smelt and other arctic species. Conservation of TMAO would provide further evidence that it has a physiological role. Urea synthesis in teleosts is normally accomplished through the purine pathway, but in fishes with high urea levels (including a few teleosts), it can occur by the ornithine cycle. Both pathways will be assayed in the smelt to determine if their high urea levels are also the result of an ornithine cycle.