The Physiology and Biochemistry of Organic Osmolytes in Arctic Fishes

9423920北极鱼对雷蒙德的抗冻性通常归因于非胶原性多肽防冻剂。1991年，人们发现，一些北极鱼类在冬季会产生高浓度的甘油，从而增加体液的冰点降幅。随后对这一现象的研究使人们认识到，其他无机离子和有机渗透分子在许多极地鱼类的抗冻能力中也发挥着重要作用，可能还有其他作用。这些溶质中最突出的是三甲胺氧化物(TMAO)和尿素，它们在极地鱼类中的分布比甘油更广泛。这些研究导致了关于这些渗透分子的合成、作用、保护和分布的几个不同寻常的发现。因此，本研究的总体目标是为了更好地了解甘油、TMAO和尿素在北极鱼类中的生物化学和生理。以前的研究表明，在熔炼过程中，甘油会发生显著的损失，这些损失可以通过一种合成途径(通过丙酮酸)来补偿，这种合成途径似乎不同于其他耐寒的、生产甘油的动物。新的研究将集中在可能参与这一途径的各种氨基酸底物，以及被认为也参与其中的三种关键酶活性的季节性变化。许多北极鱼类血清中TMAO水平的意外升高也引发了几个问题。TMAO通常是硬骨鱼通过饮食获得的，但在斯库尔特家族中，它似乎是对寒冷的反应而合成的。将通过对温暖和冷驯化的鱼类进行TMAO氧化酶测试来测试TMAO的合成。极地鱼类体内的无机离子浓度高到足以干扰细胞内的过程，例如酶的活性。TMAO可能部分抵消这种盐抑制的可能性--这方面有一些初步证据--将被调查。TMAO在降低上皮膜对低分子渗透压物质的通透性方面的可能作用也将被研究。初步证据表明，TMAO本身是保守的，而不是像大多数鱼类那样排泄出来，这将在斯库尔特和其他北极物种中进一步测试。保存TMAO将提供进一步的证据，证明它具有生理作用。硬骨鱼的尿素合成通常是通过嘌呤途径完成的，但在尿素水平较高的鱼类(包括一些硬骨鱼)中，尿素的合成可以通过鸟氨酸循环来完成。这两种途径都将在冶炼过程中进行检测，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.