The Urea Cycle in Fish

9727741安德森大多数鱼去除废物氮（来自蛋白质和能量代谢）作为氨，氨通过鳃扩散到水中。 这与大多数陆生脊椎动物相反，陆生脊椎动物将氨转化为尿素，尿素在尿液中定期储存和排泄（称为“排脲”物种）;这种转化的代谢途径称为尿素循环。 一组鱼类，即条鳃类（鲨鱼和鳐），在其组织中合成并保留高浓度的尿素，以维持海水中高盐浓度的渗透平衡。 这些物种的尿素循环类似于排尿素物种，但有一些显着的差异。 安德森博士已经确定了表达这种独特酶的基因结构，这种酶催化尿素循环中的第一个代谢步骤。 这种独特的酶也存在于一些长尾鱼类中，但显然在大多数成年鱼类中不存在，他已经确定了几种硬骨鱼中这种酶的基因结构。 令人惊讶的发现是，在几种硬骨鱼物种中，这种酶以及在某些情况下所有的尿素循环酶主要在肌肉中表达，而不是如预期的那样在肝脏中表达。 这项研究将有助于确定这种尿素循环酶的基因是否存在于所有鱼类中，以及何时、如何以及为什么表达。 该假说认为，这种酶和尿素循环仅在大多数鱼类的胚胎发生（非常早期的发育）期间表达。 氨毒性是水产养殖业的头号问题。 了解尿素循环酶在鱼类中的表达是如何调节的机制，可能会提供一个机会，遗传修饰鱼类表达尿素循环，从而增加对氨的耐受性。

9727741 Anderson Most fish remove waste nitrogen (from protein and energy metabolism) as ammonia, which diffuses across the gills into the water. This is in contrast to most terrestrial vertebrate animals, which convert ammonia to urea which is stored and excreted periodically in the urine (referred to as "ureotelic" species); the metabolic pathway for this conversion is called the urea cycle. One group of fishes, the clasmobranchs (sharks and rays), synthesize and retain high concentrations of urea in their tissues for the purpose of maintaining an osmotic balance with the high salt concentration in sea water. The urea cycle in these species is analogous to that in ureotelic species, but with some significant differences. Dr. Anderson has determined the structure of the gene that expresses this unique enzyme catalyzing the first metabolic step in the urea cycle. This same unique enzyme is found in some telcost fishes as well, but apparently is absent in most adult species, and he has determined the structure of the gene for the enzyme in several teleost fishes. A surprising finding is that is several teleost species, this enzyme and in some cases, all of the urea cycle enzymes, is expressed primarily in muscle, rather than in liver as expected. The proposed studies will help to establish if the gene for this enzyme of the urea cycle is present in all fish and when and how and why it is expressed. The hypothesis is that this enzyme and the urea cycle are expressed only during embryogenesis (very early development) in most fish. Ammonia toxicity is the number one problem in the aquaculture industry. Understanding the mechanism of how expression of the urea cycle enzymes is regulated in fish may provide an opportunity to genetically modify fish to express the urea cycle and thus increase tolerance to ammonia.