Mommsen T P, Walsh P J
Division of Biology and Living Resources, Rosenstiel School of Marine and Atmospheric Sciences, University of Miami, FL 33149.
Science. 1989 Jan 6;243(4887):72-5. doi: 10.1126/science.2563172.
Elasmobranch fishes, the coelacanth, estivating lungfish, amphibians, and mammals synthesize urea by the ornithine-urea cycle; by comparison, urea synthetic activity is generally insignificant in teleostean fishes. It is reported here that isolated liver cells of two teleost toadfishes, Opsanus beta and Opsansus tau, synthesize urea by the ornithine-urea cycle at substantial rates. Because toadfish excrete ammonia, do not use urea as an osmolyte, and have substantial levels of urease in their digestive systems, urea may serve as a transient nitrogen store, forming the basis of a nitrogen conservation shuttle system between liver and gut as in ruminants and hibernators. Toadfish synthesize urea using enzymes and subcellular distributions similar to those of elasmobranchs: glutamine-dependent carbamoyl phosphate synthethase (CPS III) and mitochondrial arginase. In contrast, mammals have CPS I (ammonia-dependent) and cytosolic arginase. Data on CPS and arginases in other fishes, including lungfishes and the coelacanth, support the hypothesis that the ornithine-urea cycle, a monophyletic trait in the vertebrates, underwent two key changes before the evolution of the extant lungfishes: a switch from CPS III to CPS I and replacement of mitochondrial arginase by a cytosolic equivalent.
软骨鱼类、腔棘鱼、夏眠肺鱼、两栖动物和哺乳动物通过鸟氨酸 - 尿素循环合成尿素；相比之下，硬骨鱼类的尿素合成活性通常微不足道。本文报道，两种硬骨蟾鱼（Opsanus beta和Opsansus tau）的离体肝细胞能以相当高的速率通过鸟氨酸 - 尿素循环合成尿素。由于蟾鱼排泄氨，不将尿素用作渗透剂，且其消化系统中脲酶水平较高，尿素可能作为一种临时的氮储存形式，如同反刍动物和冬眠动物一样，构成肝脏和肠道之间氮保存穿梭系统的基础。蟾鱼利用与软骨鱼类相似的酶和亚细胞分布来合成尿素：谷氨酰胺依赖性氨甲酰磷酸合成酶（CPS III）和线粒体精氨酸酶。相比之下，哺乳动物具有CPS I（氨依赖性）和胞质精氨酸酶。包括肺鱼和腔棘鱼在内的其他鱼类中关于CPS和精氨酸酶的数据支持了这样一种假说，即鸟氨酸 - 尿素循环作为脊椎动物的一个单系性状，在现存肺鱼进化之前经历了两个关键变化：从CPS III转变为CPS I，以及线粒体精氨酸酶被胞质中的等效物所取代。
