Division of Marine Biology and Fisheries, Rosenstiel School of Marine and Atmospheric Science, University of Miami, FL 33149-1098, USA.
Comp Biochem Physiol Part D Genomics Proteomics. 2011 Jun;6(2):187-94. doi: 10.1016/j.cbd.2011.03.001. Epub 2011 Apr 8.
Carbamoyl phosphate synthetase (CPS) catalyses the formation of carbamoyl phosphate from glutamine or ammonia, bicarbonate and ATP. There are three different isoforms of CPS that play vital roles in two metabolic pathways, pyrimidine biosynthesis (CPS II) and arginine/urea biosynthesis (CPS I and CPS III). Gene duplication has been proposed as the evolutionary mechanism creating this gene family with CPS II likely giving rise to the CPS I/III clade. In the evolutionary history of this gene family it is still undetermined when CPS I diverged from CPS III on the path to terrestriality in the vertebrates. Transitional organisms such as lungfishes are of particular interest because they are capable of respiring via gills and with lungs and therefore can be found in both aquatic and terrestrial environments. Notably, enzymatic characterization of the mitochondrial CPS isoforms in this transitional group has not led to clear conclusions. In order to determine which CPS isoform is present in transitional animals, we examined partial sequences for liver CPS amplified from five species of lungfish, and a larger fragment of CPS from one lungfish species (Protopterus annectens) and compared them to CPS isoforms from other fish and mammals. Enzyme activities for P. annectens liver were also examined. While enzyme activities did not yield a clear distinction between isoforms (virtually equal activities were obtained for either CPS I or III), CPS sequences from the lungfishes formed a monophyletic clade within the CPS I clade and separate from the CPS III clade of other vertebrates. This finding implies that the mitochondrial isoform of CPS in lungfish is derived from CPS I and is likely to have a physiological function similar to CPS I. This finding is important because it supports the hypothesis that lungfish employ a urea cycle similar to terrestrial air-breathing vertebrates.
氨甲酰磷酸合成酶（CPS）催化谷氨酰胺或氨、碳酸氢盐和 ATP 生成氨甲酰磷酸。有三种不同的 CPS 同工酶，在嘧啶生物合成（CPS II）和精氨酸/尿素生物合成（CPS I 和 CPS III）两条代谢途径中发挥着重要作用。基因复制被认为是产生这个基因家族的进化机制，CPS II 可能产生了 CPS I/III 分支。在这个基因家族的进化历史中，CPS I 何时从脊椎动物的陆地进化路径中与 CPS III 分化仍然不确定。过渡生物，如肺鱼，特别有趣，因为它们能够通过鳃和肺呼吸，因此可以在水生和陆地环境中找到。值得注意的是，对这个过渡群体中线粒体 CPS 同工酶的酶学特性的研究并没有得出明确的结论。为了确定过渡动物中存在哪种 CPS 同工酶，我们从 5 种肺鱼的肝脏中扩增了部分 CPS 序列，并从一种肺鱼（Protopterus annectens）中扩增了 CPS 的较大片段，并将其与其他鱼类和哺乳动物的 CPS 同工酶进行了比较。还检查了 P. annectens 肝脏的酶活性。虽然酶活性并没有在同工酶之间产生明显的区别（几乎相同的活性可用于 CPS I 或 CPS III），但肺鱼的 CPS 序列在 CPS I 分支内形成了一个单系分支，与其他脊椎动物的 CPS III 分支分开。这一发现意味着肺鱼中线粒体同工酶的 CPS 来源于 CPS I，并且可能具有与 CPS I 相似的生理功能。这一发现很重要，因为它支持了肺鱼采用与陆地空气呼吸脊椎动物相似的尿素循环的假说。
