Campbell J W, Vorhaben J E, Smith D D
Department of Biology, Rice University, Houston, Texas 77251.
J Exp Zool. 1987 Sep;243(3):349-63. doi: 10.1002/jez.1402430302.
The hepatic mechanism for detoxication of ammonia formed during amino acid gluconeogenesis in uricotelic vertebrates requires the intramitochondrial synthesis of glutamine by glutamine synthetase. This glutamine then serves as a precursor of uric acid in the cytosol. The evolutionary development of uricoteley thus required the localization of glutamine synthetase in liver mitochondria. The mechanism for the mitochondrial import of glutamine synthetase in uricotelic vertebrate liver is not yet known. Tortoises, extant relatives of the stem reptiles, possess both the ureotelic and uricotelic hepatic systems. It therefore seems likely that the genetic events allowing the mitochondrial localization of glutamine synthetase in liver occurred in the amniote amphibian ancestors of the stem reptiles. The selection of ureoteley by the theropsids and of uricoteley by the sauropsids were major events in the divergence and subsequent evolution of these two lines. Once established in the sauropsid line, uricoteley has persisted through to the higher reptiles, crocodilians, and birds. Uricoteley was in part responsible for the radiation of the archosaurs during the Triassic as a water-conserving mechanism in the adult, thereby allowing them to invade the arid environments of that period. Contrary to dogma, uricoteley was probably of minor significance in the development of the cleidoic egg. Neither mammalian nor avian embryonic liver tissues catabolize amino acids to any great extent, so it is inappropriate to attribute to them a kind of "waste" nitrogen metabolism.
在排尿酸脊椎动物中，氨基酸糖异生过程中形成的氨的肝脏解毒机制需要谷氨酰胺合成酶在线粒体内合成谷氨酰胺。然后这种谷氨酰胺在细胞质中作为尿酸的前体。因此，排尿酸的进化发展需要谷氨酰胺合成酶定位于肝脏线粒体中。排尿酸脊椎动物肝脏中谷氨酰胺合成酶的线粒体导入机制尚不清楚。乌龟是茎部爬行动物的现存近亲，同时拥有排尿素和排尿酸的肝脏系统。因此，允许谷氨酰胺合成酶在线粒体中定位的基因事件似乎可能发生在茎部爬行动物的羊膜两栖动物祖先中。兽孔目动物选择排尿素和蜥形纲动物选择排尿酸是这两条谱系分化及后续进化过程中的重大事件。一旦在蜥形纲谱系中确立，排尿酸一直延续到高等爬行动物、鳄鱼和鸟类。排尿酸在一定程度上是三叠纪期间主龙类辐射的原因，作为一种成体节水机制，从而使它们能够侵入那个时期的干旱环境。与传统观念相反，排尿酸在羊膜卵的发育中可能意义不大。哺乳动物和鸟类的胚胎肝脏组织都不会大量分解氨基酸，因此将一种“废物”氮代谢归因于它们是不合适的。
