Urea synthesis in the African lunglish Protopterus dolloi -: hepatic carbamoyl phosphate synthetase III and glutamine synthetase are upregulated by 6 days of aerial exposure
Urea synthesis in the African lunglish Protopterus dolloi -: hepatic carbamoyl phosphate synthetase III and glutamine synthetase are upregulated by 6 days of aerial exposure
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DOI：
10.1242/jeb.00619
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发表时间：
2003-10-01
影响因子：
2.8
通讯作者：
Ip, YK
中科院分区：
文献类型：
--
作者：
Chew, SF;Ong, TF;Ip, YK
与海洋鳐Taeniura lagma一样，非洲肺鱼Protopterus dolloi在肝脏中具有氨甲酰基磷酸盐III（CPS III），而不是像小鼠Mus musculus或其他非洲肺鱼那样在肝脏中具有氨甲酰基磷酸盐I（CPS I）。然而，与其他非洲肺鱼和四足动物相似，杜氏肺鱼的肝磷脂酶主要存在于细胞质中。谷氨酰胺合成酶活性存在于杜氏毕赤酵母肝脏的线粒体和胞质组分中。因此，我们得出结论，P. dolloi是一种更原始的现存肺鱼，介于水生鱼类和陆生四足动物之间，代表了鱼类-四足动物连续体中的一个环节。在6天的空中暴露期间，杜氏原螯虾的排氨率显著下降，仅为浸水对照的8-16%。然而，有没有显着增加氨含量在肌肉，肝脏或血浆中的样品暴露于空气中6天。这些结果表明：（1）内源性氨的产生急剧减少，（2）内源性氨被有效地解毒为尿素。事实上，暴露在空气中的鱼的肝脏中的谷氨酸盐、谷氨酰胺和赖氨酸水平显著下降，这导致总游离氨基酸含量下降。这间接证实了标本降低了其蛋白水解和/或氨基酸催化速率，以抑制内源性氨产生。同时，暴露于空气中6天的样本的肌肉（8倍）、肝脏（10.5倍）和血浆（12.6倍）中的尿素水平显著增加。此外，肝脏鸟氨酸-尿素循环（OUC）能力增加，CPS III（3.8倍）、乙酰氨基琥珀酸合成酶+裂解酶（1.8倍）和更重要的谷氨酰胺合成酶（2.2倍）的活性显著增加。这是第一份关于暴露于空气中的非洲肺鱼OUC能力和尿素合成速率上调的报告。再次浸泡后，尿素排泄率增加了22倍，与对照样品相比，这是最大的增加在鱼类在浸泡过渡，并表明，P. dolloi具有转运蛋白，促进尿素在水中的排泄。
Like the marine ray Taeniura lymma, the African lungfish Protopterus dolloi possesses carbamoyl phosphate III (CPS III) in the liver and not carbamoyl phosphate I (CPS I), as in the mouse Mus musculus or as in other African lungfish reported elsewhere. However, similar to other African lungfish and tetrapods, hepatic arginase of P. dolloi is present mainly in the cytosol. Glutamine synthetase activity is present in both the mitochondrial and cytosolic fractions of the liver of P. dolloi. Therefore, we conclude that P. dolloi is a more primitive extant lungfish, which is intermediate between aquatic fish and terrestrial tetrapods, and represents a link in the fish-tetrapod continuum. During 6 days of aerial exposure, the ammonia excretion rate in P. dolloi decreased significantly to 8-16% of the submerged control. However, there were no significant increases in ammonia contents in the muscle, liver or plasma of specimens exposed to air for 6 days. These results suggest that (1) endogenous ammonia production was drastically reduced and (2) endogenous ammonia was detoxified effectively into urea. Indeed, there were significant decreases in glutamate, glutamine and lysine levels in the livers of fish exposed to air, which led to a decrease in the total free amino acid content. This indirectly confirms that the specimen had reduced its rates of proteolysis and/or amino acid catabolism to suppress endogenous ammonia production. Simultaneously, there were significant increases in urea levels in the muscle (8-fold), liver (10.5-fold) and plasma (12.6-fold) of specimens exposed to air for 6 days. Furthermore, there was an increase in the hepatic ornithine-urea cycle (OUC) capacity, with significant increases in the activities of CPS III (3.8-fold), argininosuccinate synthetase + lyase (1.8-fold) and, more importantly, glutamine synthetase (2.2-fold). This is the first report on the upregulation of OUC capacity and urea synthesis rate in an African lungfish exposed to air. Upon re-immersion, the urea excretion rate increased 22-fold compared with that of the control specimen, which is the greatest increase among fish during emersion-immersion transitions and suggests that P. dolloi possesses transporters that facilitate the excretion of urea in water.