Smith D D, Campbell J W
Department of Biology, Rice University, Houston, TX 77251.
Proc Natl Acad Sci U S A. 1988 Jan;85(1):160-4. doi: 10.1073/pnas.85.1.160.
Mitochondrial glutamine synthetase (EC 6.3.1.2) is the primary ammonia-detoxifying enzyme in avian liver and is therefore analogous in function to carbamoyl-phosphate synthetase I (ammonia) (EC 6.3.4.16) in mammalian liver. In mammalian liver, glutamine synthetase is cytosolic and its distribution is restricted to a few hepatocytes around the terminal venules. These cells do not express carbamoyl-phosphate synthetase I. Using immunocytochemistry, we show here that there is little or no zonation of glutamine synthetase in avian liver. Rather, it is broadly distributed to most hepatocytes, much like carbamoyl-phosphate synthetase I in mammalian liver. In situ hybridization with a cloned glutamine synthetase cDNA probe showed the distribution of glutamine synthetase mRNA in both mammalian and avian liver to correspond to the distribution of immunoreactive protein. Neither glutamine synthetase nor carbamoyl-phosphate synthetase I and ornithine transcarbamoylase (EC 2.1.3.3) are strictly zoned in liver of the Texas tortoise or of an Argentine tree frog, both of which possess a complete urea cycle but which may also rely on glutamine synthetase for ammonia detoxication. These latter results suggest that the mutually exclusive expression of either carbamoyl-phosphate synthetase I or glutamine synthetase may be unique to mammalian liver.
线粒体谷氨酰胺合成酶（EC 6.3.1.2）是鸟类肝脏中主要的氨解毒酶，因此在功能上类似于哺乳动物肝脏中的氨甲酰磷酸合成酶I（氨）（EC 6.3.4.16）。在哺乳动物肝脏中，谷氨酰胺合成酶存在于胞质溶胶中，其分布局限于终末小静脉周围的少数肝细胞。这些细胞不表达氨甲酰磷酸合成酶I。通过免疫细胞化学方法，我们在此表明鸟类肝脏中谷氨酰胺合成酶几乎没有或不存在区域化分布。相反，它广泛分布于大多数肝细胞中，这与哺乳动物肝脏中的氨甲酰磷酸合成酶I非常相似。用克隆的谷氨酰胺合成酶cDNA探针进行原位杂交显示，谷氨酰胺合成酶mRNA在哺乳动物和鸟类肝脏中的分布与免疫反应性蛋白的分布一致。在德州陆龟或阿根廷树蛙的肝脏中，谷氨酰胺合成酶、氨甲酰磷酸合成酶I和鸟氨酸转氨甲酰酶（EC 2.1.3.3）都没有严格的区域化分布，这两种动物都拥有完整的尿素循环，但也可能依赖谷氨酰胺合成酶进行氨解毒。后一个结果表明，氨甲酰磷酸合成酶I或谷氨酰胺合成酶的相互排斥性表达可能是哺乳动物肝脏所特有的。
