Jernigan H M
Department of Biochemistry, University of Tennessee, Memphis 38163.
Exp Eye Res. 1990 Jun;50(6):597-601. doi: 10.1016/0014-4835(90)90100-9.
Glutamate is important to lenses as a central intermediate in amino acid metabolism, as well as for synthesis of proteins and glutathione. In rat and calf lenses, the principal source of lenticular glutamate is glutamine, which enters the lens and is deamidated to form glutamate. In contrast, monkey lenses use external glutamate more readily than glutamine. Amino acid metabolism was studied in human lenses by incubating them with amino-labeled [15N]glutamine or [15N]glutamate. The lenticular free amino acids were then isolated and analysed by gas chromatography-mass spectrometry to determine 15N-labeled products. The results were compared with those of similar experiments with lenses from other species. Label was measured in aspartate, alanine, serine and proline, as well as lenticular glutamine and glutamate. Glutamine enters lenses more readily than glutamate in all the species examined. Nevertheless, aspartate, alanine and serine were more rapidly labeled by incubating human lenses in [15N]glutamate than in [15N]glutamine. This observation is similar to reports of experiments with monkey lenses, which unlike rat lenses, preferentially utilize glutamate rather than glutamine. In contrast, human lens proline was more rapidly labeled by incubating lenses with [15N]glutamine than with [15N]glutamate. In human lenses, the relatively slow utilization of glutamine for the transamination reactions which form aspartate, alanine and serine appears to result from slow deamidation. The relative preference for glutamine over glutamate as a precursor for proline synthesis in human lenses may be related to the mitochondrial location of the enzymes involved.
谷氨酸对晶状体很重要，它是氨基酸代谢的中心中间体，也是蛋白质和谷胱甘肽合成所必需的。在大鼠和小牛晶状体中，晶状体谷氨酸的主要来源是谷氨酰胺，谷氨酰胺进入晶状体后脱酰胺形成谷氨酸。相比之下，猴晶状体比谷氨酰胺更易利用外部的谷氨酸。通过用氨基酸标记的[15N]谷氨酰胺或[15N]谷氨酸孵育人晶状体，研究了人晶状体中的氨基酸代谢。然后分离晶状体游离氨基酸，并通过气相色谱-质谱法进行分析，以确定15N标记的产物。将结果与其他物种晶状体的类似实验结果进行比较。在天冬氨酸、丙氨酸、丝氨酸和脯氨酸以及晶状体谷氨酰胺和谷氨酸中检测到标记。在所研究的所有物种中，谷氨酰胺比谷氨酸更易进入晶状体。然而，将人晶状体置于[15N]谷氨酸中孵育时，天冬氨酸、丙氨酸和丝氨酸的标记速度比置于[15N]谷氨酰胺中更快。这一观察结果与猴晶状体实验的报道相似，与大鼠晶状体不同，猴晶状体优先利用谷氨酸而非谷氨酰胺。相比之下，将晶状体与[15N]谷氨酰胺而非[15N]谷氨酸一起孵育时，人晶状体脯氨酸的标记速度更快。在人晶状体中，谷氨酰胺用于形成天冬氨酸、丙氨酸和丝氨酸的转氨反应的利用率相对较低，这似乎是由于脱酰胺速度较慢所致。人晶状体中作为脯氨酸合成前体，谷氨酰胺相对于谷氨酸的相对偏好可能与所涉及酶的线粒体定位有关。
