van Straaten Henny W M, He Youji, van Duist Marjan M, Labruyère Wil T, Vermeulen Jacqueline L M, van Dijk Paul J, Ruijter Jan M, Lamers Wouter H, Hakvoort Theodorus B M
Department of Anatomy and Embryology, University of Maastricht, Netherlands.
Biochem Cell Biol. 2006 Apr;84(2):215-31. doi: 10.1139/o05-170.
Glutamine synthetase (GS) is the only enzyme that can synthesize glutamine, but it also functions to detoxify glutamate and ammonia. Organs with high cellular concentrations of GS appear to function primarily to remove glutamate or ammonia, whereas those with a low cellular concentration appear to primarily produce glutamine. To validate this apparent dichotomy and to clarify its regulation, we determined the GS concentrations in 18 organs of the mouse. There was a >100-fold difference in GS mRNA, protein, and enzyme-activity levels among organs, whereas there was only a 20-fold difference in the GS protein:mRNA ratio, suggesting extensive transcriptional and posttranscriptional regulation. In contrast, only small differences in the GS enzyme activity : protein ratio were found, indicating that posttranslational regulation is of minor importance. The cellular concentration of GS was determined by relating the relative differences in cellular GS concentration, detected using image analysis of immunohistochemically stained tissue sections, to the biochemical data. There was a >1000-fold difference in cellular concentrations of GS between GS-positive cells in different organs, and cellular concentrations were up to 20x higher in subpopulations of cells within organs than in whole organs. GS activity was highest in pericentral hepatocytes (approximately 485 micromol.g(-1).min-(1), followed in descending order by epithelial cells in the epididymal head, Leydig cells in the testicular interstitium, epithelial cells of the uterine tube, acid-producing parietal cells in the stomach, epithelial cells of the S3 segment of the proximal convoluted tubule of the kidney, astrocytes of the central nervous tissue, and adipose tissue. GS activity in muscle amounted to only 0.4 micromol.g(-1).min(-1). Our findings confirmed the postulated dichotomy between cellular concentration and GS function.
谷氨酰胺合成酶（GS）是唯一能够合成谷氨酰胺的酶，但它也具有使谷氨酸和氨解毒的功能。GS细胞浓度高的器官似乎主要起到清除谷氨酸或氨的作用，而GS细胞浓度低的器官似乎主要是产生谷氨酰胺。为了验证这种明显的二分法并阐明其调节机制，我们测定了小鼠18个器官中的GS浓度。各器官之间GS mRNA、蛋白质和酶活性水平存在100倍以上的差异，而GS蛋白质与mRNA的比值仅相差20倍，这表明存在广泛的转录和转录后调控。相比之下，GS酶活性与蛋白质的比值仅存在微小差异，表明翻译后调控的重要性较小。通过将免疫组织化学染色组织切片的图像分析检测到的细胞GS浓度的相对差异与生化数据相关联，来确定GS的细胞浓度。不同器官中GS阳性细胞的细胞浓度相差1000倍以上，器官内细胞亚群的细胞浓度比整个器官高20倍。GS活性在中央周围肝细胞中最高（约485微摩尔·克⁻¹·分钟⁻¹），其次依次为附睾头部的上皮细胞、睾丸间质中的Leydig细胞、输卵管上皮细胞、胃中产生酸的壁细胞、肾近端曲管S3段的上皮细胞、中枢神经组织的星形胶质细胞和脂肪组织。肌肉中的GS活性仅为0.4微摩尔·克⁻¹·分钟⁻¹。我们的研究结果证实了细胞浓度与GS功能之间的假定二分法。
