Question

What are the growth conditions and mechanisms that regulate the production and activity of glutamine synthetase.

Answer 1

The scale of a single cell, organ or organism, glutamine homeostasis is to a large extent determined by the activities of glutaminase (GA) and glutamine synthetase (GS), the two enzymes that are the focus of this report. GA and GS each provide examples of regulation of gene expression at many different levels. In the case of GA, two different genes (hepatic- and kidney-type GA) encode isoforms of this enzyme. The expression of hepatic GA mRNA is increased during starvation, diabetes and high protein diet through a mechanism involving increased gene transcription. In contrast, the expression of kidney GA mRNA is increased post-transcriptionally by a mechanism that increases mRNA stability during acidosis. We found recently that several isoforms of rat and human kidney-type GA are formed by tissue-specific alternative RNA splicing. Although the implications of this post-transcriptional processing mechanism for GA activity are not yet clear, it allows for the expression of different GA isoforms in different tissues and may limit the expression of GA activity in muscle tissues by diverting primary RNA transcripts to a spliceform that produces a nonfunctional translation product. The expression of GS enzyme is also regulated by both transcriptional and post-transcriptional mechanisms. For example, the GS gene is transcriptionally activated by glucocorticoid hormones in a tissue-specific fashion. This hormonal response allows GS mRNA levels to increase in selected organs during catabolic states. However, the ultimate level of GS enzyme expression is further governed by a post-transcriptional mechanism regulating GS protein stability. In a unique form of product feedback, GS protein turnover is increased by glutamine. This mechanism appears to provide a means to index the production of glutamine to its intracellular concentration and, therefore, to its systemic demand. Herein, we also provide experimental evidence that GS protein turnover is dependent upon the activity of the 26S proteosome.

In the human body, the amino acid l-glutamine (GLN) serves as an important energetic and metabolic substrate, acting as a major respiratory fuel, gluconeogenic precursor and carrier of nitrogen. In keeping with these roles, GLN is by far the most prevalent amino acid in the blood, and GLN homeostasis seems to be rigidly controlled. Accomplishing this requires the precise control of GLN utilization and production, both within cells and whole tissues. The transport of GLN into and out of cells plays an important role in the control of this process. Ultimately, however, the balance between GLN formation and catabolism relies largely on the activity of two enzymes, glutaminase (GA) and glutamine synthetase (GS).

Glutamine synthesis is an important mechanism for elimination of ammonia in various tissues. In the liver, it primarily takes place in hepatocytes surrounding the central vein of the liver lobes. Glutamine synthetase activity is also important in muscle, kidney, and brain tissues. The brain is particularly sensitive to the presence of ammonia and, thus actively produces glutamine from ammonia.

Glutamine is hydrolyzed to glutamic acid and ammonia by action of the glutaminase. Since the glutamine synthesis reaction is irreversible, its hydrolysis is not performed by reversing the same process, but by a different mechanism. Glutaminase is expressed in periportal hepatocytes and renal tubules, where the production of ammonia and its excretion in urine is one of the mechanisms regulating acid–base balance and cation conservation.