Glutamine synthetase activity of muscle in acidosis.
Ontology highlight
ABSTRACT:
SUBMITTER: King PA
PROVIDER: S-EPMC1152536 | biostudies-other | 1983 Nov
REPOSITORIES: biostudies-other
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Project description:L-Glutamine (Gln) functions physiologically to balance the carbon and nitrogen requirements of tissues. It has been proposed that in cancer cells undergoing aerobic glycolysis, accelerated anabolism is sustained by Gln-derived carbons, which replenish the tricarboxylic acid (TCA) cycle (anaplerosis). However, it is shown here that in glioblastoma (GBM) cells, almost half of the Gln-derived glutamate (Glu) is secreted and does not enter the TCA cycle, and that inhibiting glutaminolysis does not affect cell proliferation. Moreover, Gln-starved cells are not rescued by TCA cycle replenishment. Instead, the conversion of Glu to Gln by glutamine synthetase (GS; cataplerosis) confers Gln prototrophy, and fuels de novo purine biosynthesis. In both orthotopic GBM models and in patients, (13)C-glucose tracing showed that GS produces Gln from TCA-cycle-derived carbons. Finally, the Gln required for the growth of GBM tumours is contributed only marginally by the circulation, and is mainly either autonomously synthesized by GS-positive glioma cells, or supplied by astrocytes.
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Project description:1. Glutamine synthetase activity has been determined in extracts of rat cardiac and skeletal muscle and kidney, after treatment to ensure that the rate of synthesis was proportional to time of incubation and to amount of extract added. The activity was measured by two methods, with hydroxylamine as substrate. 2. No activity was detected in rat heart extract by either method. The activity in skeletal muscle was of the order of 20mumol of glutamylhydroxamate synthesized/h per g of tissue under optimum conditions. The activity in kidney extracts was 180mumol/h per g of tissue when measured as ferric hydroxamate. 3. The activity in both skeletal-muscle and kidney extracts was inhibited by P(i). The inhibition is competitive for the muscle enzyme, with a K(i) of 12mm. For the kidney enzyme the inhibition is non-competitive, and less marked. Possible enzyme mechanisms that would lead to these types of inhibition are discussed. 4. Several observations are reported that suggest that the enzymes from muscle and kidney are not identical. 5. Growth hormone, either in vivo or in vitro, did not affect the measured glutamine synthetase activity of tissue extracts.
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