Glutamine turnover

RL Levine, CN Oliver, RH Fulks, ER Stadtman. Turnover of bacterial glutamine synthetase oxidative inactivation precedes proteolysis. Proc Natl Acad Sci (USA) 78 2120-2124, 1981. 

Each tissue, including the bloodstream, has a free amino acid pool. This amounts to a total of about 100 g. By far the largest fraction, 50-80%, is located in muscle. Kidney accounts for about 4%, liver for 10%, and the bloodstream another 4%. Glutamine and glutamate are major components of such pools. Free amino acid pools are in equilibrium with tissue protein. Tissue proteins are in a constant state of turnover, that is, biosynthesis and degradation from and to free amino acids. Only plasma proteins, which are largely synthesized in the liver, are not in equilibrium with the plasma free amino acid pool. 

This metabolic activity is achieved by a turnover of amino acids and proteins that is as rapid as that of lipids and carbohydrates. In an adult human male, 400 g of body proteins is turned over each day. Of this, 50g is used to replace digestive enzymes (Sec. 15.2), and 6g to replace hemoglobin (Sec. 15.8). The concentration of free amino acids in plasma is small (total 3.2mmol L l, of which 25 percent is glutamine), but the turnover of 400g per day of protein is equivalent to the uptake, and release back into the plasma, of 4.6 moles of a-amino-N, so that the average lifetime of an amino... 

Giorgi O and Meek J. L. (1984) v-aminobutync acid turnover m rat striatum. Effects of glutamate and kainic add / Neurochem 42, 215-220 Graham L T Jr and Aprison M H (1969) Distribution of some enzymes associated with the metabolism of aspartate, glutamate, y-aminobutyrate, and glutamine in cat spinal cord /. Neurochem 16, 559-566... 

Furthermore, to nitrite reduction as a source of stromal alkalinity, must be added the rate of ammonium uptake by chloroplasts in photorespiratory cycling and turnover of amino acids independent of photorespiration for reassimilation by glutamine synthetase and glutamate synthase. 

A metabolic alteration leading to overproduction of either PRPP or glutamine would cause an overproduction of 5 -phosphoribosylamine. Excess PRPP could result from overproduction of its precursor glucose, or from reduced use in other pathways, such as pvrimidine biosynthesis. Increased incorporation of " Qglucose and accelerated turnover of PRPP have been reported. A case of gout with low orotidylic acid pyrophosphorylase and decarboxylase activity has also been described. Whether these metabolic changes constitute the primary injury or are several steps removed from it remains to be established. 

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