Glutamine synthesis and synthetase in guinea pig kidney

Quarcoo, Ezekiel Tawia (1983) Glutamine synthesis and synthetase in guinea pig kidney. Masters thesis, Memorial University of Newfoundland.

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Abstract

It is now generally believed that kidneys of many species increase ammonia excretion (through the action of glutaminases) in the urine, in response to an acid challenge, and that the physiological significance of such enhanced ammoniagenesis is to primarily conserve extra-cellular fluid cations, especially Na⁺. The source material for urine ammonia has been identified as glutamine. However, the physiological stimulus for renal ammonia excretion, the relative importance of enzymatic and physicochemical factors in the mechanism of ammonia excretion, and the rate-limiting metabolic step(s) which is (are) modified during acidosis, are still not clear. -- In an attempt to unravel some of these mysteries, studies were conducted on renal synthesis of glutamine, and glutamine synthetase. Glutamine synthetase has the potential to regulate ammoniagenesis by removing ammonia. It is clear, therefore, that results of studies performed on a system which contains high activities of both the ammonia producing enzyme (glutaminase), and the ammonia consuming enzyme (glutamine synthetase), would be difficult to interprete in terms of net ammonia metabolism at any given time. A system with a relatively high activity of one or the other enzyme was necessary. -- Earlier studies had implicated glutamine synthetase in the regulatory events of renal ammoniagenesis in the rat. The guina pig was chosen as a suitable experimental animal model for the studies reported, because the guinea pig kidney has a relatively high activity of glutamine synthetase, and a low activity of glutaminase. -- These studies showed that the guinea pig kidney cortex tubule synthesizes glutamine rapidly at pH 7.4 (503+78μmoles/30 min/g dry wt), when incubated with glutamate and ammonium chloride. Furthermore, the high rates of glutamine synthesis were reduced by about 50% at pH 7.1, regardless of whether the lower pH was produced by decreasiag the HC0₃ concentration (metabolic acidosis) or increasing the PC0₂ (respiratory acidosis). The inhibition of glutamine synthesis was reversible. -- Subcellular fractionation studies on guinea pig kidney cortex indicated an exclusive oytosolic localization of glutamine synthetase. Such localization was not altered when the pH of the homogenizing medium was changed, since glutamine synthetase almost consistently appeared in the soluble fraction of spun homogenates, using sucrose or KC1 media.

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