Glutamine synthesis from aspartate in guinea-pig renal cortex

Abstract

1. Glutamine was found to be the main carbon and nitrogen product of the metabolism of aspartate in isolated guinea-pig kidney-cortex tubules. Glutamate, ammonia and alanine were only minor products. 2. Carbon-balance calculations and the release of 14CO2 from [U-14C]aspartate indicate that oxidation of the aspartate carbon skeleton occurred. 3. A pathway involving aspartate aminotransferase, glutamate dehydrogenase, glutamine synthetase, phosphoenolpyruvate carboxykinase, pyruvate kinase, pyruvate dehydrogenase and enzymes of the tricarboxylic acid cycle is proposed for the conversion of aspartate into glutamine. 4. Evidence for this pathway was obtained by: (i) inhibiting aspartate removal by amino-oxyacetate, an inhibitor of transaminases, (ii) the use of methionine sulphoximine, an inhibitor of glutamine synthetase, which induced a large increase in ammonia release from aspartate, (iii) the use of quinolinate, an inhibitor of phosphoenolpyruvate carboxykinase, which inhibited glutamine synthesis from aspartate, (iv) the use of alpha-cyano-4-hydroxycinnamate, an inhibitor of the mitochondrial transport of pyruvate, which caused an accumulation of pyruvate from aspartate, and (v) the use of fluoroacetate, an inhibitor of aconitase, which inhibited glutamine synthesis with concomitant accumulation of citrate from aspartate.

Fatty acids and glycerol or lactate are required to induce gluconeogenesis from alanine in isolated rabbit renal cortical tubules

“…However, on the addition of MSO, an inhibitor of glutamine synthetase, to rabbit renal tubules incubated with alanine, a massive release of ammonium into incubation media occurs, indicating that glutamate dehydrogenase activity is masked by a very efficient operation of glutamine synthetase. Similar phenomenon has also been observed in rabbit renal cortical tubules matabolizing glutamine (Dugelay and Baverel, 1991) or aspartate (Lietz and Bryla, 1995), as well as in guinea pig renal tubules incubated with alanine (Michoudet et al, 1988) and aspartate (Baverel et al, 1990). In contrast, in human renal cortical tubules which are deficient of glutamine synthetase activity, alanine is utilized for ammoniagenesis (Fouque et al, 1996).…”

“…4. The lack of ammonium production from alanine in rabbit renal tubules is probably due to a high activity of glutamine synthetase which effectively consumes ammonium ions for glutamine synthesis in both rabbit (Dugelay and Baverel, 1991) and guinea pig kidney cortex preparations (Baverel et al, 1990). However, on the addition of MSO, an inhibitor of glutamine synthetase, to rabbit renal tubules incubated with alanine, a massive release of ammonium into incubation media occurs, indicating that glutamate dehydrogenase activity is masked by a very efficient operation of glutamine synthetase.…”

Role of controlled cardiac reoxygenation in reducing nitric oxide production and cardiac oxidant damage in cyanotic infantile hearts.

“…In addition to the benefits of regulating pO2 when CPB was initiated, we speculate that several aspects of controlled cardiac reoxygenation with blood cardioplegia limited reoxygenation damage as enumerated previously (3,4) and by some recently defined mechanisms described below.…”

“…The conventional method ofstarting CPB in hypoxemic infants is to raise P02 to > 400 mmHg by mixing their blood with fluid in the extracorporeal circuit pre-circulated at hyperoxic levels. We have speculated that such abrupt reoxygenation causes an "unintended reoxygenation injury" (2) and adds to subsequent intraoperative oxidative stress due to surgical ischemia that provides a bloodless field, and limits the effectiveness of a cardioplegic strategy shown previously to reduce reperfusion damage (3,4). Our hypothesis is based on experimental evidence that cyanosis reduces endogenous myocardial antioxidants and reoxygenation enhances free radical generation (5).…”

Implications for the Kynurenine Pathway and Quinolinic Acid in Amyotrophic Lateral Sclerosis

“…High glutamate concentrations were correlated with a spinal onset of the disease, more impaired limb function and a higher rate of muscle deterioration [116] . QUIN contributes to increase extracellular glutamate concentrations by at least four different mechanisms leading to excessive microenvironment glutamate concentrations and neurotoxicity: (1) QUIN stimulates synaptosomal glutamate release by neurons [117] ; (2) QUIN can inhibit glu- tamate uptake into the synaptic vesicle by astrocyte [17] ; (3) QUIN decreases glutamate uptake by synaptic vesicles in the rat brain [17] , and (4) QUIN can also limit glutamate recycling to glutamine in the astrocyte by decreasing glutamine synthetase activity [83,118] . In terms of neuronal damage, QUIN can potentiate its own toxicity and that of other molecules involved in excitotoxicity such as glutamate in the context of energy depletion [22] .…”