Role of the malate--aspartate shuttle in renal sodium transport in the rat

Mitochondrial aspartate aminotransferase catalyses cysteine S-conjugate beta-lyase reactions

Rat liver mitochondrial aspartate aminotransferase (a homodimer) was shown to catalyse a beta-lyase reaction with three nephrotoxic halogenated cysteine S-conjugates [ S -(1,1,2,2-tetrafluoroethyl)-L-cysteine, S -(1,2-dichlorovinyl)-L-cysteine and S -(2-chloro-1,1,2-trifluoroethyl)-L-cysteine], and less effectively so with a non-toxic cysteine S-conjugate [benzothiazolyl-L-cysteine]. Transamination competes with the beta-lyase reaction, but is not favourable. The ratio of beta elimination to transamination in the presence of S -(1,1,2,2-tetrafluoroethyl)-L-cysteine and 2-oxoglutarate is >100. Syncatalytic inactivation by the halogenated cysteine S-conjugates is also observed. The enzyme turns over approx. 2700 molecules of halogenated cysteine S-conjugate on average for every monomer inactivated. Kidney mitochondria are known to be especially sensitive to toxic halogenated cysteine S-conjugates. Evidence is presented that 15-20% of the cysteine S-conjugate beta-lyase activity towards S -(1,1,2,2-tetrafluoroethyl)-L-cysteine in crude kidney mitochondrial homogenates is due to mitochondrial aspartate aminotransferase. The possible involvement of mitochondrial aspartate aminotransferase in the toxicity of halogenated cysteine S-conjugates is also discussed.

Effect of gentamicin-induced nephrotoxicity on some carbohydrate metabolic pathways in the rat renal cortex

Rats were injected with gentamicin at doses of 20, 40 and 80 mg/kg per day for 6 consecutive days. The treatment caused nephrotoxicity as evidenced by dose-related increases in serum creatinine concentration and renal tubular necrosis. The nephrotoxicity was accompanied by reduced renal cortical and fasting blood glucose levels, and by increases in serum lactate concentrations. The activities of cortical malate dehydrogenase and alanine transaminase were significantly reduced by the three doses of gentamicin. On the other hand, aspartate transaminase activity was lowered only by the highest dose of antibiotic used. However, the activity of cortical glucose-6-phosphate dehydrogenase was altered by the 20 and 40 mg/kg doses of gentamicin, but not by the 80 mg/kg dose. The two lower doses reduced the lactate content of the cortex but activated lactate dehydrogenase. The activity of isocitrate dehydrogenase was not altered by any of the gentamicin doses used.

Enalapril maleate affects 2-oxoglutarate metabolism in mitochondria from the rat kidney cortex

Enalapril maleate (EM) is the salt of N-[(S)-1-ethoxycarbonyl)-3-phenylpropyl]-L-alanyl-L-proline, used therapeutically as an anti-hypertensive agent. The effects of EM on some aspects of the energy metabolism and membrane properties of mitochondria from rat liver and kidney cortex were studied, but only the latter were significantly affected. With 0.8 mM of EM and 2-oxoglutarate as oxidizable substrate for isolated mitochondria from rat kidney cortex, the findings were: (a) inhibition of the respiratory rate in state III (37 per cent) and decrease (45 per cent) in respiratory control ratio (RCR), with only one addition of ADP; (b) reinforcement of the inhibition when a second addition of ADP was made; (c) no significant effect either on the rate of respiration in state IV or on the ADP/O ratio; (d) no effect on the ATPase activity of mitochondria from liver or kidney cortex; (e) inhibition of the transmembrane potential (delta psi) after a second addition of ADP; (f) inhibition of the 2-oxoglutarate dehydrogenase complex. It is suggested that in kidney mitochondria, EM interferes in the gluconeogenesis dependence of at least five substrates: 2-oxoglutarate, glutamine, glutamate, lactate, and pyruvate. Also, EM may inhibit Na+/H+ exchange causing natriuresis.

Contribution of amino acids in protective solutions to postischemic functional recovery of canine kidneys

Amino acids are known to increase glomerular filtration rate (GFR). There is also an early resumption of filtration following 2-h renal ischemic stress under protection by histidine-buffered histidine-tryptophan-ketoglutarate solution (HTK), possibly due in part to an amino acid effect. Hence, we have examined the possibility of further enhancing the postischemic GFR by adding 32 (ASP I; 4 mM Mg2+) or 36 (ASP II; 6 mM Mg2+) mM L-aspartate (asp) or 32 mM DL-aspartate (ASP III) to the HTK solution in place of chloride. After infusion of 500 ml 5% glucose, canine kidneys were protected by an 8-min perfusion with HTK (n = 5), ASP I (n = 4), ASP II (n = 5) or ASP III-solution (n = 3). The subsequent ischemia lasted for 2 h at 27-31 degrees C. During reperfusion, both GFR and filtration fraction (FF) were higher in kidneys protected by L-aspartate-containing solutions. ASP III showed no improvement against HTK. An additional preischemic intra-aortal application of HTK or ASP I solution just above the exit of the renal arteries prior to the intrinsic protective perfusion further raised the postischemic GFR. The present results suggest that L-aspartate but also histidine may have favorable amino acid effects in renal protective solutions in addition to known positive effects of histidine.

Postischaemic interrelations between energy metabolism and functional recovery of protected canine kidneys

Following renal ischaemia under effective protection glomerular filtration starts at a time when renal high-energy phosphates are resynthesized. The present investigation was carried out to examine the interdependence of the two processes. Therefore, canine kidneys were protected with sodium poor and nominally Ca2+ free buffered solutions. After different times of ischaemia and reperfusion the glomerular filtration rates (GFR) were determined and afterwards tissue specimens were excised for the determination of high-energy phosphates. Both the GFR and the renal energy content were higher with shorter ischaemia periods or longer reperfusion. As a rule there was a correlation between GFR and ATP, both probably mainly signifying the degree of preservation of proximal nephron sites, as the interrelation between the GFR and the renal SAN (ATP + ADP + AMP) content also signifies. However, with the addition of aspartate to the protective solution it could be demonstrated that the GFR also can rise without a concomitant increase of ATP. Under certain conditions an increase of GFR can even be associated with a lowering of the renal ATP content.

Brain alpha-ketoglutarate dehydrogenase complex: kinetic properties, regional distribution, and effects of inhibitors

The substrate and cofactor requirements and some kinetic properties of the alpha-ketoglutarate dehydrogenase complex (KGDHC; EC 1.2.4.2, EC 2.3.1.61, and EC 1.6.4.3) in purified rat brain mitochondria were studied. Brain mitochondrial KGDHC showed absolute requirement for alpha-ketoglutarate, CoA and NAD, and only partial requirement for added thiamine pyrophosphate, but no requirement for Mg2+ under the assay conditions employed in this study. The pH optimum was between 7.2 and 7.4, but, at pH values below 7.0 or above 7.8, KGDHC activity decreased markedly. KGDHC activity in various brain regions followed the rank order: cerebral cortex greater than cerebellum greater than or equal to midbrain greater than striatum = hippocampus greater than hypothalamus greater than pons and medulla greater than olfactory bulb. Significant inhibition of brain mitochondrial KGDHC was noted at pathological concentrations of ammonia (0.2-2 mM). However, the purified bovine heart KGDHC and KGDHC activity in isolated rat heart mitochondria were much less sensitive to inhibition. At 5 mM both beta-methylene-D,L-aspartate and D,L-vinylglycine (inhibitors of cerebral glucose oxidation) inhibited the purified heart but not the brain mitochondrial enzyme complex. At approximately 10 microM, calcium slightly stimulated (by 10-15%) the brain mitochondrial KGDHC. At concentrations above 100 microM, calcium (IC50 = 1 mM) inhibited both brain mitochondrial and purified heart KGDHC. The present results suggest that some of the kinetic properties of the rat brain mitochondrial KGDHC differ from those of the purified bovine heart and rat heart mitochondrial enzyme complexes. They also suggest that the inhibition of KGDHC by ammonia and the consequent effect on the citric acid cycle fluxes may be of pathophysiological and/or pathogenetic importance in hyperammonemia and in diseases (e.g., hepatic encephalopathy, inborn errors of urea metabolism, Reye's syndrome) where hyperammonemia is a consistent feature. Brain accumulation of calcium occurs in a number of pathological conditions. Therefore, it is possible that such a calcium accumulation may have a deleterious effect on KGDHC activity.

Glucose metabolism in renal tubular function

Heterogeneity of metabolic activity along the nephron points to a very varied relationship between glucose metabolism and ion transport. Glycolysis is linked closely to free-water clearance and possibly to sodium, potassium, and hydrogen ion transport. Glucose oxidation, while not the major source of renal energy, is crucial in sodium, potassium, and phosphate reabsorption. Gluconeogenesis recovers carbon compounds generated during the process of renal ammoniagenesis. Glucose synthesis and active sodium transport appear to compete for renal ATP, although no regulatory function for this competition has been identified. Glucose formed in the proximal tubule may support free-water clearance in adjacent distal tubule, but is not thought to contribute to any medullary function. The complex network of biosynthetic and catabolic pathways of glucose metabolism may have evolved in the kidney to protect the organism against wide variations in glucose demand which would otherwise be unavoidable during the course of rapidly fluctuating renal electrolyte loads.

Dissociation of gluconeogenesis from fluid and phosphate reabsorption in isolated rabbit proximal tubules

Gluconeogenesis in the kidney is a metabolic function characteristic of proximal tubules. Recent studies suggest that renal gluconeogenesis (GNG) may in some way be coupled to fluid and phosphate reabsorption in the proximal tubule. Therefore, the present studies examined more directly the relationship between GNG and transport of fluid and phosphate using isolated proximal straight tubules of rabbit kidney. Glucose production rates were determined in isolated tubules with a glucose microassay while both phosphate (Jp) and net fluid fluxes (Jv) were measured by the in vitro isolated tubule perfusion technique. Glucose production rates from individual substrates, including pyruvate, lactate, glutamate and alpha-ketoglutarate, differed significantly, but neither Jv nor Jp was altered when different substrates in the bath medium were used. Inhibition of GNG by 3-mercaptopicolinate did not alter the Jv or Jp. Acid pH (7.0) stimulated GNG and suppressed both Jv and Jp. The addition of 3-mercaptopicolinate at acid pH abolished the stimulatory effect of acid pH on GNG but had no effect on Jv or Jp. These data thus indicate that, in the isolated rabbit renal proximal tubule, GNG rates can be dissociated from both the Jv and Jp and suggest that renal GNG may not directly regulate the fluid and phosphate transport rates in the proximal straight tubule.

Use of beta-methylene-D,L-aspartate to assess the role of aspartate aminotransferase in cerebral oxidative metabolism

Several inhibitors of aspartate aminotransferase, a key enzyme of the malate-aspartate shuttle, were investigated for their effects on cerebral oxidative metabolism in vitro. beta-Methylene-D,L-aspartate (2 mM), aminooxyacetate (0.1 mM), and D,L-vinylglycine (20 mM) all significantly reduced the activity of aspartate aminotransferase and the rate of oxygen consumption of rat cerebral cortex slices respiring on glucose. In the presence of beta-methyleneaspartate, a one-to-one correlation was found between the degree of inhibition of tissue respiration and the degree of inhibition of transaminase activity. Slices of rat liver incubated in the presence of glucose and beta-methyleneaspartate showed a similar one-to-one relationship between inhibition of oxygen comsumption and inhibition of aspartate aminotransferase activity, whereas with rat kidney cortex slices, the inhibition of aspartate aminotransferase activity was greater than the inhibition of oxygen consumption. Structural analogs of beta-methyleneaspartate (D,L-beta-methyl-D,L-aspartate, gamma-methyl-D,L-glutamate, and alpha-methyl-D,L-didehydroglutamate) that did not inhibit the activity of aspartate aminotransferase similarly did not inhibit the rate of oxygen consumption by cerebral cortex slices. In the presence of beta-methyleneaspartate, pyruvate oxidation by cerebral cortex slices was inhibited to almost the same extent as was glucose oxidation, and the oxidation of succinate was decreased by approximately 20%. The artificial electron acceptor phenazine methosulfate (0.1 mM) only partially overcame the beta-methyleneaspartate-mediated inhibition of respiration with glucose as substrate. The content of ATP and phosphocreatine declined steadily in slices incubated with glucose and beta-methyleneaspartate. At 1 h the concentration of lactate and the lactate/pyruvate ratio, an indicator of the cytoplasmic redox state, increased threefold, whereas the concentrations of malate, citrate, and aspartate decreased. The findings are interpreted in the context of the hypothesis that enzymes common to the malate-aspartate shuttle and the tricarboxylic acid cycle are physically complexed in brain, so that inhibition of aspartate aminotransferase, a component of the complex, impedes the flow of carbon through both metabolic pathways.(ABSTRACT TRUNCATED AT 400 WORDS)