Inhibition Of Glucose Formation Research Articles

Differential effects of vanadium, tungsten and molybdenum on inhibition of glucose formation in renal tubules and hepatocytes of control and diabetic rabbits: beneficial action of melatonin and N-acetylcysteine.

Effect of vanadyl acetylacetonate (VAc), tungstate and molybdate on gluconeogenesis has been studied in isolated hepatocytes and kidney-cortex tubules. In renal tubules of control and alloxan-diabetic animals, the rank order of the metal-compounds-induced (i) inhibition of glucose formation from alanine+glycerol+octanoate or aspartate+glycerol+octanoate, (ii) decrease in the mitochondrial membrane potential (delta psim), (iii) increase in the hydroxyl free radicals (HFR) generation and (iv) decline in glucose-6-phosphatase activity was the following: VAc > tungstate > molybdate. Moreover, in contrast to VAc, both tungstate and molybdate at 100 microM concentration did not practically decrease glucose production in hepatocytes isolated from diabetic rabbits, and significantly increased the rate of lactate formation in renal tubules. N-acetylcysteine at 2 mM concentration partially attenuated vanadium-induced alterations in glucose formation, delta psim and the cellular glutathione redox state, whereas 0.1 mM melatonin did not abolish vanadium-induced changes in gluconeogenesis despite attenuation of vanadium effects on HFR formation and delta psim decline. However, similarly to control rabbits, following 6 days of intraperitoneal administration of both VAc (1.275 mg V/kg body weight daily) and melatonin (1 mg/kg body weight daily) to alloxan-diabetic animals, vanadium-induced elevated serum creatinine and urea levels were decreased, indicating the beneficial effect of melatonin on diabetes- and vanadium-induced nephrotoxicity in rabbits. As serum glucose levels were also significantly diminished by vanadium+melatonin treatment of diabetic animals, the combination therapy of vanadium compounds and melatonin needs a careful evaluation.

Effect of As2O3 on gluconeogenesis.

1) The effect of As2O3 and As2O5 on gluconeogenesis from various substrates in the liver and kidney of rats was investigated. 2) A concentration-dependent inhibition by As2O3 was found. The effect was not dependent on the amount of investigated material (hepatocytes or kidney tubules). For either hepatocytes or kidney tubules the extent of inhibition depended strongly on the substrate used. The highest degree of inhibition was observed in incubations with pyruvate. The inhibition of glucose formation was accompanied to a lesser extent by a diminution in O2 consumption and ATP content. The effect was also dependent on the substrate used. Maximum effect was found in incubations with pyruvate. 3) Oleate, 0.5 mmol/l, increased gluconeogenesis from pyruvate. The effect was not abolished by As2O3. 4) A decrease in the content of acetyl-CoA, 3-hydroxybutyrate, and reduced glutathione was found in suspensions of isolated rat kidney tubules or hepatocytes incubated with As2O3. 5) About 10 times higher concentrations of As2O5 were necessary to induce a similar extent of inhibition of gluconeogenesis, decrease in O2 consumption, and in ATP content as compared with As2O3. The extent of the As2O5 effect depended on the concentration of the toxicant and on the substrate used. Gluconeogenesis from pyruvate exhibited the highest sensitivity to As2O5. 6) All findings can be largely explained by inhibition of pyruvate dehydrogenase as the central target for arsenicals. The subsequent depletion of acetyl CoA results in impaired formation of reducing equivalents in the citric acid cycle, decrease in high energy phosphates and, acetyl CoA being a strong positive modulator of pyruvate carboxylase, in gluconeogenesis inhibition.(ABSTRACT TRUNCATED AT 250 WORDS)

Relationship of energy production to gluconeogenesis in renal cortical tubules.

Isolated tubules prepared by collagenase treatment of rat renal cortex retained their ultrastructural integrity and responded to added lactate and succinate with an increase in gluconeogenesis and respiration. Inhibition of the mitochondrial respiratory chain with rotenone, or energy conservation sites with oligomycin caused a marked reduction in respiration and ATP content thereby completely inhibiting net gluconeogenesis. Dissociation of gluconeogenesis from respiration was accomplished with quinolinic acid and hydrazine, inhibitors of gluconeogenesis. At 5 times 10(-3) M quinolinic acid, gluconeogenesis from succinate was inhibited approximately 50% and from lactate nearly 100%. This concentration of quinolinic acid did not affect oxygen uptake or the ATP content of tubules in the presence or absence of substrate. Hydrazine at 10(-3) M resulted in approximately 75% inhibition of glucose formation from succinate and complete inhibition from lactate without interfering with respiration or ATP content. The increased mitochondrial energy generation, as manifested by accelerated respiration was independent of gluconeogenesis. The unchanging cell ATP concentration with a higher respiratory rate upon addition of exogenous substrate bespeaks increased ATP turnover. ATP utilization for the substrate-induced enhancement of gluconeogenesis could not account for the increment in ATP hydrolysis.

Inhibition of glucose formation from fructose by phenformin in perfused guinea pig livers.

Glucose formation from fructose was inhibited by 4×10−5 mol/1 phenylethylbiguanide (DBI) in perfused guinea pig livers. The pattern of hepatic metabolite concentrations revealed a cross-over phenomenon between fructose and fructose-1 -phosphate indicating that the phosphorylation of the substrate was influenced by the biguanide. It is suggested that this effect was due to a decrease of the ATP/ADP ratio observed in the presence of DBI.

Preparation and properties of the glycogen-debranching enzyme from rabbit liver

The glycogen-debranching enzyme, oligo-α-1,4-glucan: α-1,4-glucan-4-glycosyl-transferase-amylo-1,6-glucosidase has been purified about 500-fold from rabbit liver. The enzyme preparation has been shown to have both transferase and glucosidase activities. In view of the fact that the protein moves as one band in discgel electrophoresis and possesses the two activities in constant ratio throughout its purification, it is concluded that one protein in fact possesses both activities. The molecular weight of the enzyme has been found to be about 179 000 by sucrose-density gradient centrifugation. The enzyme is strongly inhibited when assayed in Tris buffer and somewhat less so in imidazole buffer as compared with citrate or phosphate buffers in which the optimum for activity is at about pH 6 when a limit dextrin of glycogen, prepared by prior phosphorylase action, is the substrate. Enzyme activity is also reversibly inhibited by urea at concentrations below 2 M. Guanidine at 0.15 M produces 50% inhibition of glucose formation from a phosphorylase limit dextrin.

Rate-limiting steps of gluconeogenesis in liver cells as determined with the aid of fluoro-dicarboxylic acids.

Fluoromalate (an inhibitor of malate dehydrogenase and the malate carrier) or difluorooxaloacetate (an inhibitor of aspartate aminotransferase) inhibited gluconeogenesis from both pyruvate and lactate, but had no effect on glucose formation from fructose or endogenous substrates. When cells were incubated with both fluorocarboxylic acids simultaneously, an additive effect on the inhibition of glucose formation from lactate was observed, but the inhibition was not additive when pyruvate was the glucogenic precursor. Pyruvate removal was reduced 15% by difluorooxaloacetate and 44% by fluoromalate. Lactate production from pyruvate was inhibited 36% by fluoromalate but was almost unaffected by difluorooxaloacetate. The results suggest that both malate-oxaloacetate and glutamate-aspartate shuttles participate, but to different extents, in glucose formation from either pyruvate or lactate. The malateoxaloacetate shuttle predominates during gluconeogenesis from pyurvate whereas the glutamateaspartate shuttle is of greater importance when lactate is the gluconeogenic precursor. However, the inhibition by fluoromalate of glucose formation from lactate indicates that not all the reducing equivalents arising during lactate oxidation are directly available for the reductive step of gluconeogenesis. Neither of the two shuttles seems to compensate for inhibition of the alternative system. This lack of compensation implies that each shuttle is working close to or at its maximal capacity. Hence, transfer processes between mitochondria and cytoplasm must be considered as potential rate-limiting and regulatory sites for gluconeogenesis. The failure of fluoromalate to inhibit ethanol oxidation suggests that alternatives to the malate-oxaloacetate shuttle exist for the transfer to the mitochondria of reducing equivalents generated in the cytoplasmic compartment.