The distribution of hepatic metabolites and the control of the pathways of carbohydrate metabolism in animals of different dietary and hormonal status

Kinetic modeling of glucose central metabolism in hepatocytes and hepatoma cells

BACKGROUND

Kinetic modeling and control analysis of a metabolic pathway may identify the steps with the highest control in tumor cells, and low control in normal cells, which can be proposed as the best therapeutic targets.

METHODS

Enzyme kinetic characterization, pathway kinetic modeling and control analysis of the glucose central metabolism were carried out in rat (hepatoma AS-30D) and human (cervix HeLa) cancer cells and normal rat hepatocytes.

RESULTS

The glycogen metabolism enzymes in AS-30D, HeLa cells and hepatocytes showed similar kinetic properties, except for higher AS-30D glycogen phosphorylase (GP) sensitivity to AMP. Pathway modeling indicated that fluxes of glycogen degradation and PPP were mainly controlled by GP and NADPH consumption, respectively, in both hepatocytes and cancer cells. Likewise, hexose-6-phosphate isomerase (HPI) and phosphoglucomutase (PGM) exerted significant control on glycolysis and glycogen synthesis fluxes in cancer cells but not in hepatocytes. Modeling also indicated that glycolytic and glycogen synthesis fluxes could be strongly decreased when HPI and PGM were simultaneously inhibited in AS-30D cells but not in hepatocytes. Experimental assessment of these predictions showed that both the glycolytic and glycogen synthesis fluxes of AS-30D cells, but not of hepatocytes, were inhibited by oxamate, by inducing increased Fru1,6BP levels, a competitive inhibitor of HPI and PGM.

CONCLUSION

HPI and PGM seem suitable targets for decreasing glycolytic and glycogen synthesis fluxes in AS-30D cells but not in hepatocytes.

GENERAL SIGNIFICANCE

The present study identified new therapeutic targets within glucose central metabolism in the analyzed cancer cells, with no effects on non-cancer cells.

Control of the NADPH supply for oxidative stress handling in cancer cells

It has not been systematically analyzed whether the NADPH supply is a limiting factor for oxidative stress management in cancer cells. In the present work, it was determined in non-cancer and cancer cells the protein contents and kinetomics of (i) the cytosolic enzymes responsible for the NADPH production (i.e., Glc6PDH, 6PGDH, ME, IDH-1); and (ii) the two main enzymes responsible for NADPH/NADP⁺ and GSH/GSSG recycling (GR, GPx-1) associated to oxidative stress management. With these data, kinetic models were built and further validated. Rat liver and hepatoma AS-30D cytosolic fractions exhibited greater Vmax for IDH-1 than for Glc6PDH and 6PGDH whereas human cancer cells and platelets showed greater Vmax for Glc6PDH than for 6PGDH and IDH-1. The ME activity was comparatively low in all cell types tested. The Km values for the respective specific substrates were all similar among the different cell types. Most activities were lower in AS-30D cells than in liver. In contrast, IDH-1, Glc6PDH and GR activities in human cancer cells were similar or greater to those of platelets, but GPx-1 activity was severely suppressed, despite showing similar GPx-1 protein content vs. platelets. Kinetic analysis and pathway modeling revealed a previously unveiled feedback IDH-1 regulation by GSH. The oxidative stress management in cancer cells (i) was mainly controlled by GPx-1 and the main NADPH provider was Glc6PDH; and (ii) modeling indicated that NADPH supply was not a controlling step. These data suggested that Glc6PDH and GPx-1 are adequate and promising targets for anti-cancer therapeutic intervention.

Oral health, diabetes, and body weight

OBJECTIVES

The effects of apical periodontitis (AP) and periodontal disease (PD) on organ weights in rats with diabetes mellitus (DM) were evaluated.

DESIGN

Eighty male rats (Rattus norvegicus albinus, Wistar) were divided into eight groups of ten: normoglycemic (N), AP, PD, AP+PD, DM, DM+AP, DM+PD, and DM+AP+PD. DM was induced by streptozotocin; AP, by dental exposure to the oral environment; and PD, by periodontal ligature. Blood glucose concentration was measured at 0, 6, 15, and 35days; body weight, measured daily; and animals were sacrificed after 30days after induction of oral infections. Liver, kidney, pancreas, brain, heart, lungs, and gonads were each weighed. Glycemia, feed intake, organ weight, and body weight were subjected to statistical analyses (p<0.05).

RESULTS

Glycemic levels were higher in all diabetic groups after day 6, but were consistently similar in normoglycemic groups. Blood glucose was higher in DM+PD and DM+AP+PD groups than in the DM group at days 15 and 35. The feed intake was similar among all groups. Brain, heart, and gonad weights were significantly increased in DM+AP+PD. Kidney and lung weights were increased in DM, regardless of the presence of oral infections. Liver weight was reduced in AP and/or PD. Pancreas weight was reduced in DM, independent of AP or PD. Among the normoglycemic groups, there were no significant differences among organ weights.

CONCLUSION

Apical periodontitis and periodontal disease may potentiate the adverse effects of diabetes.

Phenotypic severity of homozygous GCK mutations causing neonatal or childhood-onset diabetes is primarily mediated through effects on protein stability

Mutations in glucokinase (GCK) cause a spectrum of glycemic disorders. Heterozygous loss-of-function mutations cause mild fasting hyperglycemia irrespective of mutation severity due to compensation from the unaffected allele. Conversely, homozygous loss-of-function mutations cause permanent neonatal diabetes requiring lifelong insulin treatment. This study aimed to determine the relationship between in vitro mutation severity and clinical phenotype in a large international case series of patients with homozygous GCK mutations. Clinical characteristics for 30 patients with diabetes due to homozygous GCK mutations (19 unique mutations, including 16 missense) were compiled and assigned a clinical severity grade (CSG) based on birth weight and age at diagnosis. The majority (28 of 30) of subjects were diagnosed before 9 months, with the remaining two at 9 and 15 years. These are the first two cases of a homozygous GCK mutation diagnosed outside infancy. Recombinant mutant GCK proteins were analyzed for kinetic and thermostability characteristics and assigned a relative activity index (RAI) or relative stability index (RSI) value. Six of 16 missense mutations exhibited severe kinetic defects (RAI ≤ 0.01). There was no correlation between CSG and RAI (r(2) = 0.05, P = 0.39), indicating that kinetics alone did not explain the phenotype. Eighty percent of the remaining mutations showed reduced thermostability, the exceptions being the two later-onset mutations which exhibited increased thermostability. Comparison of CSG with RSI detected a highly significant correlation (r(2) = 0.74, P = 0.002). We report the largest case series of homozygous GCK mutations to date and demonstrate that they can cause childhood-onset diabetes, with protein instability being the major determinant of mutation severity.

Divergent evolution of protein conformational dynamics in dihydrofolate reductase

Molecular evolution is driven by mutations, which may affect the fitness of an organism and are then subject to natural selection or genetic drift. Analysis of primary protein sequences and tertiary structures has yielded valuable insights into the evolution of protein function, but little is known about evolution of functional mechanisms, protein dynamics and conformational plasticity essential for activity. We characterized the atomic-level motions across divergent members of the dihydrofolate reductase (DHFR) family. Despite structural similarity, E. coli and human DHFRs use different dynamic mechanisms to perform the same function, and human DHFR cannot complement DHFR-deficient E. coli cells. Identification of the primary sequence determinants of flexibility in DHFRs from several species allowed us to propose a likely scenario for the evolution of functionally important DHFR dynamics, following a pattern of divergent evolution that is tuned by the cellular environment.

Regulation of metabolic pathways in liver and kidney during experimental diabetes: Effects of antidiabetic compounds

Diabetes has been classified as a disease of glucose overproduction by tissues, mainly liver and glucose underutilization by insulin requiring tissues like liver, adipose and muscle due to lack of insulin. There is, however, glucose over utilization in tissues not dependent on insulin for glucose transport like kidney, nerve and brain. There are serious complications due to this excess glucose in these tissues and their reversal is important for a good metabolic control and normalisation of other parameters. Insulin, trace metals and some plant extracts have been used to see the reversal effects of the complications of diabetes in liver and kidney in experimental diabetes. Almost complete reversal of the metabolic changes has been achieved in the activities of key enzymes of metabolic pathways in liver and kidney and an effective glucose control has been achieved suggesting a combination of therapies in the treatment of metabolic disturbance of the diabetic state.

Alternative pathways of glucose utilization in developing rat spinal cord

The activities of alternative pathways of glucose utilization in the developing rat spinal cord were evaluated from the release of (14)CO(2) and the incorporation of [(14)C] into lipids from differentially labelled glucose. Total lipid synthesis had peak activity at 15 days post-partum corresponding to the period of peak myelination in rat spinal cord. The activities of the glycolytic route, tricarboxylic acid cycle and fully activated pentose phosphate pathway were highest up to 20 days post-partum. After this period myelin (which is biochemically relatively inert) will constitute a larger proportion of the mass of the cord and this may contribute to the lower observed rates of the above pathways during later stages of development. Treatment of 20 day old rats with 6-aminonicotinamide resulted in spastic paralysis of the rats and pronounced inhibition of the pentose phosphate pathway indicating that this pathway, although low in activity (less than 4% of total glucose oxidation) has an important role in developing rat spinal cord.

Inactivation of the AMP-activated protein kinase by glucose in cardiac myocytes: a role for the pentose phosphate pathway

Incubation of adult rat cardiac myocytes with increasing glucose concentrations decreased phosphorylation (αThr172) and activity of AMPK (AMP-activated protein kinase). The effect could be demonstrated without measurable changes in adenine nucleotide contents. The glucose effect was additive to the decrease in AMPK activity caused by insulin, was attenuated by adrenaline, was not mimicked by glucose analogues, lactate or pyruvate and was not due to changes in myocyte glycogen content. AMPK activity was decreased by xylitol and PMS (phenazine methosulfate) and was increased by the glucose-6-phosphate dehydrogenase inhibitor DHEA (dehydroepiandrosterone) and by thiamine. PMS and DHEA respectively, increased and decreased CO2 formation by the PPP (pentose phosphate pathway). AMPK activity was inversely related to the myocyte content of Xu5P (xylulose 5-phosphate), an intermediate of the non-oxidative arm of the PPP. Endothall, an inhibitor of PP2A (protein phosphatase 2A), abolished the glucose effect on AMPK activity. Further studies are needed to define the ‘active component’ that mediates the glucose effect and whether its site of action is PP2A.

Metabolic-flux analysis of hybridoma cells under oxidative and reductive stress using mass balances

Hybridoma cells were grown at steady state under both reductiveand oxidative stress and the intracellular fluxes weredetermined by mass-balancing techniques. By decreasing the dissolved oxygen pressure (pO(2)) in the bioreactor, the reduced formof nicotinamide adenine nucleotide (NADH) was enhanced relativeto the oxidized form (NAD(+)). Oxidative stress, as a resultof which the NAP(P)(+)/NAD(P)H-ratio increases, was generatedby both the enhancement of the pO(2) to 100% air saturationand by the addition of the artificial electron acceptorphenazine methosulphate (PMS) to the culture medium. It wasfound that fluxes of dehydrogenase reactions by which NAD(P)H isproduced decreased under hypoxic conditions. For example, thedegradation rates of arginine, isoleucine, lysine and theglutamate dehydrogenase flux were significantly lower at oxygenlimitation, and increased at higher pO(2) levels and when PMSwas added to the culture medium. In contrast, the prolinesynthesis reaction, which requires NADPH, decreased under PMSstress. The flux of the NADH-requiring lactate dehydrogenase reaction also strongly decreased from 19 to 3,4 pmol/cell/day,under oxygen limitation and under PMS stress, respectively. Thedata show that metabolic-flux balancing can be used to determinehow mammalian respond to oxidative and reduction stress.

Cytosolic NADPH balancing in Penicillium chrysogenum cultivated on mixtures of glucose and ethanol

The in vivo flux through the oxidative branch of the pentose phosphate pathway (oxPPP) in Penicillium chrysogenum was determined during growth in glucose/ethanol carbon-limited chemostat cultures, at the same growth rate. Non-stationary (13)C flux analysis was used to measure the oxPPP flux. A nearly constant oxPPP flux was found for all glucose/ethanol ratios studied. This indicates that the cytosolic NADPH supply is independent of the amount of assimilated ethanol. The cofactor assignment in the model of van Gulik et al. (Biotechnol Bioeng 68(6):602-618, 2000) was supported using the published genome annotation of P. chrysogenum. Metabolic flux analysis showed that NADPH requirements in the cytosol remain nearly the same in these experiments due to constant biomass growth. Based on the cytosolic NADPH balance, it is known that the cytosolic aldehyde dehydrogenase in P. chrysogenum is NAD(+) dependent. Metabolic modeling shows that changing the NAD(+)-aldehyde dehydrogenase to NADP(+)-aldehyde dehydrogenase can increase the penicillin yield on substrate.

Rapid separation of particulate components and soluble cytoplasm of isolated rat-liver cells

A method is described for the rapid separation of mitochondria (plus other particulate components) from the soluble cytoplasm of isolated rat-liver cells. The cells were incubated briefly with a low concentration of digitonin. After rapid centrifugation, the pellet contained more than 90% of the total adenylate kinase and glutamate dehydrogenase activities and the supernatant at least 80% of the lactate dehydrogenase activity. About 60% of total adenine nucleotides in hepatocytes were found in the soluble cytoplasm. The ATP/ADP ratio in the particulate fraction 80 s after exposure to digitonin of hepatocytes metabolizing alanine was 2.0-2.4, and that in the soluble cytoplasm 6-19. In the presence of atractyloside, these values were 3.5-4.4 and 1.3-2.2, respectively.

Endothelial inflammation induced by excess glucose is associated with cytosolic glucose 6-phosphate but not increased mitochondrial respiration

AIMS/HYPOTHESIS

Exposure of endothelial cells to high glucose levels suppresses responses to insulin, including induction of endothelial nitric oxide synthase activity, through pro-inflammatory signalling via the inhibitor of nuclear factor kappaB (IkappaB)alpha-nuclear factor kappaB (NF-kappaB) pathway. In the current study, we aimed to identify metabolic responses to glucose excess that mediate endothelial cell inflammation and insulin resistance. Since endothelial cells decrease their oxygen consumption rate (OCR) in response to glucose, we hypothesised that increased mitochondrial function would not mediate these cells' response to excess substrate.

METHODS

The effects of glycolytic and mitochondrial fuels on metabolic intermediates and end-products of glycolytic and oxidative metabolism, including glucose 6-phosphate (G6P), lactate, CO(2), NAD(P)H and OCR, were measured in cultured human microvascular endothelial cells and correlated with IkappaBalpha phosphorylation.

RESULTS

In response to increases in glucose concentration from low to physiological levels (0-5 mmol/l), production of G6P, lactate, NAD(P)H and CO(2) each increased as expected, while OCR was sharply reduced. IkappaBalpha activation was detected at glucose concentrations >5 mmol/l, which was associated with parallel increases of G6P levels, whereas downstream metabolic pathways were insensitive to excess substrate.

CONCLUSIONS/INTERPRETATION

Phosphorylation of IkappaBalpha by excess glucose correlates with increased levels of the glycolytic intermediate G6P, but not with lactate generation or OCR, which are inhibited well below saturation levels at physiological glucose concentrations. These findings suggest that oxidative stress due to increased mitochondrial respiration is unlikely to mediate endothelial inflammation induced by excess glucose and suggests instead the involvement of G6P accumulation in the adverse effects of hyperglycaemia on endothelial cells.

Combined n-benzoyl-d-phenylalanine and metformin treatment reverses changes in the fatty acid composition of streptozotocin diabetic rats

The present investigation was carried out to evaluate the effect of N-benzoyl-D-phenylalanine (NBDP) and metformin on blood glucose, plasma insulin, and on the fatty acid composition of total lipids in the livers and kidneys of control and experimental diabetic rats. When compared with nondiabetic control rats, neonatal streptozotocin (nSTZ) diabetic rats showed a significant increase in blood glucose and decreased plasma insulin. Analysis of fatty acids revealed a significant increase in the concentration of palmitic, stearic, and oleic acids in liver and kidney, whereas linolenic and arachidonic acids were significantly decreased. In diabetic rats, the oral administration of combined NBDP/metformin for 6 wk decreased the high concentrations of palmitic, stearic, and oleic acids and elevated the low levels of linolenic and arachidonic acids. The results suggest that the NBDP/metformin combination exhibits both antidiabetic and antihyperlipidemic effects in nSTZ diabetic rats and prevents the fatty acid changes produced during diabetes.

Disruption of the coenzyme binding site and dimer interface revealed in the crystal structure of mitochondrial aldehyde dehydrogenase "Asian" variant

Mitochondrial aldehyde dehydrogenase (ALDH2) is the major enzyme that oxidizes ethanol-derived acetaldehyde. A nearly inactive form of the enzyme, ALDH2*2, is found in about 40% of the East Asian population. This variant enzyme is defined by a glutamate to lysine substitution at residue 487 located within the oligomerization domain. ALDH2*2 has an increased Km for its coenzyme, NAD+, and a decreased kcat, which lead to low activity in vivo. Here we report the 2.1 A crystal structure of ALDH2*2. The structure shows a large disordered region located at the dimer interface that includes much of the coenzyme binding cleft and a loop of residues that form the base of the active site. As a consequence of these structural changes, the variant enzyme exhibits rigid body rotations of its catalytic and coenzyme-binding domains relative to the oligomerization domain. These structural perturbations are the direct result of the inability of lysine 487 to form important stabilizing hydrogen bonds with arginines 264 and 475. Thus, the elevated Km for coenzyme exhibited by this variant probably reflects the energetic penalty for reestablishing this site for productive coenzyme binding, whereas the structural alterations near the active site are consistent with the lowered Vmax.

Changes of pentose phosphate pathway flux in vivo in Corynebacterium glutamicum during leucine-limited batch cultivation as determined from intracellular metabolite concentration measurements

Corynebacterium glutamicum is an important organism for the industrial production of amino acids such as lysine. In the present study time-dependent changes in the oxidative pentose phosphate pathway activity, an important site of NADPH regeneration in C. glutamicum, are investigated, whereby intracellular metabolite concentrations and specific enzyme activities in two isogenic leucine auxotrophic strains differing only in the regulation of their aspartate kinases were compared. After leucine limitation only the strain with a feedback-resistant aspartate kinase began to excrete lysine into the culture medium. Concomitantly, the intracellular NADPH to NADP concentration ratio increased from 2 to 4 in the non-producing strain, whereas it remained constant at about 1.2 in the lysine-producing strain. From these data the in'vivo flux through the pentose phosphate pathway was calculated. These results were used to approximate the total NADPH regeneration by glucose-6-phosphate dehydrogenase, 6-phosphogluconate dehydrogenase and isocitrate dehydrogenase, which agreed fairly well with the calculated demands for biomass formation and lysine biosynthesis. The analysis allowed to conclude that NADPH regeneration in the pentose phosphate pathway is essential for lysine biosynthesis in C. glutamicum.

Bimodal activation of acetyl-CoA carboxylase by glutamate

Acetyl-CoA carboxylase (ACC) catalyzes the formation of malonyl-CoA, an essential substrate for fatty acid biosynthesis and a potent inhibitor of fatty acid oxidation. Here, we provide evidence that glutamate may be a physiologically relevant activator of ACC. Glutamate induced the activation of both major isoforms of ACC, prepared from rat liver, heart, or white adipose tissue. In agreement with previous studies, a type 2A protein phosphatase contributed to the effects of glutamate on ACC. However, the protein phosphatase inhibitor microcystin LR did not abolish the effects of glutamate on ACC activity. Moreover, glutamate directly activated purified preparations of ACC when protein phosphatase activity was excluded. Phosphatase-independent ACC activation by glutamate was also reflected by polymerization of the enzyme as judged by size-exclusion chromatography. The sensitivity of ACC to direct activation by glutamate was diminished by treatment in vitro with AMP-activated protein kinase or cAMP-dependent protein kinase or by beta-adrenergic stimulation of intact adipose tissue. We conclude that glutamate, an abundant intracellular amino acid, induces ACC activation through complementary actions as a phosphatase activator and as a direct allosteric ligand for dephosphorylated ACC. This study supports the general hypothesis that amino acids fulfill important roles as signal molecules as well as intermediates in carbon and nitrogen metabolism.

Evidence of enhancement of malate-aspartate shuttle activity in beta cells of streptozotocin-induced non-insulin-dependent diabetic rats

Glucose-induced insulin secretion is selectively impaired in beta cells from animals with non-insulin-dependent diabetes mellitus (NIDDM). This study was performed to clarify whether the malate-aspartate shuttle among the glucose metabolic pathways is intact in beta cells of NIDDM rats. The insulin secretory capacity of the islets and the K(ATP) channel activity in single beta cells were measured in control and NIDDM rats injected with streptozotocin (STZ) during the neonatal period, using a radioimmunoassay and patch-clamp technique. The increase of insulin secretion induced by 11.1 mmol/L glucose or 10 mmol/L dihydroxyacetone (DHA) was significantly reduced in NIDDM islets, suggesting an impaired glycerol-phosphate shuttle. The application of glyceraldehyde (10 mmol/L) in NIDDM or control islets elicited an increase in insulin secretion, but the difference between the 2 groups was indistinguishable. On the contrary, the increase of insulin secretion and the inhibition of K(ATP) channel activity induced by aspartate, which preferentially participates in the malate-aspartate shuttle, were significantly greater in NIDDM versus the control. However, intracellularly applied aspartate in the inside-out mode did not inhibit K(ATP) channel activity. These findings show that malate-aspartate shuttle activity is potentiated in pancreatic beta cells of NIDDM rats, suggesting the development of a compensatory mechanism for the reduced activity of the glycerol-phosphate shuttle in NIDDM.

Change in the lipid profile, lipogenic and related enzymes in the livers of experimental diabetic rats: effect of insulin and vanadate

Administration of sodium orthovanadate to diabetic animals exhibits insulin-like effects and has been effective in the reversal of biochemical complications. This study evaluates the effect of sodium orthovanadate (0.6 mg/ml) treatment for 21 days on the hepatic glucose homeostasis and lipid metabolism in alloxan diabetic rats. The activities of two lipogenic enzymes, glucose-6-phosphate dehydrogenase and malic enzyme; and related enzymes, hexokinase and glucose-6-phosphatase were measured in the liver cytosolic fractions of diabetic rats and diabetic rats treated separately with insulin and sodium orthovanadate. The total lipids, triglycerides and cholesterol levels were estimated in the livers of the diabetic and the treated rats. The activities of both the lipogenic enzymes and hexokinase isozymes were significantly decreased, whereas the activity of glucose-6-phosphatase was significantly increased in the diabetic liver. During diabetes, the levels of total lipids and triglycerides increased significantly with a decrease in the cholesterol levels in the liver. Insulin and vanadate were able to restore the altered enzyme activities to almost control levels. Both insulin and vanadate were found to partially restore the altered levels of total lipids, triglycerides and cholesterol in the livers of diabetic rats. The results indicate that vanadate administration to diabetic animals normalizes blood glucose and causes marked improvement of altered lipid metabolism during diabetes. The present study and earlier reports suggest the possible use of vanadate as insulin replacement in the therapy of diabetes when administered at pharmacological doses.

Characterization of the transacylase activity of rat liver 60-kDa lysophospholipase-transacylase. Acyl transfer from the sn-2 to the sn-1 position

Rat liver 60-kDa lysophospholipase-transacylase catalyzes not only the hydrolysis of 1-acyl-sn-glycero-3-phosphocholine, but also the transfer of its acyl chain to a second molecule of 1-acyl-sn-glycero-3-phosphocholine to form phosphatidylcholine (H. Sugimoto, S. Yamashita, J. Biol. Chem. 269 (1994) 6252-6258). Here we report the detailed characterization of the transacylase activity of the enzyme. The enzyme mediated three types of acyl transfer between donor and acceptor lipids, transferring acyl residues from: (1) the sn-1 to -1(3); (2) sn-1 to -2; and (3) sn-2 to -1 positions. In the sn-1 to -1(3) transfer, the sn-1 acyl residue of 1-acyl-sn-glycero-3-phosphocholine was transferred to the sn-1(3) positions of glycerol and 2-acyl-sn-glycerol, producing 1(3)-acyl-sn-glycerol and 1,2-diacyl-sn-glycerol, respectively. In the sn-1 to -2 transfer, the sn-1 acyl residue of 1-acyl-sn-glycero-3-phosphocholine was transferred to not only the sn-2 positions of 1-acyl-sn-glycero-3-phosphocholine, but also 1-acyl-sn-glycero-3-phosphoethanolamine, producing phosphatidylcholine and phosphatidylethanolamine, respectively. 1-Acyl-sn-glycero-3-phospho-myo-inositol and 1-acyl-sn-glycero-3-phosphoserine were much less effectively transacylated by the enzyme. In the sn-2 to -1 transfer, the sn-2 acyl residue of 2-acyl-sn-glycero-3-phosphocholine was transferred to the sn-1 position of 2-acyl-sn-glycero-3-phosphocholine and 2-acyl-sn-glycero-3-phosphoethanolamine, producing phosphatidylcholine and phosphatidylethanolamine, respectively. Consistently, the enzyme hydrolyzed the sn-2 acyl residue from 2-acyl-sn-glycero-3-phosphocholine. By the sn-2 to -1 transfer activity, arachidonic acid was transferred from the sn-2 position of donor lipids to the sn-1 position of acceptor lipids, thus producing 1-arachidonoyl phosphatidylcholine. When 2-arachidonoyl-sn-glycero-3-phosphocholine was used as the sole substrate, diarachidonoyl phosphatidylcholine was synthesized at a rate of 0.23 micromol/min/mg protein. Thus, 60-kDa lysophospholipase-transacylase may play a role in the synthesis of 1-arachidonoyl phosphatidylcholine needed for important cell functions, such as anandamide synthesis.

Inhibition and uncoupling of oxidative phosphorylation by nonsteroidal anti-inflammatory drugs: study in mitochondria, submitochondrial particles, cells, and whole heart

The effects of the anti-inflammatory drugs diclofenac, piroxicam, indomethacin, naproxen, nabumetone, nimesulide, and meloxicam on mitochondrial respiration, ATP synthesis, and membrane potential were determined. Except for nabumetone and naproxen, the other drugs stimulated basal and uncoupled respiration, inhibited ATP synthesis, and collapsed membrane potential in mitochondria incubated in the presence of either glutamate + malate or succinate. Plots of membrane potential versus ATP synthesis (or respiration) showed proportional variations in both parameters, induced by different concentrations of nimesulide, meloxicam, piroxicam, or indomethacin, but not by diclofenac. The activity of the adenine nucleotide translocase was blocked by diclofenac and nimesulide; diclofenac also slightly inhibited mitochondrial ATPase activity. Naproxen did not affect any of the mitochondrial parameters measured. Nabumetone inhibited respiration, ATP synthesis, and membrane potential in the presence of glutamate + malate, but not with succinate. NADH oxidation in submitochondrial particles also was inhibited by nabumetone. Nabumetone inhibited O2 uptake in intact cells and in whole heart, whereas the other five drugs stimulated respiration. These observations revealed that in situ mitochondria are an accessible target. Except for diclofenac, a negative inotropic effect on cardiac contractility was induced by the drugs. The data indicated that nimesulide, meloxicam, piroxicam, and indomethacin behaved as mitochondrial uncouplers, whereas nabumetone exerted a specific inhibition of site 1 of the respiratory chain. Diclofenac was an uncoupler too, but it also affected the adenine nucleotide translocase and the H+-ATPase.

Functional evidence for a monocarboxylate transporter (MCT) in strial marginal cells and molecular evidence for MCT1 and MCT2 in stria vascularis

The transport of lactate, pyruvate and other monocarboxylates across plasma membranes of metabolically active cells such as strial marginal cells (SMC) may be important under aerobic conditions as well as under ischemic and hypoxic conditions. This study addresses the question whether SMC from the gerbil contain a membrane transport mechanism for monocarboxylates. The type of transporter was identified in functional studies by monitoring uptake of monocarboxylates into SMC through measurement of the cytosolic pH (pHi) with the pH-sensitive dye 2',7'-bis-(2-carboxyethyl)-5(6)-carboxyfluorescein (BCECF). Further, subtypes of the functionally identified transporter which are present in stria vascularis were identified as transcripts by cloning and sequencing the reverse-transcription polymerase chain reaction (RT-PCR) products. All functional experiments were performed under nominally HCO3--free conditions. The monocarboxylates acetate and pyruvate (each 20 mM) induced an acidification of pHi. In contrast, the dicarboxylate malonate (20 mM) had no significant effect on pHi. Alpha-cyano-4-hydroxycinnamate (CHC; 5 mM), a blocker of H+/monocarboxylate cotransporter (MCT), reduced reversibly the acidification induced by 5 mM pyruvate. In contrast, 1 microM DIDS, a blocker of band-3 protein, had no significant effect on the acidification induced by 20 mM acetate. The presence of the transcripts for each of the MCT subtypes, MCT1 and MCT2, was determined by RT-PCR of stria vascularis from gerbil. RT-PCR performed with primers for the MCT1 and MCT2 subtypes on total RNA from stria vascularis revealed PCR products of the predicted sizes. Sequence analysis confirmed that amplified MCT1 and MCT2 cDNA fragments encoded a nucleotide sequence of MCT1 and MCT2, respectively. These observations suggest that SMC contain a MCT and that stria vascularis contains RNA for the subtypes MCT1 and MCT2 subtypes.

Glycolytic pathway, redox state of NAD(P)-couples and energy metabolism in lens in galactose-fed rats: effect of an aldose reductase inhibitor

PURPOSE

The present study was aimed at evaluating early changes in glycolysis, the redox state of free cytosolic NAD(P)-couples, and the adenine nucleotide system in lens in both control and 50% galactose-fed rats, with the possibility of preventing these with an aldose reductase inhibitor (ARI).

METHODS

Experiments were performed on male Sprague-Dawley rats fed the galactose diet for 2-14 days. The levels of glucose, galactose, glycolytic intermediates, alpha-glycerophosphate, malate, NAD, ATP, ADP, AMP were assayed spectrofluorometrically in individual lenses by enzymatic procedures, while galactitol and myo-inositol were quantified by GC-MS. Free cytosolic NAD+/NADH, NADP+/NADPH, and ATP/ADP x P(i) (phosphate potential) were estimated from lactate dehydrogenase, malic enzyme, and triose phosphate isomerase-glyceraldehyde 3-phosphate dehydrogenase-3-phosphoglycerate kinase systems. Lactate and pyruvate production by lenses of both control and galactose-fed rats was measured in a set of in vitro incubation studies (2 hr, 37 degrees C, Krebs bicarbonate-Hepes buffer, pH 7.45, with 5mM glucose or 5mM glucose + 30 mM galactose, respectively).

RESULTS

Lens galactitol levels in 2, 4, 6, 8, 10, and 14-day galactose-fed rats were 48 +/- 8, 58 +/- 9, 68 +/- 8, 73 +/- 5, 81 +/- 20, and 75 +/- 11 mmol/g wet weight (mean +/- SD), respectively. NAD+/NADH ratios were indistinguishable from controls after 2-6 days on the galactose diet, but fell dramatically between 8 and 10 days, and did not correlate with polyol accumulation per se. The pattern of glycolytic intermediates (no change in G6P, F6P, and 3-PG, increase in GA3P, decrease in FDP, PEP, pyruvate, and lactate), as well as reduced in vitro lactate and pyruvate production, suggest inhibition of glycolysis at the sites of phosphofructokinase, glyceraldehyde 3-phosphate dehydrogenase, enolase, and pyruvate kinase. ATP levels as well as total ATP/ADP, ATP/ADP x Pi, adenylate charge, and cytosolic phosphate potential were decreased in galactose-fed rats, while galactose 1-phosphate and a-glycerophosphate levels as well as NADP+/NADPH ratio were increased. Lens galactitol levels were reduced approximately 57% in 10-day galactose-fed rats treated with the ARI (tolrestat, 100 mg/kg bwt/day, 6-day pretreatment); the changes in the lower segment of glycolysis, alpha-glycerophosphate levels, redox state of NAD-couples, and energy metabolism were partially prevented while NADP+/ NADPH ratios were unchanged and galactose 1-phosphate levels were further increased.

CONCLUSIONS

Depressed glycolysis in lens in galactose-fed rats is consistent with decreased NAD+/NADPH and adenine nucleotide phosphorylation. Early changes in lens glucose utilization, redox state of NAD-couples, and energy metabolism in this model of galactosemia are similar to those in diabetes, are at least in part mediated by aldose reductase involved mechanisms, and can be partially prevented by an aldose reductase inhibitor.

Uremia-induced disturbances in hepatic carbohydrate metabolism: enhancement by sucrose feeding

A high-sucrose (S) diet accentuates anorexia and stunts growth in uremic (U) rats, and an oral S load induces a greater hyperfructosemia in U rats than in control (C) rats. Four studies were performed to determine the roles of S feeding and an acute S load on liver carbohydrate (CHO) metabolism in U and C rats (eight to 10 rats per group). We also examined the plasma responses to either water or a S load. Levels of the main metabolites of glycolysis, gluconeogenesis, and glycogenesis were measured under basal conditions (7 hours' postmeal) in U and C rats fed either a cornstarch diet (study I) or S diet (study II) and at 30 and 60 minutes after an intragastric S load (studies III and IV) in s-fed U and C rats. The weight gain, food intake, and plasma creatinine and urea levels of the rats in the four studies were comparable. Weight gain and liver weight (g/100 g body weight) were lower in U than in C rats. In the plasma, baseline levels of lactate were decreased by uremia and S feeding and those of glucose (G) were increased by S feeding. The increases in plasma G and fructose (F) levels after a S load were greater in U rats than in C rats, whereas those of plasma lactate were comparable. In the liver under basal conditions, uremia markedly decreased levels of glycogen, F-1,6-diphosphate (F-1,6-diP), F-2,6-diP, 3-glycero-phosphate (3-glycero-P), dihydroxyacetone phosphate (DHAP), pyruvate, lactate, and adenosine triphosphate (ATP), and the phosphorylation state (ATP/adenosine diphosphate [ADP] x inorganic phosphorus [PI]), increased phosphoenolpyruvate (PEP), ADP, and Pi levels, but did not affect the cytosolic redox state (pyruvate/lactate). In addition to uremia, S feeding further decreased levels of glycogen, F-2,6-diP, 3-glycero-P, and ATP. After S loading, liver F levels increased more in U than in C rats, but glycogen and 3-glycero-P levels increased less in U than in C rats. Liver lactate and pyruvate levels increased more in U than in C rats, and the pyruvate/lactate and DHAP/3-glycero-P ratios were higher in U than in C rats after a S load. The ATP level and the phosphorylation state in U rats increased 30 minutes later in U than in C rats. Our findings indicate that uremia causes a depletion in liver glycogen, which is enhanced by S feeding and could be partially attributed to decreased glycogen synthesis.(ABSTRACT TRUNCATED AT 400 WORDS)

The guanine nucleotide-exchange factor, eIF-2B

Eukaryotic initiation factor eIF-2B catalyses the exchange of guanine nucleotides on another translation initiation factor, eIF-2, which itself mediates the binding of the initiator Met-tRNA to the 40S ribosomal subunit during translation initiation. eIF-2B promotes the release of GDP from inactive [eIF-2.GDP] complexes, thus allowing formation of the active [eIF-2.GTP] species which subsequently binds the Met-tRNA. This guanine nucleotide-exchange step, and thus eIF-2B activity, are known to be an important control point for translation initiation. The activity of eIF-2B can be modulated in several ways. The best characterised of these involves the phosphorylation of the alpha-subunit of eIF-2 by specific protein kinases regulated by particular ligands. Phosphorylation of eIF-2 alpha leads to inhibition of eIF-2B. This mechanism is involved in the control of translation under a variety of conditions, including amino acid deprivation in yeast (Saccharomyces cerevisiae) where it causes translational upregulation of the transcription factor GCN4, and in virus-infected animal cells, where it involves a protein kinase activated by double-stranded RNA. There is now also growing evidence for direct regulation of eIF-2B. This appears likely to involve the phosphorylation of its largest subunit. Under certain circumstances eIF-2B may also be regulated by allosteric mechanisms. eIF-2B is a heteropentamer (subunits termed alpha, beta, gamma, delta and epsilon) and is thus more complex than most other guanine nucleotide-exchange factors. The genes encoding all five subunits have been cloned in yeast (exploiting the GCN4 regulatory system): all but the alpha appear to be essential for eIF-2B activity. However, this subunit may confer sensitivity to eIF-2 alpha phosphorylation. cDNAs encoding the alpha, beta, delta and epsilon subunits have been cloned from mammalian sources. There is substantial homology between the yeast and mammalian sequences. Attention is now directed towards understanding the roles of individual subunits in the function and regulation of eIF-2B.

Effects of albumin and adenosine phosphates on iron transfer from ferric lactate

Ferric lactate is known to modify Ca2+ uptake by the cells. To enlighten the role of protein and ATP in this phenomenon, iron transfer from ferric lactate to albumin and adenosine polyphosphates was determined by electrophoresis. The order of iron affinity was ATP > ADP > AMP for the polyphosphates, and albumin does not compete for iron binding with the polyphosphates. The iron transfer to ATP was also observed in vivo by adsorption chromatography of the adenosine polyphosphates fraction from blood plasma of mice injected with ferric lactate plus ATP. In vitro iron and calcium uptake by Ehrlich ascites tumor cells showed that albumin and ATP decreased iron uptake, whereas calcium incorporation is diminished by albumin but augmented by ATP. This difference might be explained by albumin binding of ferric lactate that is inhibited from reaching cell structures, whereas ATP, known to be an inhibitor of iron polymerization, facilitates it.

Derivatives of cinnamic acid interact with the nucleotide binding site of mitochondrial aldehyde dehydrogenase. Effects on the dehydrogenase reaction and stimulation of esterase activity by nucleotides

A wide variety of cinnamic acid derivatives are inhibitors of the low Km mitochondrial aldehyde dehydrogenase. Two of the most potent inhibitors are alpha-cyano-3,4-dihydroxythiocinnamamide (Ki0.6 microM) and alpha-cyano-3,4,5-trihydroxycinnamonitrile (Ki2.6 microM). With propionaldehyde as substrate the inhibition by these compounds was competitive with respect to NAD+. alpha-Fluorocinnamate was a much less effective inhibitor of the enzyme, with mixed behaviour towards NAD+, but with a major competitive component. These cinnamic acid derivatives were ineffective as inhibitors of the aldehyde dehydrogenase-catalysed hydrolysis of p-nitrophenyl acetate, but inhibited the ability of NAD+ and NADH to activate this activity. Inhibition of the stimulation of esterase activity was competitive with respect to NAD+ and NADH, and the derived Ki values were the same as for inhibition of dehydrogenase activity. NAD+, but not acetaldehyde, could elute the low Km aldehyde dehydrogenase from alpha-cyanocinnamate-Sepharose, to which the enzyme binds specifically (Poole RC and Halestrap AP, Biochem J 259: 105-110, 1989). The cinnamic acid derivatives have little effect on lactate dehydrogenase, glyceraldehyde-3-phosphate dehydrogenase or a high Km aldehyde dehydrogenase present in rat liver mitochondria. It is concluded that some cinnamic acid derivatives are potent inhibitors of the low Km aldehyde dehydrogenase, by competing with NAD+/NADH for binding to the enzyme. They are much less effective as inhibitors of other NAD(+)-dependent dehydrogenases.

Concentrations of D-lactate and its related metabolic intermediates in liver, blood, and muscle of diabetic and starved rats

This is a report investigating the methylglyoxal (MG) bypass in animals, by which D-lactate is produced from triosephosphate via MG. Rats were made diabetic using streptozotocin or starved for 72 h. D-Lactate and various metabolites related to it, such as L-lactate, pyruvate, methylglyoxal, glucose, and inorganic phosphate, were measured in the blood plasma, liver, and skeletal muscle of the rats. Diabetic and starved rats had significantly higher levels of D-lactate in plasma, liver, and skeletal muscle compared with the control group. In contrast, pyruvate levels in plasma, liver, and skeletal muscle was markedly lower than normal in diabetic and starved rats. L-Lactate level lowered markedly in plasma, liver, and skeletal muscle of starved rats and elevated in liver of diabetic rats. Differences between plasma L-lactate level for diabetes and control were not significant. MG level was significantly elevated in plasma and depressed in livers and muscles of starved rats as well as livers of diabetic rats. Hepatic glycerol content was markedly increased in those states. Enzyme activities related to D- and L-lactate, such as pyruvate kinase, phosphofructokinase, aldolase, and glyoxalase I, were measured in the livers of these rats. Pyruvate kinase activity decreased in these states, but other enzyme activities showed no significant changes. D-Lactate was much more excreted than L-lactate in the urine of diabetic and fasted rats compared with normal rats.

Regulation of the malic enzyme and fatty acid synthase genes in chick embryo hepatocytes in culture: corticosterone and carnitine regulate responsiveness to triiodothyronine

Triiodothyronine (T3) added to chick embryo hepatocytes between 20 and 68 h of culture caused a 30- to 40-fold increase in malic enzyme activity. This T3 response decreased as a function of time; after 1 week in culture, a 48-h incubation with T3 had no effect on hepatocyte malic enzyme activity. Neither corticosterone nor carnitine had a significant effect on malic enzyme activity in the absence of T3 at any time or on the response of malic enzyme to T3 during the first 68 h of culture; both stimulated responsiveness to T3 subsequent to 68 h. The effects of corticosterone and carnitine on malic enzyme activity were additive, suggesting different mechanisms. Corticosterone and carnitine regulated abundance of malic enzyme mRNA. For corticosterone, at least, this effect was due to regulation of transcription. Abundance of fatty acid synthase mRNA was also stimulated by T3 in chick embryo hepatocytes in culture, and its responsiveness to T3 decreased with time. Corticosterone and carnitine stimulated responsiveness to T3 at times subsequent to 68 h. Corticosterone had no effect on binding of T3 to nuclear receptors. Intracellular accumulation of long-chain fatty acids or long-chain acyl-CoAs probably did not cause the loss of responsiveness to T3 or the stimulation of that responsiveness by corticosterone or carnitine because adding serum albumin (0.5%) or long-chain fatty acids (0.25-0.5 mM) to the medium was without effect. Corticosterone and carnitine may control the levels of other metabolic intermediates or protein factors which, in turn, regulate the transcriptional response of the lipogenic genes to T3.

Iron transfer from urate-Fe(III) to citrate and ATP

1. Urate, citrate and ATP, which form stable complexes with ferric ions, are proposed to function as low mol. wt iron binding agents in humans. 2. Citrate and ATP were found to readily take up iron from the urate-Fe(III) complex; the study suggests that citrate and ATP may be physiologically more important iron binding agents than urate.

Acceleration of purine synthesis in mouse liver by glycogenolytic hormones

Administration (ip) into fed mice of glucagon, epinephrine, vasopressin, oxytocin, angiotensin II, and dibutyryl cyclic AMP (dbcAMP) resulted in a rapid (within 2.5 to 15 min) elevation of PRPP content (two- to threefold) and in acceleration of the rate of de novo purine synthesis (twofold). Inhibition of the epinephrine-stimulated glycogenolysis by 2,5-anhydromannitol diminished markedly the acceleration effect of the hormone on the rate of purine synthesis. Administration of the hormones caused a rapid rise in the liver content of glucose 6-phosphate (G6P) by 15-70% but did not increase the ribose 5-phosphate (R5P) content. Liver ATP content was not affected. The hormones did not cause direct activation of PRPP synthetase, as gauged by the specific activity of the enzyme, its Km for substrates R5P and ATP, and its sensitivity to inhibition by ADP and GDP. The hormones did not increase the liver content of the enzyme activators Pi and Mg2+. The results suggest that the glycogenolytic hormones accelerate purine synthesis by a metabolic mechanism associated with the enhancement of glycogenolysis. PRPP synthesis is probably enhanced by the glycogenolysis-induced alterations in the cellular content of some metabolites other than R5P.

A computer model of pancreatic islet glycolysis

We have modeled an experiment with perifused pancreatic islet cells using our BIOSSIM language. The experiment and the resulting model are concerned with glucose uptake and glycolysis by the beta-cells of pancreatic islets. Although glycolysis appears to be involved in insulin release, we do not have enough information to represent insulin release in detail. The rapid entry of glucose into the beta-cell is promoted by a carrier having a very high tissue capacity. Phosphorylation of glucose by the low affinity enzyme glucokinase appears to be limiting for glycolysis. The effects of several hexose diphosphate activators of phosphofructokinase are modeled. Model behavior is described. The kinetic parameters of the enzyme submodels are given. Because of the difficulties of preparing large amounts of experimental material, information on pancreatic islet metabolism is limited. This model is a plausible explanation of the experimental results. Recent work on the genetically engineered glucose transporter and glucokinase is discussed.

Time course changes in glycogen accretion, 6-phosphogluconate, fructose-2,6-bisphosphate, and lipogenesis upon refeeding a high sucrose diet to starved rats

1. Starved rats refed 60% sucrose diets were used to determine in vivo lipogenesis and levels of hepatic metabolites. 2. Fatty acid synthesis increased 11-fold 4 hr after refeeding. 3. Glycogen rose from 3 to 100 mg/g liver after 8 hr. 4. Fructose-2,6-bisphosphate rose to 6 nmol/g at 1 hr and remained constant. 5. 6-Phosphogluconate increased from 10 to 45 nmol/g liver after 2 hr and remained constant.

Interstrain differences in organization of metabolic processes in the rat liver--I. The dynamics of changes in the contents of adenine nucleotides, glycogen and fatty acyl-CoAs in the course of short-term starvation in the livers of rats of Wistar, August and WAG strains

1. The metabolic patterns in the livers of rats of the Wistar, August and Wag strains were evaluated 4, 8, 12 and 24 hr after food withdrawal. 2. In the fed state (4 hr) there were large differences in the liver's contents of ATP, phosphate potential values, glycogen contents and blood FFA. These distinctions disappeared in the fasted state (12 hr). 3. There are large differences between the strains in the dynamics of transition of the liver metabolic patterns from the fed to the starved states. 4. The results obtained show that the three strains of the laboratory animals strongly differ in the organization of the liver energy metabolism.

Substrate and inhibitor specificity of monocarboxylate transport into heart cells and erythrocytes. Further evidence for the existence of two distinct carriers

A range of short-chain aliphatic monocarboxylates, both unsubstituted and substituted with hydroxy, chloro and keto groups, were shown to inhibit transport of L-lactate and pyruvate into both guinea-pig cardiac myocytes and rat erythrocytes. The carrier of heart cells exhibited a higher affinity (approx. 10-fold) for most of the monocarboxylates than did the erythrocyte carrier. A notable exception was L-lactate, whose Km for both carriers was similar. The K1 values of the two carriers for inhibitors such as phenylpyruvate and alpha-cyanocinnamate derivatives were also different. The high affinity of the heart cell carrier for ketone bodies and acetate may be physiologically important, since these substrates are used as fuels by the heart.