Impaired glucose-1,6-biphosphate production due to bi-allelic PGM2L1 mutations is associated with a neurodevelopmental disorder

We describe a genetic syndrome due to PGM2L1 deficiency. PGM2 and PGM2L1 make hexose-bisphosphates, like glucose-1,6-bisphosphate, which are indispensable cofactors for sugar phosphomutases. These enzymes form the hexose-1-phosphates crucial for NDP-sugars synthesis and ensuing glycosylation reactions. While PGM2 has a wide tissue distribution, PGM2L1 is highly expressed in the brain, accounting for the elevated concentrations of glucose-1,6-bisphosphate found there. Four individuals (three females and one male aged between 2 and 7.5 years) with bi-allelic inactivating mutations of PGM2L1 were identified by exome sequencing. All four had severe developmental and speech delay, dysmorphic facial features, ear anomalies, high arched palate, strabismus, hypotonia, and keratosis pilaris. Early obesity and seizures were present in three individuals. Analysis of the children's fibroblasts showed that glucose-1,6-bisphosphate and other sugar bisphosphates were markedly reduced but still present at concentrations able to stimulate phosphomutases maximally. Hence, the concentrations of NDP-sugars and glycosylation of the heavily glycosylated protein LAMP2 were normal. Consistent with this, serum transferrin was normally glycosylated in affected individuals. PGM2L1 deficiency does not appear to be a glycosylation defect, but the clinical features observed in this neurodevelopmental disorder point toward an important but still unknown role of glucose-1,6-bisphosphate or other sugar bisphosphates in brain metabolism.

Rational design of substrate binding pockets in polyphosphate kinase for use in cost-effective ATP-dependent cascade reactions

Adenosine-5'-triphosphate (ATP) is the energy equivalent of the living system. Polyphosphate (polyP) is the ancient energy storage equivalent of organisms. Polyphosphate kinases (PPKs) catalyze the polyP formation or ATP formation, to store energy or to regenerate ATP, respectively. However, most PPKs are active only in the presence of long polyPs, which are more difficult and more expensive to generate than the short polyPs. We investigated the PPK preference towards polyPs by site-directed mutagenesis and computational simulation, to understand the mechanism and further design enzymes for effective ATP regeneration using short polyPs for in vitro cascade reactions, which are highly desired for research and applications. The results suggest that the short polyPs inhibit PPK by blocking the ADP-binding pocket. Structural comparison between PPK (Corynebacterium glutamicum) and PPK (Sinorhizobium meliloti) indicates that three amino acid residues, i.e., lysine, glutamate, and threonine, are involved in the activity towards short polyP by fixing the adenosine group of ADP in between the subunits of the dimer, while the terminal phosphate group of ADP still offers an active site, which presents a binding pocket for ADP. A proposed triple mutant PPK (SMc02148-KET) demonstrates significant activity towards short polyP to form ATP from ADP. The obtained high glutathione titer (38.79 mM) and glucose-6-phosphate titer (87.35 mM) in cascade reactions with ATP regeneration using the triple mutant PPK (SMc02148-KET) reveal that the tailored PPK establishes the effective ATP regeneration system for ATP-dependent reactions.

In response to 'Can sugars be produced from fatty acids? A test case for pathway analysis tools'

MOTIVATION

In their article entitled 'Can sugars be produced from fatty acids? A test case for pathway analysis tools' de Figueiredo and co-authors assess the performance of three pathway prediction tools (METATOOL, PathFinding and Pathway Hunter Tool) using the synthesis of glucose-6-phosphate (G6P) from acetyl-CoA in humans as a test case. We think that this article is biased for three reasons: (i) the metabolic networks used as input for the respective tools were of very different sizes; (ii) the 'assessment' is restricted to two study cases; (iii) developers are inherently more skilled to use their own tools than those developed by other people. We extended the analyses led by de Figueiredo and clearly show that the apparent superior performance of their tool (METATOOL) is partly due to the differences in input network sizes. We also see a conceptual problem in the comparison of tools that serve different purposes. In our opinion, metabolic path finding and elementary mode analysis are answering different biological questions, and should be considered as complementary rather than competitive approaches.

SUPPLEMENTARY INFORMATION

Supplementary data are available at Bioinformatics online.

[Participation of adenylate kinase in the regulation of chloroplasts energy exchange]

The influence of adenylate kinase on the rates of glucose-6-phosphate synthesis and ferricyanide reduction in a system containing chloroplasts, hexokinase, and ADP at low concentration during photophosphorylation has been studied. It has been found that the addition of adenylate kinase into the reaction medium under phosphorylation results in a simultaneous increase in the rate of ferricyanide reduction and glucose-6-phosphate synthesis. In this case, the ratio of glucose-6-phosphate formed to the quantity of ferricyanide reduced was close to unity as the concentration of adenylate kinase in the medium increased. The concentrations of glucose-6-phosphate and ferricyanide reduced in the system sharply increased with time; at the same time, no significant decrease in ADP concentration and AMP accumulation by the methods available was found. Hence, the limiting factors in these reactions are not the concentrations but the rates of diffusion of the substrates. Presumably, diffusion limitations in the system are eliminated owing to the participation of adenylate kinase. The results obtained are discussed in terms of the model according to which the regulation of the diffusion of adenine nucleotides and the control of regeneration of ATP according to its requirements in correlation with other regulation mechanisms can occur in chloroplasts upon adenylate kinase functioning by direct and reverse connection of the shuttle type.

Hepatocyte-like cells from directed differentiation of mouse bone marrow cells in vitro

AIM

To design the effective directed differentiation medium to differentiate bone marrow cells into hepatocyte-like cells.

METHODS

Bone marrow cells were cultured in the directed differentiation media including fibroblast growth factor-4 (FGF-4) and oncostatin M (OSM). Hepatocyte-like cells from directed differentiation of bone marrow cells were identified through cell morphology, RNA expressions by reverse transcriptase-polymerase chain reaction (RT-PCR), protein expressions by Western blot, and hepatocellular synthesis and metabolism functions by albumin ELISA, Periodic acid-Shiff staining and urea assay.

RESULTS

Some epithelial-like cells or polygonal cells appeared and increased in the course of the cell directed differentiation. Hepatocyte nucleur factor-3beta (HNF-3beta, albumin (ALB), cytokeratin 18 (CK18), transthyretin (TTR), glucose-6-phosphate (G-6-Pase), and tyrosine aminotransferase (TAT) mRNA were expressed in the course of the directed differentiation. The directed differentiated cells on d 21 expressed HNF-3? ALB, and CK18 proteins. The directed differentiated cells produced albumin and synthesized urea in a time-dependent manner. They could also synthesize glycogen.

CONCLUSION

Our differentiation media, including FGF-4 and OSM, are effective to differentiate bone marrow cells into hepatocyte-like cells, which could be used for hepatocyte resources for bioartificial liver or hepatocyte transplantation.

Enhanced glucose cycling and suppressed de novo synthesis of glucose-6-phosphate result in a net unchanged hepatic glucose output in ob/ob mice

AIMS/HYPOTHESIS

Leptin-deficient ob/ob mice are hyperinsulinaemic and hyperglycaemic; however, the cause of hyperglycaemia remains largely unknown.

METHODS

Glucose metabolism in vivo in 9-h fasted ob/ob mice and lean littermates was studied by infusing [U-(13)C]-glucose, [2-(13)C]-glycerol, [1-(2)H]-galactose and paracetamol for 6 h, applying mass isotopomer distribution analysis on blood glucose and urinary paracetamol-glucuronide.

RESULTS

When expressed on the basis of body weight, endogenous glucose production (109+/-23 vs 152+/-27 micromol.kg(-1).min(-1), obese versus lean mice, p<0.01) and de novo synthesis of glucose-6-phosphate (122+/-13 vs 160+/-6 micromol.kg(-1).min(-1), obese versus lean mice, p<0.001) were lower in ob/ob mice than in lean littermates. In contrast, glucose cycling was greatly increased in obese mice (56+/-13 vs 26+/-4 micromol.kg(-1).min(-1), obese versus lean mice, p<0.001). As a result, total hepatic glucose output remained unaffected (165+/-31 vs 178+/-28 micromol.kg(-1).min(-1), obese vs lean mice, NS). The metabolic clearance rate of glucose was significantly lower in obese mice (8+/-2 vs 18+/-2 ml.kg(-1).min(-1), obese versus lean mice, p<0.001). Hepatic mRNA levels of genes encoding for glucokinase and pyruvate kinase were markedly increased in ob/ob mice.

CONCLUSIONS/INTERPRETATION

Unaffected total hepatic glucose output in the presence of hyperinsulinaemia reflects hepatic insulin resistance in ob/ob mice, which is associated with markedly increased rates of glucose cycling. Hyperglycaemia in ob/ob mice primarily results from a decreased metabolic clearance rate of glucose.

Molecular and biochemical characterization of a fructose-6-phosphate-forming and ATP-dependent fructokinase of the hyperthermophilic archaeon Thermococcus litoralis

Close to an operon encoding an ABC transporter for maltose and trehalose, Thermococcus litoralis contains a gene whose encoded sequence showed similarity to sugar kinases. We cloned this gene, now called frk, and expressed it as a C-terminal His-tag version in Escherichia coli. We purified the recombinant protein, identified it as an ATP-dependent and fructose-6-phosphate-forming fructokinase (Frk) and determined its biochemical properties. At its optimal temperature of 80 degrees C, the apparent Km and Vmax values of Frk were 2.3 mM and 730 U/mg protein for fructose at saturating ATP concentration, and 0.81 mM and 920 U/mg protein for ATP at saturating fructose concentration. The enzyme did not lose activity at 80 degrees C for 4 h. Under denaturating conditions in SDS-PAGE, it exhibited a molecular mass of 35 kDa. Gel-filtration chromatography revealed a molecular mass of 58 kDa, indicating a dimer under nondenaturating, in vitro conditions.

Hepatic de novo synthesis of glucose 6-phosphate is not affected in peroxisome proliferator-activated receptor alpha-deficient mice but is preferentially directed toward hepatic glycogen stores after a short term fast

Apart from impaired beta-oxidation, Pparalpha-deficient (Pparalpha(-/-)) mice suffer from hypoglycemia during prolonged fasting, suggesting alterations in hepatic glucose metabolism. We compared hepatic glucose metabolism in vivo in wild type (WT) and Pparalpha(-/-) mice after a short term fast, applying novel isotopic methods. After a 9-h fast, mice were infused with [U-(13)C]glucose, [2-(13)C]glycerol, [1-(2)H]galactose, and paracetamol for 6 h, and blood and urine was collected in timed intervals. Plasma glucose concentrations remained constant and were not different between the groups. Hepatic glycogen content was 69 +/- 11 and 90 +/- 31 microM/g liver after 15 h of fasting in WT and Pparalpha(-/-) mice, respectively. The gluconeogenic flux toward glucose 6-phosphate was not different between the groups (i.e. 157 +/- 9 and 153 +/- 9 microM/kg/min in WT and Pparalpha(-/-) mice, respectively). The gluconeogenic flux toward plasma glucose, however, was decreased in PPARalpha(-/-) mice (i.e. 142 +/- 9 versus 124 +/- 13 microM/kg/min) (p < 0.05), accounting for the observed decrease (-15%) in hepatic glucose production in Pparalpha(-/-) mice. Expression of the gene encoding glucose-6-phosphate hydrolase (G6ph) was lower in the PPARalpha(-/-) mice compared with WT mice. In conclusion, Pparalpha(-/-) mice were able to maintain a normal total gluconeogenic flux to glucose 6-phosphate during moderate fasting, despite their inability to up-regulate beta-oxidation. However, this gluconeogenic flux was directed more toward glycogen, leading to a decreased hepatic glucose output. This was associated with a down-regulation of the expression of G6ph in PPARalpha-deficient mice.

Quantification of hepatic carbohydrate metabolism in conscious mice using serial blood and urine spots

In vivo studies of hepatic carbohydrate metabolism in (genetically modified) conscious mice are hampered by limitations of blood and urine sample sizes. We developed and validated methods to quantify stable isotope dilution and incorporation in small blood and urine samples spotted onto filter paper. Blood glucose and urinary paracetamol-glucuronic acid were extracted from filter paper spots reproducibly and with high yield. Fractional isotopomer distributions of glucose and paracetamol-glucuronic acid when extracted from filter paper spots were almost identical to those isolated from the original body fluids. Rates of infusion of labeled compounds could be adjusted without perturbing hepatic glucose metabolism. This approach was used in mice to find the optimal metabolic condition for the study of hepatic carbohydrate metabolism. In fed mice, no isotopic steady state was observed during a 6-h label-infusion experiment. In 9-h-fasted mice, isotopic steady state was reached after 3 h of label infusion and important parameters in hepatic glucose metabolism could be calculated. The rate of de novo glucose-6-phosphate synthesis was 143 +/- 17 micromol kg(-1) min(-1) and partitioning to plasma glucose was 79.0 +/- 5.2%. In 24-h-fasted mice, abrupt changes were noticed in whole body and in hepatic glucose metabolism at the end of the experiment.

Sucrose synthase catalyzes the de novo production of ADPglucose linked to starch biosynthesis in heterotrophic tissues of plants

By using barley seeds, developmental changes of ADPglucose (ADPG)-producing sucrose synthase (SS) and ADPG pyrophosphorylase (AGPase) have been compared with those of UDPglucose (UDPG), ADPG, sucrose (Suc) and starch contents. Both ADPG-synthesizing SS and AGPase activity patterns were found to correlate well with those of ADPG and starch contents. Remarkably, however, maximal activities of ADPG-synthesizing SS were found to be several fold higher than those of AGPase throughout seed development, the highest rate of starch accumulation being well accounted for by SS. Kinetic analyses of SS from barley endosperms and potato tubers in the Suc cleavage direction showed similar K(m) values for ADP and UDP, whereas apparent affinity for Suc was shown to be higher in the presence of UDP than with ADP. Moreover, measurements of transglucosylation activities in starch granules incubated with purified SS, ADP and [U-(14)C]Suc revealed a low inhibitory effect of UDP. The ADPG and UDPG contents in the transgenic S-112 SS and starch deficient potato mutant [Zrenner et al. (1995) Plant J. 7: 97] were found to be 35% and 30% of those measured in wild-type plants, whereas both glucose-1-phosphate and glucose-6-phosphate contents were found to be normal as compared with those of wild-type plants. The overall results thus strongly support a novel gluconeogenic mechanism reported previously [Pozueta-Romero et al. (1999) CRIT: Rev. Plant Sci. 18: 489] wherein SS catalyses directly the de novo production of ADPG linked to starch biosynthesis in heterotrophic tissues of plants.

Glucose 6-phosphate produced by gluconeogenesis and by glucokinase is equally effective in activating hepatic glycogen synthase

Glucose 6-phosphate (Glc-6-P) produced in cultured hepatocytes by direct phosphorylation of glucose or by gluconeogenesis from dihydroxyacetone (DHA) was equally effective in activating glycogen synthase (GS). However, glycogen accumulation was higher in hepatocytes incubated with glucose than in those treated with DHA. This difference was attributed to decreased futile cycling through GS and glycogen phosphorylase (GP) in the glucose-treated hepatocytes, owing to the partial inactivation of GP induced by glucose. Our results indicate that the gluconeogenic pathway and the glucokinase-mediated phosphorylation of glucose deliver their common product to the same Glc-6-P pool, which is accessible to liver GS. As observed in the treatment with glucose, incubation of cultured hepatocytes with DHA caused the translocation of GS from a uniform cytoplasmic distribution to the hepatocyte periphery and a similar pattern of glycogen deposition. We hypothesize that Glc-6-P has a major role in glycogen metabolism not only by determining the activation state of GS but also by controlling its subcellular distribution in the hepatocyte.

Regulation of sucrose and starch synthesis in wheat (Triticum aestivum L.) leaves: role of fructose 2,6-bisphosphate

Fructose 2,6-bisphosphate (F26BP) is a competitive inhibitor of the cytosolic fructose 1,6-bisphosphatase (cytFBPase, EC 3.1.3.11). In spinach (Spinacia oleracea L.) leaves it is a significant component of the complex regulatory network that co-ordinates rates of photosynthesis, sucrose synthesis and starch synthesis. However the role of F26BP has only been studied in plants that predominantly store starch in their leaves and its role in other species is not clear. This paper examines the significance of F26BP in the regulation of photosynthetic carbon metabolism in the intact leaves of wheat (Triticum aestivum L.), a plant that accumulates predominantly sucrose. The approach taken was to vary rates of photosynthesis and then correlate measurements of F26BP and a range of other metabolites with rates of carbohydrate synthesis obtained from (14)CO(2)-feeding experiments performed under physiological conditions. It was found that: (i) Amounts of 3-phosphoglycerate and fructose-6-phosphate are correlated with the amount of F26BP. (ii) F26BP is involved in inhibiting cytFBPase at low light and low CO(2), but other factors, for example triose-phosphate, must also be involved. (iii) Amounts of both F26BP and substrate are involved in co-ordinating rates of photosynthesis and sucrose synthesis, but the relative importance of these depends on the conditions. (iv) Amounts of F26BP do not correlate with the partitioning of fixed carbon between sucrose and starch. Together these data suggest that the amount of F26BP in wheat is regulated by mechanisms similar to those in spinach, and that the metabolite is one of the factors involved in co-ordinating sucrose synthesis and photosynthesis. However F26BP does not appear to be involved in regulating the partitioning of fixed carbon between sucrose and starch in wheat under the experimental conditions examined.

Liver glycogen synthase but not the muscle isoform differentiates between glucose 6-phosphate produced by glucokinase or hexokinase

Using adenovirus-mediated gene transfer into FTO-2B cells, a rat hepatoma cell line, we have overexpressed hexokinase I (HK I), glucokinase (GK), liver glycogen synthase (LGS), muscle glycogen synthase (MGS), and combinations of each of the two glucose-phosphorylating enzymes with each one of the GS isoforms. FTO-2B cells do not synthesize glycogen even when incubated with high doses of glucose. Adenovirus-induced overexpression of HK I and/or LGS, two enzymes endogenously expressed by these cells, did not produce a significant increase in the levels of active GS and the total glycogen content. In contrast, GK overexpression led to the glucose-dependent activation of endogenous or overexpressed LGS and to the accumulation of glycogen. Similarly overexpressed MGS was efficiently activated by the glucose-6-phosphate (Glc-6-P) produced by either endogenous or overexpressed HK I and by overexpressed GK. These results indicate the existence of at least two pools of Glc-6-P in the cell, one of them is accessible to both isoforms of GS and is replenished by the action of GK, whereas LGS is excluded from the cellular compartment where the Glc-6-P produced by HK I is directed. These findings are interpreted in terms of the metabolic role that the two pairs of enzymes, HK I-MGS in the muscle and GK-LGS in the hepatocyte, perform in their respective tissues.

A radiometric assay for glutamine:fructose-6-phosphate amidotransferase

Glutamine:fructose-6-phosphate amidotransferase (GFAT) catalyzes the first step in the biosynthesis of amino sugars by transferring the amino group from l-glutamine to the acceptor substrate, fructose 6-phosphate, generating the products glucosamine 6-phosphate and glutamic acid. We describe a method for the synthesis and purification of the substrate, fructose 6-phosphate, and methods for a radiometric assay of human GFAT1 that can be performed in either of two formats: a small disposable-column format and a high-throughput 96-well-plate format. The method performed in the column format can detect 1 pmol of glucosamine 6-phosphate, much less than that required by previously published assays that measure GlcN 6-phosphate. The column assay demonstrates a broad linear range with low variability. In both formats, the assay is linear with time and enzyme concentration and is highly reproducible. This method greatly improves the sensitivity and speed with which GFAT1 activity can be measured and facilitates direct kinetic measurement of the transferase activity.

From Lobry de Bruyn to enzyme-catalyzed ammonia channelling: molecular studies of D-glucosamine-6P synthase

This review summarizes the state of knowledge on D-glucosamine-6P synthesis catalyzed by glucosamine-6P synthase. The mechanisms of L-glutamine hydrolysis, ammonia transfer and fructose-6P conversion into D-glucosamine-6P are analyzed with the E. coli enzyme in light of recent X-ray structures. With 92 references this paper covers the literature up to June 2001 and emphasizes the potential implication of the mammalian glucosamine-6P synthase in type 2 diabetes.

Presence of glutamine:fructose-6-phosphate amidotransferase for glucosamine-6-phosphate synthesis in endothelial cells: effects of hyperglycaemia and glutamine

AIMS/HYPOTHESIS

Recent studies show that glucosamine infusion impairs endothelium-dependent blood flow in normoglycaemic rats. The pathophysiological relevance of this finding, however, depends on whether de novo glucosamine synthesis occurs in endothelial cells. The aim of this study was to test the hypothesis of whether glutamine:fructose-6-phosphate amidotransferase (the first and key regulatory enzyme in hexosamine synthesis) is present for endothelial glucosamine synthesis.

METHODS

Bovine venular, bovine aortic, human microvascular, human umbilical vein, and rat coronary microvascular endothelial cells were used to measure glutamine:fructose-6-phosphate amidotransferase activity. To determine glucosamine-6-phosphate synthesis in intact cells, they were incubated for 1 h in Krebs bicarbonate buffer containing 5, 15 or 30 mmol/l [U-14C]glucose and 0.5, 2 or 4 mmol/l glutamine. The [14C]Glucosamine-6-phosphate and its end products ([14C]UDP-N-acetylglucosamine and [14C]UDP-Nacetylgalactosamine) were separated by HPLC.

RESULTS

There were high glutamine:fructose-6-phosphate amidotransferase activities in all endothelial cells studied. Exposure of cells to 15 to 30 mmol/l glucose or 2 to 4 mmol/l glutamine increased enzyme activity. Glucosamine-6-phosphate, UDP-N-acetylglucosamine and UDP-N-acetylgalactosamine syntheses increased with increasing extracellular concentrations of glucose from 5 to 30 mmol/l or of glutamine from 0.5 to 4 mmol/l.

CONCLUSION/INTERPRETATION

Our results show the presence of glutamine:fructose-6-phosphate amidotransferase for de novo glucosamine synthesis in endothelial cells and the modulation of this pathway by hyperglycaemia and glutamine. As glucosamine inhibits endothelial nitric oxide synthesis, these findings could have important implications for impaired endothelium-dependent relaxation and vascular dysfunction in diabetes mellitus.

Regulation of glutamine:fructose-6-phosphate amidotransferase activity by high glucose and transforming growth factor beta in rat mesangial cells

BACKGROUND

The hexosamine biosynthesis pathway acts as a cellular glucose sensor and mediates many of the adverse effects of glucose. Increased flux through this pathway results in insulin resistance in rat fibroblasts and transgenic mice and upregulation of transforming growth factor beta (TGF-beta) transcriptional activity in rat kidney cells. The first and rate-limiting step in this pathway, which is responsible for the metabolism of glucose to glucosamine, is catalyzed by glutamine:fructose-6-phosphate amidotransferase (GFA).

METHODS

Because of the known effects of hyperglycemia on mesangial cell (MC) function and growth factor regulation, we examined the regulation of GFA by glucose and TGF-beta in cultured SV40 rat MCs. GFA activity was assayed in cytosolic extracts of MCs using high-performance liquid chromatography.

RESULTS

Culturing in 10 and 25 mM of glucose for 24 hours resulted in 33.4% (P < 0.025) and 43.5% (P < 0.05) decreases in GFA activity when compared with cells cultured at 1 to 5 mM of glucose. The downregulation in GFA activity by high glucose (HG) required at least 6 hours in culture and persisted for several days. HG effects were not a result of osmolar changes or glucose-induced differences in glucose uptake. Like HG, treatment of MCs with TGF-beta (2 ng/mL) for 4 hours resulted in a 30% (P < 0.05) decrease in GFA activity in cells cultured at 1 mM glucose, but the effects of TGF-beta were not additive to those of HG. TGF-beta-mediated downregulation of GFA activity was inhibited by a TGF-beta-neutralizing antibody, but HG's effects were not. Insulin-like growth factor-1 (IGF-1) had similar effects as TGF-beta, but GFA activity was not regulated by angiotensin II.

CONCLUSIONS

GFA activity is downregulated by HG, TGF-beta, and IGF-1 in rat MCs. Downregulation of this cellular glucose sensor may be a protective mechanism against the harmful effects of excess glucose as seen in diabetes.

Hexokinase activity alters sugar-nucleotide formation in maize root homogenates

Two pools of hexokinase activities differing in sensitivity to ADP inhibition were characterised in maize roots. In order to evaluate how glucose utilisation could be affected by these hexokinases, glucose-6-P and NDP-5'-sugar levels were measured after a D-[U-14C]glucose pulse in root extracts in the presence of 0 or 1 mM ADP. Analysis of radio-labelled activated sugars by paper chromatography revealed that: (1) without ADP, nearly 20% of the 14C appeared in NDP-5'-sugars; (2) 0.1 mM ADP inhibited 14C-NDP-5'-sugar formation by 85%; and (3) with 1 mM ADP, 14C-NDP-5'-sugars were undetectable, but substantial (14%) 14C accumulated as glucose-6-P. Mannoheptulose, a hexokinase inhibitor, blocked the NDP-5'-sugar formation, but did not modify the amount of 14C-glucose-6-P in root extracts either with or without ADP. The analysis of the hexokinase activities with 0.8 mM glucose in maize root extracts showed that: (1) mitochondrial hexokinase activity was totally inhibited by 30 mM mannoheptulose; and (2) the cytosolic hexokinase was inhibited by only 30%. These data suggest that NDP-5'-sugar synthesis is sensitive to ADP fluctuations and that mannoheptulose affects preferentially the mitochondrial-bound hexokinase, but the cytosolic form is less sensitive. We propose that the mitochondrial hexokinase is the main energy charge sensor in this pathway in maize.

Enhanced catalytic activity of hexokinase by work-induced mitochondrial binding in fast-twitch muscle of rat

Using a teased muscle fiber preparation, we determined the activity of mitochondrially bound hexokinase in rat fast-twitch muscle under control conditions and after low-frequency stimulation periods for up to 2 h. As compared to soluble hexokinase, mitochondrial binding led to stimulation of glucose 6-phosphate production. Low-frequency stimulation greatly enhanced glucose 6-phosphate formation which was 100% and 250% elevated after 1 and 2 h, respectively. These observations point to a mechanism which rapidly increases the catalytic activity of hexokinase through binding to the mitochondrial surface.

A comparison of AMP degradation in the perfused rat heart during 2-deoxy-D-glucose perfusion and anoxia. Part I: The release of adenosine and inosine

AMP degradation is studied in two models of the Langendorff-perfused rat heart which generate a large release of purines: the 2-deoxy-D-glucose (2DG)-perfused heart and the anoxic heart. In the 2DG model, mitochondrial energy generation is quasi-normal, despite a very low ATP concentration. Furthermore, inorganic phosphate (Pi) concentration is low, an important difference with anoxia where Pi is very high, up to 82 mM. Coronary release of purines is measured by high performance liquid chromatography, and myocardial metabolite content by 31P nuclear magnetic resonance spectroscopy. In the 2DG-perfused hearts with glucose or acetate, the purine release consists nearly exclusively of inosine [up to 130 nmol/(min x gww)] while adenosine is less than 1 nmol/(min x gww). A possible interpretation is that AMP degradation proceeds mainly through deamination to inosine monophosphate by AMP deaminase (the IMP pathway). In contrast, the purine release in anoxia (100% N2) contains comparable quantities of adenosine and inosine [respectively 30 and 20 nmol/(min x gww)], indicating that part of AMP is dephosphorylated directly to adenosine. Comparison with the 2DG model suggests that the release of adenosine in the anoxic heart is a result of inhibition of AMP deaminase by Pi.