Nucleoside-diphosphate derivatives of 2-deoxy-D-glucose in animal cells

The Anticancer Activity of Monosaccharides: Perspectives and Outlooks

A major hallmark of cancer is the reprogramming of cellular metabolism from oxidative phosphorylation (OXPHOS) to glycolysis, a phenomenon known as the Warburg effect. To sustain high rates of glycolysis, cancer cells overexpress GLUT transporters and glycolytic enzymes, allowing for the enhanced uptake and consumption of glucose. The Warburg effect may be exploited in the treatment of cancer; certain epimers and derivatives of glucose can enter cancer cells and inhibit glycolytic enzymes, stunting metabolism and causing cell death. These include common dietary monosaccharides (ᴅ-mannose, ᴅ-galactose, ᴅ-glucosamine, ʟ-fucose), as well as some rare monosaccharides (xylitol, ᴅ-allose, ʟ-sorbose, ʟ-rhamnose). This article reviews the literature on these sugars in in vitro and in vivo models of cancer, discussing their mechanisms of cytotoxicity. In addition to this, the anticancer potential of some synthetically modified monosaccharides, such as 2-deoxy-ᴅ-glucose and its acetylated and halogenated derivatives, is reviewed. Further, this article reviews how certain monosaccharides can be used in combination with anticancer drugs to potentiate conventional chemotherapies and to help overcome chemoresistance. Finally, the limitations of administering two separate agents, a sugar and a chemotherapeutic drug, are discussed. The potential of the glycoconjugation of classical or repurposed chemotherapy drugs as a solution to these limitations is reviewed.

Glucose and mannose analogs inhibit KSHV replication by blocking N-glycosylation and inducing the unfolded protein response

Kaposi's sarcoma-associated herpesvirus (KSHV) is the etiological agent for Kaposi's sarcoma (KS), an HIV/AIDS-associated malignancy. Effective treatments against KS remain to be developed. The sugar analog 2-deoxy- d-glucose (2-DG) is an anticancer agent that is well-tolerated and safe in patients and was recently demonstrated to be a potent antiviral, including KSHV and severe acute respiratory syndrome coronavirus 2. Because 2-DG inhibits glycolysis and N-glycosylation, identifying its molecular targets is challenging. Here we compare the antiviral effect of 2-DG with 2-fluoro-deoxy- d-glucose, a glycolysis inhibitor, and 2-deoxy-fluoro- d-mannose (2-DFM), a specific N-glycosylation inhibitor. At doses similar to those clinically achievable with 2-DG, the three drugs impair KSHV replication and virion production in iSLK.219 cells via downregulation of viral structural glycoprotein expression (K8.1 and gB), being 2-DFM the most potent KSHV inhibitor. Consistently with the higher potency of 2-DFM, we found that d-mannose rescues KSHV glycoprotein synthesis and virus production, indicating that inhibition of N-glycosylation is the main antiviral target using d-mannose competition experiments. Suppression of N-glycosylation by the sugar drugs triggers ER stress. It activates the host unfolded protein response (UPR), counteracting KSHV-induced inhibition of the protein kinase R-like endoplasmic reticulum kinase branch, particularly activating transcription factor 4 and C/EBP homologous protein expression. Finally, we demonstrate that sugar analogs induce autophagy (a prosurvival mechanism) and, thus, inhibit viral replication playing a protective role against KSHV-induced cell death, further supporting their direct antiviral effect and potential therapeutic use. Our work identifies inhibition of N-glycosylation leading to ER stress and UPR as an antienveloped virus target and sugar analogs such as 2-DG and the newly identified 2-DFM as antiviral drugs.

Cellular toxicity of the metabolic inhibitor 2-deoxyglucose and associated resistance mechanisms

Most malignant cells display increased glucose absorption and metabolism compared to surrounding tissues. This well-described phenomenon results from a metabolic reprogramming occurring during transformation, that provides the building blocks and supports the high energetic cost of proliferation by increasing glycolysis. These features led to the idea that drugs targeting glycolysis might prove efficient in the context of cancer treatment. One of these drugs, 2-deoxyglucose (2-DG), is a synthetic glucose analog that can be imported into cells and interfere with glycolysis and ATP generation. Its preferential targeting to sites of cell proliferation is supported by the observation that a derived molecule, 2-fluoro-2-deoxyglucose (FDG) accumulates in tumors and is used for cancer imaging. Here, we review the toxicity mechanisms of this drug, from the early-described effects on glycolysis to its other cellular consequences, including inhibition of protein glycosylation and endoplasmic reticulum stress, and its interference with signaling pathways. Then, we summarize the current data on the use of 2-DG as an anti-cancer agent, especially in the context of combination therapies, as novel 2-DG-derived drugs are being developed. We also show how the use of 2-DG helped to decipher glucose-signaling pathways in yeast and favored their engineering for biotechnologies. Finally, we discuss the resistance strategies to this inhibitor that have been identified in the course of these studies and which may have important implications regarding a medical use of this drug.

The induction of HAD-like phosphatases by multiple signaling pathways confers resistance to the metabolic inhibitor 2-deoxyglucose

Anti-cancer strategies that target the glycolytic metabolism of tumors have been proposed. The glucose analog 2-deoxyglucose (2DG) is imported into cells and, after phosphorylation, becomes 2DG-6-phosphate, a toxic by-product that inhibits glycolysis. Using yeast as a model, we performed an unbiased mass spectrometry-based approach to probe the cellular effects of 2DG on the proteome and study resistance mechanisms to 2DG. We found that two phosphatases that target 2DG-6-phosphate were induced upon exposure to 2DG and participated in 2DG detoxification. Dog1 and Dog2 are HAD (haloacid dehalogenase)-like phosphatases, which are evolutionarily conserved. 2DG induced Dog2 by activating several signaling pathways, such as the stress response pathway mediated by the p38 MAPK ortholog Hog1, the unfolded protein response (UPR) triggered by 2DG-induced ER stress, and the cell wall integrity (CWI) pathway mediated by the MAPK Slt2. Loss of the UPR or CWI pathways led to 2DG hypersensitivity. In contrast, mutants impaired in the glucose-mediated repression of genes were 2DG resistant because glucose availability transcriptionally repressed DOG2 by inhibiting signaling mediated by the AMPK ortholog Snf1. The characterization and genome resequencing of spontaneous 2DG-resistant mutants revealed that DOG2 overexpression was a common strategy underlying 2DG resistance. The human Dog2 homolog HDHD1 displayed phosphatase activity toward 2DG-6-phosphate in vitro and its overexpression conferred 2DG resistance in HeLa cells, suggesting that this 2DG phosphatase could interfere with 2DG-based chemotherapies. These results show that HAD-like phosphatases are evolutionarily conserved regulators of 2DG resistance.

The wonders of 2-deoxy-D-glucose

Through the eons of time, out of all possible configurations, nature has selected glucose not only as a vital source of energy to sustain life but also as the molecule who's structure supplies the appropriate elements required for a cell to grow and multiply. This understanding, at least in part, explains the profound effects that the analog of glucose, 2-deoxy-d-glucose, has been shown to have on as common and widespread diseases as cancer, viral infection, aging-related morbidity, epilepsy, and others. This review is confined to summarizing some of the salient findings of this remarkable compound as they relate mainly to cancer.

Inhibition of Akt potentiates 2-DG-induced apoptosis via downregulation of UPR in acute lymphoblastic leukemia

The ability to pair the regulation of metabolism and cellular energetics with oncogenes and tumor suppressor genes provides cancer cells with a growth and survival advantage over normal cells. We investigated the mechanism of cell death induced by 2-deoxy-D-glucose (2-DG), a sugar analog with dual activity of inhibiting glycolysis and N-linked glycosylation, in acute lymphoblastic leukemia (ALL). We found that, unlike most other cancer phenotypes in which 2-DG only inhibits cell proliferation under normoxic conditions, ALL lymphoblasts undergo apoptosis. Bp-ALL cell lines and primary cells exhibited sensitivity to 2-DG, whereas T-ALL cells were relatively resistant, revealing phenotypic differences within ALL subtypes. Cotreatment with D-mannose, a sugar essential for N-linked glycosylation, rescues 2-DG-treated ALL cells, indicating that inhibition of N-linked glycosylation and induction of ER stress and the unfolded protein response (UPR) is the predominant mechanism of 2-DG's cytotoxicity in ALL. 2-DG-treated ALL cells exhibit upregulation of P-AMPK, P-Akt, and induction of ER stress/UPR markers (IRE1α, GRP78, P-eIF2α, and CHOP), which correlate with PARP cleavage and apoptosis. In addition, we find that pharmacologic and genetic Akt inhibition upregulates P-AMPK, downregulates UPR, and sensitizes ALL cells to remarkably low doses of 2-DG (0.5 mmol/L), inducing 85% cell death and overcoming the relative resistance of T-ALL. In contrast, AMPK knockdown rescues ALL cells by upregulating the prosurvival UPR signaling. Therefore, 2-DG induces ALL cell death under normoxia by inducing ER stress, and AKT and AMPK, traditionally thought to operate predominantly on the glycolytic pathway, differentially regulate UPR activity to determine cell death or survival.

Under normoxia, 2-deoxy-D-glucose elicits cell death in select tumor types not by inhibition of glycolysis but by interfering with N-linked glycosylation

In tumor cells growing under hypoxia, inhibiting glycolysis with 2-deoxy-d-glucose (2-DG) leads to cell death, whereas under normoxic conditions cells similarly treated survive. Surprisingly, here we find that 2-DG is toxic in select tumor cell lines growing under normal oxygen tension. In contrast, a more potent glycolytic inhibitor, 2-fluorodeoxy-d-glucose, shows little or no toxicity in these cell types, indicating that a mechanism other than inhibition of glycolysis is responsible for their sensitivity to 2-DG under normoxia. A clue to this other mechanism comes from previous studies in which it was shown that 2-DG interferes with viral N-linked glycosylation and is reversible by exogenous addition of mannose. Similarly, we find that 2-DG interferes with N-linked glycosylation more potently in the tumor cell types that are sensitive to 2-DG under normoxia, which can be reversed by exogenous mannose. Additionally, 2-DG induces an unfolded protein response, including up-regulation of GADD153 (C/EBP-homologous protein), an unfolded protein response-specific mediator of apoptosis, more effectively in 2-DG-sensitive cells. We conclude that 2-DG seems to be toxic in select tumor cell types growing under normoxia by inhibition of N-linked glycosylation and not by glycolysis. Because in a phase I study 2-DG is used in combination with an anticancer agent to target hypoxic cells, our results raise the possibility that in certain cases, 2-DG could be used as a single agent to selectively kill both the aerobic (via interference with glycosylation) and hypoxic (via inhibition of glycolysis) cells of a solid tumor.

Differential toxic mechanisms of 2-deoxy-D-glucose versus 2-fluorodeoxy-D-glucose in hypoxic and normoxic tumor cells

The dependence of hypoxic tumor cells on glycolysis as their main means of producing ATP provides a selective target for agents that block this pathway, such as 2-deoxy-D-glucose (2-DG) and 2-fluoro-deoxy-D-glucose (2-FDG). Moreover, it was demonstrated that 2-FDG is a more potent glycolytic inhibitor with greater cytotoxic activity than 2-DG. This activity correlates with the closer structural similarity of 2-FDG to glucose than 2-DG, which makes it a better inhibitor of hexokinase, the first enzyme in the glycolytic pathway. In contrast, because of its structural similarity to mannose, 2-DG is known to be more effective than 2-FDG in interfering with N-linked glycosylation. Recently, it was reported that 2-DG, at a relatively low dose, is toxic to certain tumor cells, even under aerobic conditions, whereas 2-FDG is not. These results indicate that the toxic effects of 2-DG in selected tumor cells under aerobic conditions is through inhibition of glycosylation rather than glycolysis. The intention of this minireview is to discuss the effects and potential clinical impact of 2-DG and 2-FDG as antitumor agents and to clarify the differential mechanisms by which these two glucose analogues produce toxicity in tumor cells growing under anaerobic or aerobic conditions.

Unlike 2-deoxy-D-glucose, 3-O-methyl-D-glucose does not induce long-term potentiation in rat hippocampal slices

Equimolar replacement of 10 mM glucose by 2-deoxy-D-glucose (2-DG) causes substantial depression followed by a sharp and sustained potentiation of CA1 field EPSPs. In the present experiments, similar applications of 3-O-methyl-D-glucose, which is also taken up by cells but is not phosphorylated, had only a weak blocking action and elicited no potentiation. Possible explanations for the marked effects of 2-DG include a more rapid block of glycolysis and the production of phosphorylated derivatives of 2-DG.

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.

Multiplication of Legionella pneumophila in HeLa cells in the presence of cytoskeleton and metabolic inhibitors

A study has been carried out on the action of cytoskeleton and metabolic inhibitors on intracellular multiplication in HeLa cells of a virulent strain of Legionella pneumophila serogroup 6. The effects of the substances were separately tested on both penetration and intracellular multiplication of L. pneumophila. Only cytochalasin A and 2-deoxy-D-glucose (2dG) affected bacterial internalisation, whereas intracellular multiplication was inhibited by cytochalasins A, B, C, D and J (D being the most active) and by 2dG with a dose-response effect. The action of 2dG was counteracted by 50 mM glucose. Experiments carried out with cytochalasin D and a rhodamine-phalloidin conjugate showed the involvement of cytoskeletal elements in intracellular multiplication of Legionella; compounds acting on microtubules had no effect.

Assessment of glucosylifosfamide mustard biodistribution in rats with prostate adenocarcinomas by means of in vivo 31P NMR and in vitro uptake experiments

A combined in vitro/in vivo study was performed to evaluate the possible application of phosphorus (31P) NMR spectroscopy for therapy monitoring and to investigate glucosylifosfamide mustard (Glc-IPM) transport and biodistribution by radiotracer techniques. Dynamic in vivo 31P NMR measurements were performed in rats with prostate adenocarcinoma after i.v. injection of 1 mmol/kg body weight (bw) of ifosfamide (IFO) (n = 4) and 1 mmol/kg bw (n = 4) or 2.15 mmol/kg bw (n = 9) of Glc-IPM. In a biodistribution study with 14C-labeled Glc-IPM and a final dose of 0.8 mmol Glc-IPM/kg bw, the animals were killed 5, 30, 60, and 120 min after drug administration, an ethanol extraction was performed from several tissues, and the dose per g tissue was calculated. The same tumor cell line was used in saturation and competition experiments to further elucidate the transport mechanism. The 31P NMR signals of IFO and Glc-IPM showed no overlap with the endogenous phosphorus peaks. A rapid washout with a half-life between 25.9 +/- 5.6 min for the lower dose and 34.3 +/- 4.2 min for the higher dose of Glc-IPM was observed in the tumor. No statistically significant change of the pH value was observed during the examination period. The beta-nucleoside 5'-triphosphate (NTP)/inorganic phosphate (Pi) signal intensity ratio showed a tendency to decrease but without statistical significance. A rapid elimination was demonstrated by both the noninvasive NMR technique and the biodistribution study. No saturation was found in vitro for the Glc-IPM uptake, even at the concentration of 5 mM. Furthermore, the Glc-IPM uptake was not inhibited by the presence of 2-deoxyglucose and vice versa. The data show that the pharmacokinetics of Glc-IPM in the tumor can be followed in vivo by 31P NMR. The results presented are evidence for diffusion as the transport mechanism for Glc-IPM in this tumor model. However, the better visualization of Glc-IPM as compared to ifosfamide may be due to metabolic trapping of a negatively charged metabolite after deglycosylation.

Synthesis of 2-deoxy-D-galactose containing gangliosides in vivo

Incorporation of 2-deoxy-D-galactose into the oligosaccharide moieties of different gangliosides of rat liver was examined. After intraperitoneal administration of 2-deoxy-D-galactose it was shown by GLC/MS analysis that this hexose analogue is metabolized and incorporated into all the gangliosides investigated, and predominantly into GM3 and GD3. In both of these gangliosides, 25-55% of the galactose residues were substituted by 2-deoxy-D-galactose. The epimer, 2-deoxy-D-glucose, was not detectable.

Synthesis of deoxyglucose-1-phosphate, deoxyglucose-1,6-bisphosphate, and other metabolites of 2-deoxy-D-[14C]glucose in rat brain in vivo: influence of time and tissue glucose level

When the kinetics of interconversion of deoxy[14C]glucose ([14C]DG) and [14C]DG-6-phosphate ([14C]DG-6-P) in brain in vivo are estimated by direct chemical measurement of precursor and products in acid extracts of brain, the predicted rate of product formation exceeds the experimentally measured rate. This discrepancy is due, in part, to the fact that acid extraction regenerates [14C]DG from unidentified labeled metabolites in vitro. In the present study, we have attempted to identify the 14C-labeled compounds in ethanol extracts of brains of rats given [14C]DG. Six 14C-labeled metabolites, in addition to [14C]DG-6-P, were detected and separated. The major acid-labile derivatives, DG-1-phosphate (DG-1-P) and DG-1,6-bisphosphate (DG-1,6-P2), comprised approximately 5 and approximately 10-15%, respectively, of the total 14C in the brain 45 min after a pulse or square-wave infusion of [14C]DG, and their levels were influenced by tissue glucose concentration. Both of these acid-labile compounds could be synthesized from DG-6-P by phosphoglucomutase in vitro. DG-6-P, DG-1-P, DG-1,6-P2, and ethanol-insoluble compounds were rapidly labeled after a pulse of [14C]DG, whereas there was a 10-30-min lag before there was significant labeling of minor labeled derivatives. During the time when there was net loss of [14C]DG-6-P from the brain (i.e., between 60 and 180 min after the pulse), there was also further metabolism of [14C]DG-6-P into other ethanol-soluble and ethanol-insoluble 14C-labeled compounds. These results demonstrate that DG is more extensively metabolized in rat brain than commonly recognized and that hydrolysis of [14C]DG-1-P can explain the overestimation of the [14C]DG content and underestimation of the metabolite pools of acid extracts of brain. Further metabolism of DG does not interfere with the autoradiographic DG method.

Metabolites of 2-deoxy-[14C]glucose in plasma and brain: influence on rate of glucose utilization determined with deoxyglucose method in rat brain

The [14C]deoxyglucose ([14C]DG) method depends upon quantitative trapping of metabolites in brain at the site of phosphorylation, and in the usual procedure it is assumed that all the label in plasma is in free DG. Our previous finding of labeled nonacidic derivatives of DG in plasma raised the possibility that some metabolites of DG might not be fully retained in body tissues and therefore cause overestimation of the integrated specific activity of the precursor pool determined from assay of label in plasma and/or underestimation of the true size of the metabolite fraction in brain. In the present study, metabolism of DG in rat tissues by secondary pathways was examined and found to be more extensive than previously recognized. When 14C-labeled compounds in ethanol extracts of either plasma or brain were separated by anion exchange HPLC, eight fractions were obtained. 14C-labeled metabolites in plasma were detected after a 35-min lag and gradually increased in amount with time after an intravenous pulse. In brain, deoxyglucose-6-phosphate was further metabolized, mainly to deoxyglucose-1-phosphate and deoxyglucose-1,6-phosphate. These are acid-labile compounds and accounted for approximately 20% of the 14C in the metabolite pool in brain. The rate constants for net loss of 14C from the metabolite pool between 45 and 180 min after a pulse were similar (0.4-0.5%/min) in vivo and in intact postmortem brain. The rate constant for loss of deoxyglucose-6-phosphate (DG-6-P) in vivo (approximately 0.7%/min) was, however, about twice that for postmortem brain, suggesting that a significant fraction of the DG-6-P lost in vivo is due to its further metabolism by energy-dependent reactions. 14C-labeled metabolites of [14C]DG in plasma and brain do not interfere with determination of local rates of glucose utilization in brain in normal, conscious rats by the autoradiographic method if the prescribed procedures and a 45-min experimental period are used.

Incorporation of the hexose analogue 2-deoxy-D-galactose into membrane glycoproteins in HepG2 cells

The incorporation of 2-deoxy-D-galactose into the oligosaccharide moieties of glycoproteins and the consequences of 2-deoxy-D-galactose treatment on the fucosylation of glycoproteins were investigated in the human hepatoma cell line HepG2. Using different methods, it was shown that treatment of HepG2 cells with 2-deoxy-D-galactose leads to an incorporation of 2-deoxy-D-galactose and a decrease of L-fucose incorporation into the oligosaccharides of glycoproteins. The extent of labeling by L-[3H]fucose was determined by removing L-[3H]fucose from labeled cells with the aid of a purified alpha 1,2-fucosidase from Aspergillus niger. Using this method, it was shown that 2-deoxy-D-galactose markedly inhibits alpha 1,2-fucosylation. Measurement of the amount of 2-deoxy-D-galactose incorporated, however, showed that replacement of D-galactose by 2-deoxy-D-galactose does not entirely account for the decrease in alpha 1,2-fucosylation. In addition, a hitherto unreported compensatory increase of alpha 1,3/alpha 1,4-fucosylation was found to occur when alpha-1,2-fucosylation was inhibited by treatment with 2-deoxy-D-galactose.

Direct measurement of the lambda of the lumped constant of the deoxyglucose method in rat brain: determination of lambda and lumped constant from tissue glucose concentration or equilibrium brain/plasma distribution ratio for methylglucose

Steady-state distribution spaces of 2-[14C]deoxyglucose ([14C]DG), glucose, and 3-O-[14C]methylglucose at various concentrations of glucose in brain and plasma ranging from hypoglycemic to hyperglycemic levels have been determined by direct chemical analyses in the brains of conscious rats. The hexose concentrations were measured chemically in freeze-blown brain extracted with ethanol to avoid the degradation of acid-labile products of [14C]DG back to free [14C]DG that has been found to occur with the more commonly used perchloric acid extraction of brain. Corrections were also made for nonphosphorylatable, labeled products of [14C]DG found in the nonacidic fractions of the brain extracts, which were previously included with the assayed [14C]DG, and for the contribution of the hexose contents in the blood in the brain, which was found to be particularly critical for the determination of the glucose distribution space, especially in hypoglycemic states. From the measured contents of the hexoses in brain and plasma, the relationships of the tissue concentrations and distribution spaces of each of the hexoses and of the lambda (i.e., ratio of tissue distribution space of DG to that of glucose) of the DG method to the tissue glucose concentration were derived. The lambda was then quantitatively related to the measured equilibrium ratio for [14C]methylglucose over the full range of brain and plasma glucose levels. By combining these new data with the values for the lumped constant, the factor that converts the rate of DG phosphorylation to glucose phosphorylation, previously determined in rats over the same range of plasma glucose levels, the phosphorylation coefficient was calculated and the lumped constant graphed as a function of the measured distribution space in brain for [14C]methylglucose.

Acid lability of metabolites of 2-deoxyglucose in rat brain: implications for estimates of kinetic parameters of deoxyglucose phosphorylation and transport between blood and brain

The steady-state brain/plasma distribution ratios of [14C]deoxyglucose ([14C]DG) for hypoglycemic rats previously determined by measurement of DG concentrations in neutralized acid extracts of freeze-blown brain and plasma exceeded those predicted by simulations of kinetics of the DG model. Overestimation of the true size of the precursor pool of [14C]DG for transport and phosphorylation could arise from sequestration of [14C]DG within brain compartments and/or instability of metabolites of [14C]DG and regeneration of free [14C]DG during the experimental period or extraction procedure. In the present study, the concentrations of [14C]DG and glucose were compared in samples of rat brain and plasma extracted in parallel with perchloric acid or 65% ethanol containing phosphate-buffered saline. The concentrations of both hexoses in acid extracts of brain were higher than those in ethanol, whereas hexose contents of plasma were not dependent on the extraction procedure. The magnitude of overestimation of DG content (about 1.2-to fourfold) varied with glucose level and was highest in extracts isolated from hypoglycemic rats; contamination of the [14C]DG fraction with 14C-labeled nonacidic metabolites also contributed to this overestimation. Glucose concentrations in acid extracts of brain exceeded those of the ethanol extracts by less than 40% for normal and hypoglycemic rats.

Mode of action of 9-(4-hydroxy-3-hydroxymethylbut-1-yl)guanine (BRL 39123) against herpes simplex virus in MRC-5 cells

The metabolism and mode of action of 9-(4-hydroxy-3-hydroxymethylbut-1-yl)guanine (BRL 39123) were studied in herpes simplex virus type 1 (HSV-1)-infected and uninfected MRC-5 cells and compared with those of acyclovir. In uninfected cells incubated with 10 microM acyclonucleoside for 4 h, no phosphorylation of either BRL 39123 or acyclovir was detected. In contrast, in HSV-1-infected cells, both BRL 39123 and acyclovir were phosphorylated up to the triphosphate esters. Phosphorylation of BRL 39123 occurred much more rapidly and proceeded to a greater extent than did that of acyclovir. Furthermore, following the removal of acyclonucleoside from the culture medium, the intracellular triphosphate ester of BRL 39123 was much more stable than was that of acyclovir, the half-lives being about 10 and 0.7 h, respectively. BRL 39123 treatment effectively inhibited the formation of HSV-1 DNA in infected MRC-5 cells, 50% inhibitory concentrations of BRL 39123 and acyclovir being 0.04 microgram/ml (0.16 microM) and 0.15 microgram/ml (0.67 microM), respectively. In addition, BRL 39123 was shown to be more effective than acyclovir at inhibiting viral DNA synthesis following short treatment times, presumably reflecting the greater stability of BRL 39123 triphosphate. Neither BRL 39123 nor acyclovir inhibited cellular DNA synthesis in uninfected cells at concentrations of up to 100 micrograms/ml.

Studies on the energy metabolism of opossum Didelphis virginiana erythrocytes--III. Metabolic depletion with 2-deoxyglucose markedly accelerates methemoglobin reduction in opossum but not in human erythrocytes

1. Glucose-depleted, nitrite-treated erythrocytes reduce ferriheme in vitro in an environment 100 mM to 2-deoxy-D-glucose at a rate of 2.4 microM/ml cells/hr (opossum) and 0.37 microM/ml cells/hr (human). 2. During the process of methemoglobin reduction the breakdown of adenine ribonucleotides is more rapid in opossum (0.9 microM/g hg/hr) than in human (0.36 microM/g hg/hr) erythrocytes. 3. Radiolabelled ribose from [U-14C] ATP is catabolized exclusively to [14C] lactate in opossum, and to [14C] pyruvate and [14C] lactate in human red cells.

Evidence for gluconeogenesis in the amphibian retina

Evidence for the existence of a gluconeogenic pathway was provided in the amphibian retina. It was found that [3H]glutamate was converted to [3H]glucose derived from [3H]glutamate was incorporated into glycogen. The rate for this incorporation was found to be essentially the same in both light- and dark-adapted retinas: 0.147 vs. 0.142 nmol (mg protein X 2 hr)-1, respectively. However, the rate of incorporation was found to decline progressively with time. The rate for the incorporation of label derived from glutamate into glycogen was found to be considerably less than that for [3H]glucose: 10.2 nmol (mg protein X 2 hr)-1. The activity of a key gluconeogenic enzyme, fructose-1,6-bisphosphatase, also was demonstrated in retinal supernatants, approximately 1 nmol (mg X min)-1, and the activity of this enzyme was found to be inhibited both by adenosine monophosphate and by fructose-2,6-bisphosphate.

The antiherpes drug (E)-5-(2-bromovinyl)-2'-deoxyuridine (BVdU) interferes with formation of N-linked and of O-linked oligosaccharides of the herpes simplex virus type 1 glycoprotein C

In HSV-1 infected cell the antiherpes drug (E)-5-(2-bromovinyl)-2'-deoxyuridine (BVdU) exerted at least three different effects on glycosylation of glycoprotein gC. First, an overall decrease of protein glycosylation occurred due to inhibition of synthesis of the lipid-linked oligosaccharides, precursors of N-linked oligosaccharides of gC. Second, an inhibition of processing of N-linked oligosaccharides occurred after the acquisition of endo H-resistance, and possibly due to inhibition of galactose incorporation. Third, a small inhibition of incorporation of glucosamine into O-linked oligosaccharides, and, may be associated with this, a change in the proportion of two different classes of O-linked oligosaccharides of gC, namely those with terminal N-acetylgalactosamine and those with terminal sialic acid.

Role of 2-deoxy-D-glucose in the inhibition of phagocytosis by mouse peritoneal macrophage

2-Deoxy-D-glucose inhibits Fc and complement receptor-mediated phagocytosis of mouse peritoneal macrophages. To understand the mechanism of this inhibition, we analyzed the 2-deoxy-D-glucose metabolites in macrophages under phagocytosis inhibition conditions and conditions of phagocytosis reversal caused by glucose, mannose and 5-thio-D-glucose, and compared their accumulations under these conditions. Macrophages metabolized 2-deoxy-D-glucose to form 2-deoxy-D-glucose 6-phosphate, 2-deoxy-D-glucose 1-phosphate, UDP-2-deoxy-D-glucose, 2-deoxy-D-glucose 1, 6-diphosphate, 2-deoxy-D-gluconic acid and 2-deoxy-6-phospho-D-gluconic acid. The level of bulk accumulation as well as the accumulation of any of these 2-deoxy-D-glucose metabolites did not correlate with changes in macrophage phagocytosis capacities caused by the reversing sugars. 2-Deoxy-D-glucose inhibited glycosylation of thioglycolate-elicited macrophage by 70-80%. This inhibition did not cause phagocytosis inhibition, since (1) the reversal of phagocytosis by 5-thio-D-glucose was not followed by increases in the incorporation of radiolabelled galactose, glucosamine, N-acetylgalactosamine or fucose; (2) cycloheximide at a concentration that inhibited glycosylation by 70-80% did not affect macrophage phagocytosis. The inhibition of protein synthesis by 2-deoxy-D-glucose similarly could not account for phagocytosis inhibition, since cycloheximide, when used at a concentration that inhibited protein synthesis by 95%, did not affect phagocytosis. 2-Deoxy-D-glucose lowered cellular nucleoside triphosphates by 70-99%, but their intracellular levels in the presence of different reversing sugars did not correlate with the magnitude of phagocytosis reversal caused by these sugars. The results show that 2-deoxy-D-glucose inhibits phagocytosis by a mechanism distinct from its usual action of inhibiting glycosylation, protein synthesis and depleting energy supplies, mechanisms by which 2-deoxy-D-glucose inhibits other cellular processes.

Uridylate-trapping sugar analogs in combination with inhibitors of uridylate synthesis de novo and 5-fluorouridine

The combination of 3 types of antipyrimidines was studied in AS-30D hepatoma cells in suspension culture and in the rat in vivo. Cellular UTP and CTP pools can be depleted most effectively by combining an inhibitor of de novo UMP synthesis with sugar analogs diverting UMP to UDP-sugar analogs. The following UMP-trapping sugar analogs were employed: D-galactosamine, D-galactosone, D-glucosone, and D-glucosamine. These D-galactose and D-glucose analogs intensified the depletion of UTP and CTP pools induced by the following inhibitors of de novo UMP synthesis: acivicin, PALA, lapachol, pyrazofurin, and 6-azauridine. The sugar analogs, in the absence of inhibitors of the de novo pathway, enhanced the rate of de novo UMP synthesis several-fold, as indicated by incorporation of 14CO2 into intermediates and products of the pathway and by the expansion of the acid-soluble uracil nucleotide pool. Reduction of UTP and CTP contents to less than 5 and 15% of control, respectively, by D-galactosamine and PALA resulted in a decrease of the rate of RNA synthesis to 19% of control as calculated from the changes in specific activities of [14C]CTP and of [14C]cytidine in RNA after labeling with [14C]uridine. Hepatoma cells released uridine and cytidine into the extracellular fluid. This release was reduced to one third in UTP-deficient cells, indicating that pyrimidine nucleoside excretion is regulated by pyrimidine nucleotide levels, possibly by UTP and CTP regulation of uridine kinase. Determination of the rates of de novo pyrimidine synthesis, of the formation of RNA pyrimidines, and of pyrimidine nucleoside excretion indicates that de novo synthesis provides only about 67% of the pyrimidines required for the consuming processes. The difference, as well as the dilution of labeled pyrimidine nucleotide pools under conditions of a blocked de novo pathway, suggests a considerable salvage of pyrimidine nucleosides derived from RNA. This salvage of pyrimidines may be intracellular and/or by an excretion and re-uptake process. Depletion of UTP and CTP pools, induced in hepatoma cells by D-galactosamine and 6-azauridine, leads to growth inhibition in suspension culture; this inhibition becomes irreversible in an increasing percentage of cells, killing all cells after 20 hr of UTP deficiency. The enhanced uptake of 5-fluorouridine by UTP-deficient cells was associated with an increase of FUMP incorporation into RNA up to 4-fold and with stronger inhibition of cell growth.(ABSTRACT TRUNCATED AT 400 WORDS)

Effect of 2-deoxy-D-glucose on maintenance in culture of neonatal B cell of rat

The effect of 2-deoxy-D-glucose on maintenance in culture of B cells of the neonatal rat was examined by supplementation of Medium 199 containing 5.5 mM glucose with 1 mM 2-deoxy-D-glucose. Islets maintained in medium with 5.5 mM glucose (basal medium) for 7 d underwent remarkable decreases in glucose sensitivity, and the levels of insulin in the medium dropped. By contrast, addition of 2-deoxy-D-glucose promoted a higher insulin content in medium and an increase in the glucose-induced insulin release and biosynthesis. Moreover, the addition of the deoxysugar caused a selective deletion of fibroblasts and prevented the deterioration of islet cells in basal medium, yielding clusters mostly consisting of islet cells at the end of culture.

2-Deoxyglucose incorporation into rat brain glycogen during measurement of local cerebral glucose utilization by the 2-deoxyglucose method

The incorporation of 14C into glycogen in rat brain has been measured under the same conditions that exist during the measurement of local cerebral glucose utilization by the autoradiographic 2-[14C]deoxyglucose method. The results demonstrate that approximately 2% of the total 14C in brain 45 min after the pulse of 2-[14C]deoxyglucose is contained in the glycogen portion, and, in fact, incorporated into alpha-1-4 and alpha-1-6 deoxyglucosyl linkages. When the brain is removed by dissection, as is routinely done in the course of the procedure of the 2-[14C]deoxyglucose method to preserve the structure of the brain for autoradiography, the portion of total brain 14C contained in glycogen falls to less than 1%, presumably because of postmortem glycogenolysis which restores much of the label to deoxyglucose-phosphates. In any case, the incorporation of the 14C into glycogen is of no consequence to the validity of the autoradiographic deoxyglucose method, not because of its small magnitude, but because 2-[14C]deoxyglucose is incorporated into glycogen via [14C]deoxyglucose-6-phosphate, and the label in glycogen represents, therefore, an additional "trapped" product of deoxyglucose phosphorylation by hexokinase. With the autoradiographic 2-[14C]deoxyglucose method, in which only total 14C concentration in the brain tissue is measured by quantitative autoradiography, it is essential that all the labeled products derived directly or indirectly from [14C]deoxyglucose phosphorylation by hexokinase be retained in the tissue; their chemical identity is of no significance.

Identification of a glycosidase activity with apparent specificity for 2-deoxy-D-glucose in glycosidic linkage

2-Deoxy-D-glucose (dGlc) is a carbohydrate with significant activity as an inhibitor of glucose metabolism and as a precursor in the synthesis of glycosylated macromolecules; several of the enzymes associated with its metabolism remain uncharacterized. In the present report, the partial purification and some of the properties of a mammalian enzyme that appears to be relatively specific for the hydrolysis of dGlc bound in glycosidic linkage is described. The physiological function of this enzymatic activity is unknown. In addition, dGlc has been shown to be taken up by HTC cells in culture and incorporated into macromolecular bound form, both as dGlc and as 2-deoxygalactose which is formed from dGlc.

Glucose requirement for induction by sodium butyrate of the glycoprotein hormone alpha subunit in HeLa cells

Butyric acid produces multiple effects on mammalian cells in culture, including alterations in morphology, depression of growth rate, increased histone acetylation, and modified production of various proteins and enzymes. The latter effect is exemplified by the induction in HeLa cells of the glycoprotein hormone alpha subunit by millimolar concentrations of the fatty acid. This report demonstrates that increased subunit accumulation in response to sodium butyrate is strikingly dependent on the presence of glucose (or mannose) in the growth medium. In contrast, basal levels of subunit synthesis are only marginally affected when the culture medium is supplemented with one of a variety of hexoses. An increase in the accumulation of HeLa alpha does not occur in medium containing pyruvate as the energy source, and sustained induction requires the simultaneous and continued presence of both glucose and butyrate. The effects of butyrate on HeLa cell morphology and subunit induction can be separated, since the latter is glucose-dependent while the former is not. Failure of butyrate to induce alpha in medium containing pyruvate does not result from restricted subunit secretion, since the levels of intracellular alpha are not increased disproportionately relative to those in the medium. The hexoses which support induction of HeLa alpha (glucose greater than or equal to mannose greater than galactose greater than fructose) are identical to those which have been shown previously to stimulate the glucosylation of lipid-linked oligosaccharides and enhance the synthesis of certain glycoproteins. Labeling of various glycosylation intermediates with [3H]mannose indicates that in glucose medium there is a decrease in the level of radioactivity associated with both dolicholpyrophosphoryl oligosaccharide and cellular glycoproteins and a concomitant increase in the fraction of label recovered in secreted glycoproteins. Butyrate also causes a decrease in [3H]mannose-labeled cellular glycoproteins and an increase in tritiated extracellular glycoproteins, particularly in glucose medium. Likewise, glucose stimulates the incorporation of [3H]glucosamine into immunoprecipitable alpha subunit relative to the bulk of HeLa-secreted glycoproteins, and this is further enhanced by butyrate. However, as demonstrated by lectin chromatography of conditioned media, a nonglycosylated subunit does not accumulate in pyruvate medium, either in the absence or presence of butyrate.(ABSTRACT TRUNCATED AT 400 WORDS)

Poliovirus and vesicular stomatitis virus replication in the presence of 6-diazo-5-oxo-L-norleucine or 2-deoxy-D-glucose

The effects of 6-diazo-5-oxo-L-norleucine (DON), 2-deoxy-D-glucose (DOG), and tunicamycin (TM) on the replication of poliovirus (PV) and vesicular stomatitis virus (VSV) were examined. During a 48-hr replication period, TM, DON, and DOG inhibit VSV plaque formation in HEp-2 cells by 99.9%, 99.8%, and 99.9% respectively. Inhibition of VSV by DON is reversed with glutamine. Although all three agents are known to affect glycoprotein synthesis, DON and DOG also inhibit plaque formation of viruses devoid of structural glycoproteins. Thus, plaque formation of PV types 1 and 3 and Coxsackie B3 virus is delayed in HEp-2 and Buffalo green monkey kidney cells during exposure to these agents. Since these viruses do not contain glycoproteins and since concentrations up to 10 micrograms TM/ml cause no significant inhibition of PV, DON and DOG are affecting another viral or cellular process. Inhibition of PV replication by DON is reversed by addition of 25 mM glutamine or marginally by exposure to a combination of 5 mM concentrations of cytidine, uridine, adenosine monophosphate, and guanosine monophosphate. Inhibition of PV replication by DOG is reversed with 5 mM uridine alone. During DON exposure of HEp-2 cells infected with PV, the amount of 3H-uridine incorporation at 5.5 hr postinfection (pi) is reduced to 53% of untreated controls, an amount 11% greater than incorporation in cultures infected with PV but not treated with DON. These data indicate that the inhibition of PV replication by DON or DOG occurs at the level of viral RNA synthesis, while the primary target of these agents during VSV replication is probably glycosylation.

Metabolism of 3-deoxy-3-fluoro-D-mannose and 4-deoxy-4-fluoro-D-mannose by Saccharomyces cerevisiae S288C

Incubation of Saccharomyces cerevisiae S288C with 4-deoxy-4-fluoro-D-[1-14C]-mannose resulted in the formation of three metabolites that were characterized as 4-deoxy-4-fluoro-D-[1-14C]mannose 1,6-bisphosphate, 4-deoxy-4-fluoro-D-[1-14C]-mannose 6-phosphate and GDP-4-deoxy-4-fluoro-D-[1-14C]mannose. In addition, radioactive material was incorporated into a particulate fraction composed primarily of cell-wall polysaccharides. Compared with the 4-fluoro sugar, 3-deoxy-3-fluoro-D-[1-14C]mannose was not transported into yeast cells as well, and its conversion into sugar nucleotide was much less efficient. Metabolites that were isolated after incubation with the 3-fluoro analogue were identified as 3-deoxy-3-fluoro-D-[1-14C]mannose 1,6-bisphosphate, 3-deoxy-3-fluoro-D-[1-14C]mannose 6-phosphate and GDP-3-deoxy-3-fluoro-D-[1-14C]mannose. Little radioactivity was transferred into the cell-wall fraction.

2-Deoxyglucose as a substrate for glutathione regeneration in human and ruminant red blood cells

2-Deoxyglucose was found to be a substrate for the regeneration of GSH from GSSG in the red blood cells of humans, sheep, goats and cattle. The regeneration rate with 2-deoxyglucose varied from 15 to 56% of the rate with glucose. It is likely that 2-deoxyglucose is phosphorylated to 2-deoxyglucose-6-phosphate by HK and is then oxidized to 2-deoxygluconate-6-phosphate by G6PD. The latter reaction would produce the NADPH required for GSH regeneration.

Selection and characterization of a glucokinase-deficient mutant of Tetrahymena thermophila

We have isolated a mutant of Tetrahymena thermophila that is resistant to inhibition of growth by the glucose analog 2-deoxyglucose. The mutant exhibits a deficiency in a cytoplasmic glucokinase. This enzymatic defect and the attendant inability to convert 2-deoxyglucose to toxic phosphorylated derivatives is apparently the sole basis for the mutant phenotype since transport of glucose and 2-deoxyglucose is unimpaired; there is no elevation of glucose-6-phosphatase activity, which could decrease the level of toxic 2-deoxyglucose metabolites. Genetic analyses have shown that the mutant allele is recessive and inherited as a single Mendelian mutation. The glucokinase-deficient strain described here is useful for the selection of other mutants in this organism and for the investigation of various cellular processes initiated or modulated by glucose and its analogs. We have exploited the molecular defect in this strain to investigate the initial steps in the cyclic AMP-mediated repression of galactokinase gene expression which is caused by glucose.

Enzymatic deoxyglucosylation of ceramides by microsomes of BHK-21 cells. The effect of deoxyglucose treatment and herpesvirus infection

The microsomal fractions of cultured hamster fibroblasts (BHK-21 cells) catalyze the incorporation of glucose from UDPglucose or of deoxyglucose from UDPdeoxyglucose into a reaction mixture with liposomes consisting of ceramide and phosphatidylcholine. The microsomal fractions also catalyze the transfer of glucose from UDPglucose to endogenous acceptors. The specific activity of ceramide deoxyglucoside or ceramide glucoside formation was significantly higher when microsomal preparations obtained from deoxyglucose-treated or herpesvirus-infected BHK-21 cells were used as the glucosyltransferase source. Deoxyglucose was incorporated from UDPdeoxyglucose into hydroxy- and nonhydroxy-fatty acid-containing ceramides at approximately the same rate. Competitive inhibition of deoxyglucosylation of ceramides by UDPglucose suggests that both reactions were catalyzed by the same enzyme, viz. UDPglucose:ceramide glucosyltransferase. This inhibition of glycosphingolipid synthesis may account, in part, for the inhibitory effect of deoxyglucose on lipid-containing viruses.

Uptake and localization of 3H-2 deoxy-D-glucose by retinal photoreceptors

Following dark incubation of isolated retinas of Xenopus laevis in Ringer solution supplemented with 3H-2-Deoxy-D-glucose (2DG), virtually all of the uptake of the label was by the inner segments and synaptic bases of the photoreceptor cells. Autoradiographs prepared from conventionally fixed tissue showed the same cellular distribution of label as those prepared from identically incubated, unfixed, freeze-dried retinas. However, fixation removed about 77% of the total counts. This fixation-labile, soluble fraction was identified as being primarily 2DG-6 phosphate by thin-layer chromatography. The remaining insoluble fraction corresponded in distribution to glycogen grains. In cones, glycogen is stored primarily in the paraboloid, whereas in rods it is distributed throughout the inner segment and synaptic base. EM autoradiographs illustrated that these were the sites over which fixation-resistant 2DG label was localized. Measurements of radioactivity associated with extracts of retinal glycogen following 2DG incubation demonstrated that a disproportionately high fraction of total counts were associated with the glycogen fraction. We conclude that in the amphibian retina 2DG may be incorporated into glycogen.

Hexose transport in normal and SV40-transformed human endothelial cells in culture

The mechanism of glucose entry into human vascular endothelial cells was studied in monolayer cultures of normal (primary) and virally (SV40) transformed umbilical vein endothelium. Radioisotopic uptake studies with the glucose analogues 2-deoxy-D-glucose, and 3-O-methyl-D-glucose, and the nonmetabolizable stereoisomer L-glucose, indicated the presence of a saturable, stereospecific hexose carrier mechanism in both cell types. In other experiments with D-glucose and 3-O-methyl-D-glucose, the phenomenon of countertransport was demonstrable. Hexose transport was not affected by KCN, dinitrophenol, or ouabain, but was inhibited by phloretin and phlorizin in a pattern consistent with facilitated diffusion. Kinetic constants were obtained for both 2-deoxy-D-glucose and 3-O-methyl-D-glucose uptake. Similar Km values (range, 3.3-4.7 mM) were noted with normal and transformed cells, whereas the apparent Vmax was 0.56 nmol/microliter cytosol/minute for primary cells and 1.7-2.5 nmol/mu cytosol/minute for transformed cells. Under standard culture conditions, as well as following 18 hours of serum deprivation, insulin at concentrations up to 10(-5) M did not appear to influence hexose uptake in either cell type. Metabolism of 14C(U)-D-glucose to 14CO2 also was not stimulated by insulin. The presence of an insulin-insensitive, facilitated transport system for glucose in vascular endothelium has relevance for glucose metabolism in this tissue, and potentially for the association of certain vascular diseases (e.g., diabetic microangiopathy, atherosclerosis) with altered glucose homeostasis.

Formation of 2-deoxyglucose-containing lipid-linked oligosaccharides. Interference with glycosylation of glycoproteins

Crude membrane preparations from chick embryo cells catalyse the formation of dolichyl-di-N-acetylchitobiosyl diphosphate [Dol-PP-(GlcNAc)2] from uridine diphosphate N-acetylglucosamine (UDP-GlcNAc). The formation of this glycolipid was stimulated by exogenous dolichyl phosphate and inhibited by tunicamycin. Adding GDP-mannose to the cell-free system containing Dol-PP-(GlcNAc)2 by preincubation led to the formation of a lipid-linked oligosaccharide, containing 8--9 sugar residues. The formation of lipid-linked oligosaccharides was inhibited by GDP-2-deoxy-D-glucose (GDP-dGlc): in this case Dol-PP-(Glc-NAc)2-dGlc accumulated. Subsequent additions of mannosyl residues to this trisaccharide-lipid to form lipid-linked oligosaccharides were not possible. Concomitantly the glycosylation of proteins was blocked. Partially inhibitory conditions were obtained by adding both GDP-dGlc and GDP-Man with an excess of GDP-dGlc. Glycosylation of proteins was observed but the glycopeptides did not contain 2-deoxyglucosyl residues. Also in these cases 2-deoxyglucose-containing glycolipids accumulated. The main glycolipid formed under these conditions was Dol-PP-(GlcNAc)2-Man-dGlc. Lipid-linked oligosaccharides containing 2-deoxyglucose were formed under these conditions, although in small amounts, but were not transferred to protein. So the molecular basis of the inhibitory action of 2-deoxyglucose on glycosylation of protein is the incorporation of 2-deoxyglucosyl residues during early phases of the biosynthesis of the lipid-linked oligosaccharides.

Acceleration of methaemoglobin reduction by riboflavin in human erythrocytes

The effect of riboflavin on nitrite treated erythrocytes from normal subjects and patients with hereditary methaemoglobinaemia due to the deficiency of NADH-cytochrome b5 reductase was studied in the presence of glucose, 2-deoxy-D-glucose or lactate. When glucose or 2-deoxy-D-glucose was used as a substrate for these erythrocytes, the rate of methaemoglobin reduction in these cells was accelerated more than two-fold in the presence of riboflavin. The acceleration was dependent on the concentration of riboflavin and was suppressed by the addition of atebrin. The stimulative effect of riboflavin was, however, not observed when lactate was used in place of glucose or 2-deoxy-D-glucose. On the basis of these results, the acceleration of methaemoglobin reduction by riboflavin was considered to be due to the activation of NADPH-flavin reductase (Yubisui et al, 1977) in erythrocytes by the reagent. The availability of riboflavin for patients with methaemoglobinaemia due to the deficiency of NADH-cytochrome b5 reductase and for those with toxic methaemoglobinaemia is discussed in relation to methaemoglobin reducing systems in erythrocytes.