Inhibition of transplasma membrane electron transport by monoclonal antibodies to the transferrin receptor

The oxidative function of diferric transferrin

There is evidence for an unexpected role of diferric transferrin as a terminal oxidase for the transplasma membrane oxidation of cytosolic NADH. In the original studies which showed the reduction of iron in transferrin by the plasma membranes NADH oxidase, the possible role of the reduction on iron uptake was emphasized. The rapid reoxidation of transferrin iron under aerobic conditions precludes a role for surface reduction at neutral pH for release of iron for uptake at the plasma membrane. The stimulation of cytosolic NADH oxidation by diferric transferrin indicates that the transferrin can act as a terminal oxidase for the transplasma membrane NADH oxidase or can bind to a site which activates the oxidase. Since plasma membrane NADH oxidases clearly play a role in cell signaling, the relation of ferric transferrin stimulation of NADH oxidase to cell control should be considered, especially in relation to the growth promotion by transferrin not related to iron uptake. The oxidase can also contribute to control of cytosolic NAD concentration, and thereby can activate sirtuins for control of ageing and growth.

Multifunctional roles of enolase in Alzheimer's disease brain: beyond altered glucose metabolism

Enolase enzymes are abundantly expressed, cytosolic carbon-oxygen lyases known for their role in glucose metabolism. Recently, enolase has been shown to possess a variety of different regulatory functions, beyond glycolysis and gluconeogenesis, associated with hypoxia, ischemia, and Alzheimer's disease (AD). AD is an age-associated neurodegenerative disorder characterized pathologically by elevated oxidative stress and subsequent damage to proteins, lipids, and nucleic acids, appearance of neurofibrillary tangles and senile plaques, and loss of synapse and neuronal cells. It is unclear if development of a hypometabolic environment is a consequence of or contributes to AD pathology, as there is not only a significant decline in brain glucose levels in AD, but also there is an increase in proteomics identified oxidatively modified glycolytic enzymes that are rendered inactive, including enolase. Previously, our laboratory identified alpha-enolase as one the most frequently up-regulated and oxidatively modified proteins in amnestic mild cognitive impairment (MCI), early-onset AD, and AD. However, the glycolytic conversion of 2-phosphoglycerate to phosphoenolpyruvate catalyzed by enolase does not directly produce ATP or NADH; therefore it is surprising that, among all glycolytic enzymes, alpha-enolase was one of only two glycolytic enzymes consistently up-regulated from MCI to AD. These findings suggest enolase is involved with more than glucose metabolism in AD brain, but may possess other functions, normally necessary to preserve brain function. This review examines potential altered function(s) of brain enolase in MCI, early-onset AD, and AD, alterations that may contribute to the biochemical, pathological, clinical characteristics, and progression of this dementing disorder.

Proteomic analysis of specific brain proteins in aged SAMP8 mice treated with alpha-lipoic acid: implications for aging and age-related neurodegenerative disorders

Free radical-mediated damage to neuronal membrane components has been implicated in the etiology of Alzheimer's disease (AD) and aging. The senescence accelerated prone mouse strain 8 (SAMP8) exhibits age-related deterioration in memory and learning along with increased oxidative markers. Therefore, SAMP8 is a suitable model to study brain aging and, since aging is the major risk factor for AD and SAMP8 exhibits many of the biochemical findings of AD, perhaps as a model for and the early phase of AD. Our previous studies reported higher oxidative stress markers in brains of 12-month-old SAMP8 mice when compared to that of 4-month-old SAMP8 mice. Further, we have previously shown that injecting the mice with alpha-lipoic acid (LA) reversed brain lipid peroxidation, protein oxidation, as well as the learning and memory impairments in SAMP8 mice. Recently, we reported the use of proteomics to identify proteins that are expressed differently and/or modified oxidatively in aged SAMP8 brains. In order to understand how LA reverses the learning and memory deficits of aged SAMP8 mice, in the current study, we used proteomics to compare the expression levels and specific carbonyl levels of proteins in brains from 12-month-old SAMP8 mice treated or not treated with LA. We found that the expressions of the three brain proteins (neurofilament triplet L protein, alpha-enolase, and ubiquitous mitochondrial creatine kinase) were increased significantly and that the specific carbonyl levels of the three brain proteins (lactate dehydrogenase B, dihydropyrimidinase-like protein 2, and alpha-enolase) were significantly decreased in the aged SAMP8 mice treated with LA. These findings suggest that the improved learning and memory observed in LA-injected SAMP8 mice may be related to the restoration of the normal condition of specific proteins in aged SAMP8 mouse brain. Moreover, our current study implicates neurofilament triplet L protein, alpha-enolase, ubiquitous mitochondrial creatine kinase, lactate dehydrogenase B, and dihydropyrimidinase-like protein 2 in process associated with learning and memory of SAMP8 mice.

Functions of vitamin C as a mediator of transmembrane electron transport in blood cells and related cell culture models

Vitamin C (ascorbic acid) is an important physiological antioxidant. Within cells, it is practically always present in the reduced form. Several enzymatic and nonenzymatic mechanisms have been reported to maintain this status. In the extracellular environment, oxidation of ascorbate leads to loss of vitamin because the oxidized form, dehydroascorbic acid, is unstable under physiological conditions. The intermediate ascorbate free radical, although rather long-lived for a free radical, quickly disproportionates into the two other forms, also leading to loss of vitamin. Protection from loss can only be achieved by cellular regeneration mechanisms, i.e., by uptake of dehydroascorbic acid and either storage or recycling, and by plasma-membrane mediated reduction of extracellular free radical or dehydroascorbic acid. Moreover, intracellular ascorbate can also serve as an electron donor for transmembrane reduction of external electron acceptors. However, the physiological significance of this function is as yet unknown. The results presented in the literature are sometimes conflicting as to the relative contributions of these different possibilities, which seem to differ in different cell types. In this short review, the various pathways of regeneration of ascorbate and their relative contributions to the avoidance of vitamin loss in plasma or cell culture medium are discussed.

Purification of a dichlorophenol-indophenol oxidoreductase from rat and bovine synaptic membranes: tight complex association of a glyceraldehyde-3-phosphate dehydrogenase isoform, TOAD64, enolase-gamma and aldolase C

NADH-dichlorophenol-indophenol oxidoreductases (PMOs) were purified from synaptic plasma membranes or synaptic vesicles (small recycling vesicles) from both bovine and rat brains and from a neuroblastoma cell line, NB41A3. Several isoforms could be identified in purified plasma membranes and vesicles. Purification of the enzyme activity involved protein extraction with detergents, (NH4)2SO4 precipitation, chromatography under stringent conditions and native PAGE. PMO activity could be attributed to a very tight complex of several proteins that could not be separated except by SDS/PAGE. SDS/PAGE resolved the purified complex into at least five proteins, which could be micro-sequenced and identified unambiguously as hsc70, TOAD64 and glyceraldehyde-3-phosphate dehydrogenase tightly associated with the brain-specific proteins aldolase C and enolase-gamma. Enzyme activity could be purified from both synaptic plasma membranes and recycling vesicles, yields being much greater from the latter source. Highly purified plasma membranes (prepared from a neuroblastoma cell line NB41A3 by iminobiotinylation of intact cells and affinity purification with avidin and anti-avidin antibodies under very stringent conditions) also displayed PMO activity tightly associated with TOAD64. The association of PMO in a tight complex was confirmed by its immunoprecipitation from cellular and membrane extracts of NB41A3 using antibodies directed against any component protein of the complex followed by immunodetection with antibodies directed against the other members. Antibodies also inhibited the enzyme activity synergistically. In addition, induction of the different components of the complex during dichlorophenol-indophenol stress was demonstrated by the S1 RNase-protection assay in synchronized NB41A3 cells. The role of the complex in membrane fusion and cellular response to extracellular oxidative stress during growth and development is discussed.

Ascorbate is the major electron donor for a transmembrane oxidoreductase of human erythrocytes

Ascorbic acid is an important antioxidant in human blood. Erythrocytes contribute to the antioxidant capacity of blood by regenerating ascorbate and possibly by exporting ascorbate-derived reducing equivalents through a transmembrane oxidoreductase. The role of ascorbate as an electron donor to the latter enzyme was tested in human erythrocytes and ghosts using nitroblue tetrazolium as an electron acceptor. Although nitroblue tetrazolium was not directly reduced by ascorbate, erythrocyte ghosts facilitated reduction of nitroblue tetrazolium in the presence of ascorbate and ascorbate derivatives containing a reducing double bond. The resulting blue monoformazan product was deposited directly in ghost membranes. Ascorbate-induced monoformazan deposition showed several features of an enzyme-mediated process, including hyperbolic dependence on substrate and acceptor concentrations, as well as sensitivity to enzyme proteolysis, detergent solubilization, and sulfhydryl reagents. Incubation of intact erythrocytes with nitroblue tetrazolium caused deposition of the monoformazan in ghost membranes prepared from the cells. This deposition reflected the intracellular ascorbate content and was inhibited by extracellular ferricyanide, a known electron acceptor for the transmembrane oxidoreductase. Although nitroblue tetrazolium did not cross the cell membrane, like the cell-impermeant ferricyanide, it oxidized intracellular [14C]ascorbate to [14C]dehydroascorbate, which then exited the cells. In resealed ghosts, both monoformazan deposition and ferricyanide reduction were proportional to the intravesicular ascorbate concentration. NADH was only about half as effective as a donor for the enzyme as ascorbate in both open and resealed ghosts. These results suggest that not only can ascorbate donate electrons to a transmembrane oxidoreductase, but that it may be the major donor in intact erythrocytes.

Bombesin stimulates transplasma-membrane electron transport by Swiss 3T3 cells

Bombesin, a mitogenic neuropeptide, stimulates transplasmalemma reduction of diferric transferrin or ferricyanide by Swiss 3T3 cells. The stimulation of diferric transferrin reduction occurs in the range of bombesin concentrations that stimulate proliferation of Swiss 3T3 cells. Diferric transferrin reduction by the 3T3 cells is accompanied by increased proton release from the cells and bombesin increases the differic transferrin-stimulated proton release twofold. Insulin increases the diferric transferrin reductase response and increases growth stimulation with bombesin. The effect of bombesin on the transmembrane electron transport is a new aspect of its effect on the plasma membrane in addition to increase in phosphatidylinositol turnover and protein kinase c activation. The electron transport can provide an independent mechanism of activation of the Na+/H+ exchange or it can change the redox state of pyridine nucleotide in the cytoplasm.

The role of iron and iron binding proteins in lymphocyte physiology and pathology

Knowledge concerning the roles of iron and iron binding proteins in lymphocyte physiology and pathology has developed rapidly over the last few years. The genes for the major iron binding proteins have been cloned and sequenced and are now being studied with respect to transcriptional and posttranscriptional regulatory mechanisms. T cells, B cells, macrophages, and natural killer cells appear to differ from one another in the ways in which they synthesize and utilize iron binding proteins and in the amount of iron they take up and store. This suggests that differential modulation of iron-dependent metabolic functions is an intrinsic part of the distinctive physiology of each cellular component of the immune system and that the distribution of iron between those components is a carefully managed facet of the immune response. Since the immune response does not seem to be dramatically impaired by alterations in iron supplies that adversely affect other organs, it may well be that the cells of the immune system are especially adapted to have both high-priority access to iron when supply is low and high-level protection against iron-related toxicity when supply is in excess.

Characterization of the anti-cancer activity of transferrin-adriamycin conjugates

The anthracycline anti-cancer drug adriamycin (Adr) was coupled to human transferrin (Trf) by using a glutaraldehyde technique. The effect of Trf-Adr conjugates and unconjugated Adr on human cells was determined by using normal peripheral blood mononuclear cells and chronic myelogenous K562 cells. Cytotoxicity was determined by using an assay that measures the conversion of a tetrazolium salt (MTT) into a purple product (formazan) by mitochondrial dehydrogenases in viable cells. We found that free Adr at a concentration of 1 x 10(-7) had little effect on K562 cells, while Trf-Adr conjugates inhibited 75% of cellular activity. When normal peripheral blood mononuclear cells were tested against Trf-Adr conjugates, the 50% inhibitory concentration was found to be 1.4-1.7 x 10(-6) M, at which concentration greater than 85% of K562 cells were inhibited. Interactions of Trf-Adr conjugates with plasma membrane energy-producing systems are the proposed mechanisms of cytotoxicity.

Inhibition of transplasma membrane electron transport by transferrin-adriamycin conjugates

Transplasma membrane electron transport from HeLa cells, measured by reduction of ferricyanide or diferric transferrin in the presence of bathophenanthroline disulfonate, is inhibited by low concentrations of adriamycin and adriamycin conjugated to diferric transferrin. Inhibition with the conjugate is observed at one-tenth the concentration required for adriamycin inhibition. The inhibitory action of the conjugate appears to be at the plasma membrane since (a) the conjugate does not transfer adriamycin to the nucleus, (b) the inhibition is observed within three minutes of addition to cells, and (c) the inhibition is observed with NADH dehydrogenase and oxidase activities of isolated plasma membranes. Cytostatic effects of the compounds on HeLa cells show the same concentration dependence as for enzyme inhibition. The adriamycin-ferric transferrin conjugate provides a more effective tool for inhibition of the plasma membrane electron transport than is given by the free drug.