Carbon balance studies with various substrates in human erythrocytes.

Carbon metabolism in erythrocytes has been found to be in balance with a variety of substrates studied. The contribution of the 2,3-bisphosphoglycerate pathway to carbon metabolism depends on the rates of carbon utilization and increases with increasing metabolic rates. Net decrease of the 2,3-bisphosphoglycerate level acts as a NAD regenerating system thus facilitating uptake of polyalcohols such as xylitol.     

Interrelationship between energy metabolism from various substrates and the 2,3-bisphosphoglycerate bypass in human erythrocytes

Metabolism of the substrates D-ribose, xylitol, D-Xylulose, D-fructose, D-glucose and mixtures of these compounds were studied in human erythrocytes. The metabolic rates obtained with the various substrates affected the intracellular levels of ATP and 2,3-bisphosphoglycerate. Small amounts of substrate utilization resulted in a decrease of the ATP and more pronounced of the 2,3-bisphosphoglycerate concentration while carbon utilization rates beyound 14 microgram atom C/ml packed cells/120 min yielded constant levels of ATP and 2,3-bisphosphoglycerate. From these results it can be concluded that a carbon utilization rate of 14 microgram atom C/ml cells/120 min is able to cover the ATP requirement of the red cells under steady state conditions. Based on the carbon utilization rates obtained with the various substrates and the rates of 2,3-bisphosphoglycerate decomposition an attempt is made to calculate the contribution of the 2,3-bisphosphoglycerate bypass to substrate metabolism. In case of xylitol as substrate the decrease in the 2,3-bisphosphoglycerate content provides the regeneration of NAD thus facilitating uptake and metabolism of xylitol.     

Reductive metabolism of nitroprusside in rat hepatocytes and human erythrocytes.

The metabolism of nitroprusside by hepatocytes or subcellular fractions involves a one-electron reduction of nitroprusside to the corresponding metal-nitroxyl radical. Thiol compounds also reduced nitroprusside to the metal-nitroxyl radical apparently via a thiol adduct. The nitroprusside reduction by microsomes was shown to be due to cytochrome P450 reductase as an antibody to cytochrome P450 reductase inhibits the microsomal reduction of nitroprusside, and the inhibitors of cytochrome P450 such as carbon monoxide or metyrapone had no effect. The reduction of nitroprusside by mitochondria in the presence of NADH or NADPH also produced the metal-nitroxyl radical. In hepatocytes, both mitochondria and the cytochrome P450 reductase are involved in the reduction of nitroprusside. The reductive metabolism of nitroprusside was found to produce toxic by-products, namely, free cyanide anion and hydrogen peroxide. We have also detected thiyl radicals formed in the thiol compound reduction of NP. We propose that cyanide and hydrogen peroxide are important toxic species formed in the metabolism of nitroprusside. The rate of reductive metabolism of nitroprusside by rat hepatocytes was much higher than with human erythrocytes. Therefore the major site of nitroprusside metabolism in vivo may be liver and not blood as originally proposed.     

The purine metabolism of human erythrocytes

This review summarizes currently available information about a crucial part of erythrocyte metabolism, that is, purine nucleotide conversions and their relationships with other conversion pathways. We describe the cellular resynthesis, interconversion, and degradation of purine compounds, and also the regulatory mechanisms in the conversion pathways. We also mention purine metabolism disorders and their clinical consequences. The literature is fragmentary because studies have concentrated only on selected aspects of purine metabolism; hence the need for a synthetic approach. Published in Russian in Biokhimiya, 2006, Vol. 71, No. 5, pp. 581–591.     

Effect of exogenous hydrogen peroxide on human erythrocytes

Circulating erythrocytes are drastically susceptible to peroxidative reactions. To examine the extent of the damage induced by exogenous H2O2 we limited the catalase activity in order to study the extent of lysis, the lipid peroxidation and namely the behaviour of membrane micro-viscosity. Our data showed that the erythrocytes can efficiently scavenge exogenous H2O2 without significant damage of the cells and/or their membranes. These findings could confirm the important role of the erythrocytes as extracellular-antioxidant defense.     

Vanadate affects glucose metabolism of human erythrocytes

Vanadate causes a rapid breakdown of 2,3-bisphosphoglycerate in intact erythrocytes. This metabolite is nearly stoichiometrically transformed into pyruvate, which changes the cell redox state and enhances the glycolytic flux. The results show that the vanadate effect on 2,3-bisphosphoglycerate, also evident in hemolysates, is attributable to the stimulation of a phosphatase activity of the phosphoglycerate mutase. In agreement with others (J. Carreras, F. Climent, R. Bartrons and G. Pons (1982) Biochim. Biophys. Acta705, 238–242), vanadate is thought to destabilize the phosphoryl form of this enzyme which shows competitive inhibition between the ion and 2,3-bisphosphoglycerate in the mutase reaction. A competitive inhibition between vanadate and glucose 1,6-bisphosphate is also found for phosphoglucomutase, without evidence for phosphatase activity toward the bisphosphate cofactor.     

Role of aminotransferases in glutamate metabolism of human erythrocytes

Human erythrocytes require a continual supply of glutamate to support glutathione synthesis, but are unable to transport this amino acid across their cell membrane. Consequently, erythrocytes rely on de novo glutamate biosynthesis from α-ketoglutarate and glutamine to maintain intracellular levels of glutamate. Erythrocytic glutamate biosynthesis is catalyzed by three enzymes, alanine aminotransferase (ALT), aspartate aminotransferase (AST), and glutamine aminohydrolase (GA). Although the presence of these enzymes in RBCs has been well documented, the relative contributions of each pathway have not been established. Understanding the relative contributions of each biosynthetic pathway is critical for designing effective therapies for sickle cell disease, hemolytic anemia, pulmonary hypertension, and other glutathione-related disorders. In this study, we use multidimensional (1)H-(13)C nuclear magnetic resonance (NMR) spectroscopy and multiple reaction mode mass spectrometry (MRM-MS) to measure the kinetics of de novo glutamate biosynthesis via AST, ALT, and GA in intact cells and RBC lysates. We show that up to 89% of the erythrocyte glutamate pool can be derived from ALT and that ALT-derived glutamate is subsequently used for glutathione synthesis.     

Adenosine uptake, transport, and metabolism in human erythrocytes

Using rapid kinetic techniques, we have determined the kinetics of zero-trans influx and equilibrium exchange of adenosine, and its uptake and in situ phosphorylation at 25 degrees C in human erythrocytes which were pretreated with 2'-deoxycoformycin to inhibit deamination of adenosine. Both the Km and Vmax for adenosine transport were about 300 times higher than those for the in situ phosphorylation of adenosine (Km about 0.2 microM), so that the first order rate constants for both processes were about the same. In contrast, the first order rate constant for adenosine deamination by untreated, intact cells was about 20% of that of adenosine transport or phosphorylation. These kinetic properties of the various steps, in combination with substrate inhibition of adenosine phosphorylation above 1 microM adenosine, assure that, at extracellular concentrations of physiological relevance (less than 1 microM), adenosine is very rapidly and efficiently salvaged by the erythrocytes and converted to ATP, whereas at extracellular concentrations of 10 microM or higher, practically all adenosine transported into the cells is deaminated. When the concentration of adenosine was 0.1 microM, a 10% (v/v) suspension of erythrocytes depleted the extracellular fluid of adenosine within 1 min of incubation at 25 degrees C.     

Studies in the enzymology of glutathione metabolism in human erythrocytes

Spectrophotometric assay methods are described for glutathione synthetase, gamma-glutamylcysteine synthetase and gamma-glutamyl transpeptidase of erythrocytes. The contents of these enzymes in normal human erythrocytes are reported. Erythrocyte glutathione synthetase is inhibited by ADP; this inhibition is competitive with respect to ATP. gamma-Glutamylcysteine synthetase is subject to feedback inhibition by GSH, and is also inhibited by NADH, and to a lesser extent by NAD(+) and NADPH. This enzyme is irreversibly inactivated by cysteamine.     

Antihemolytic effect of various benzodiazepines on human erythrocytes]

The benzodiazepines, medazepam, chlorodiazepoxyde, and oxazepam, act antihemolytically just as chloropromazine in hypotonic NaCl solution on human erythrocytes. The effect depends on concentration and is biphasic with the exception of oxazepam. The maximal antihemolytic concentrations are for medazepam 9.1-10-5 M, for chlorodiazepoxyde 4.6-10-4 M and, respectively, 9.1-10-4 M for diazepam and oxazepam. No linear correlation exists between the concentration at maximal hemolytic protection and the lipid solubility.     

Selective reduction of xylose to xylitol from a mixture of hemicellulosic sugars

The biocatalytic reduction of d-xylose to xylitol requires separation of the substrate from l-arabinose, another major component of hemicellulosic hydrolysate. This step is necessitated by the innate promiscuity of xylose reductases, which can efficiently reduce l-arabinose to l-arabinitol, an unwanted byproduct. Unfortunately, due to the epimeric nature of d-xylose and l-arabinose, separation can be difficult, leading to high production costs. To overcome this issue, we engineered an E. coli strain to efficiently produce xylitol from d-xylose with minimal production of l-arabinitol byproduct. By combining this strain with a previously engineered xylose reductase mutant, we were able to eliminate l-arabinitol formation and produce xylitol to near 100% purity from an equiweight mixture of d-xylose, l-arabinose, and d-glucose.