The effect of normal red cell constituents on the activities of red cell enzymes

Red Blood Cell 2,3-Diphosphoglycerate Decreases in Response to a 30 km Time Trial Under Hypoxia in Cyclists

Red blood cell 2,3-diphosphoglycerate (2,3-DPG) is one of the factors of rightward-shifted oxygen dissociation curves and decrease of Hb-O₂ affinity. The reduction of Hb-O₂ affinity is beneficial to O₂ unloading at the tissue level. In the current literature, there are no studies about the changes in 2,3-DPG level following acute exercise in moderate hypoxia in athletes. For this reason, the aim of this study was to analyze the effect of prolonged intense exercise under normoxic and hypoxic conditions on 2,3-DPG level in cyclists. Fourteen male trained cyclists performed a simulation of a 30 km time trial (TT) in normoxia and normobaric hypoxia (FiO₂ = 16.5%, ~2,000 m). During the TT, the following variables were measured: power, blood oxygen saturation (SpO₂), and heart rate (HR). Before and immediately after exercise, the blood level of 2,3-DPG and acid–base equilibrium were determined. The results showed that the mean SpO₂ during TT in hypoxia was 8% lower than in normoxia. The reduction of SpO₂ in hypoxia resulted in a decrease of average power by 9.6% (p < 0.001) and an increase in the 30 km TT completion time by 3.8% (p < 0.01) compared to normoxia. The exercise in hypoxia caused a significant (p < 0.001) decrease in 2,3-DPG level by 17.6%. After exercise in normoxia, a downward trend of 2,3-DPG level was also observed, but this effect was not statistically significant. The analysis also revealed that changes of acid–base balance were significantly larger (p < 0.05) after exercise in hypoxia than in normoxia. In conclusion, intense exercise in hypoxic conditions leads to a decrease in 2,3-DPG concentration, primarily due to exercise-induced acidosis.

The glycolytic enzyme glyceraldehyde-3-phosphate dehydrogenase works as an arsenate reductase in human red blood cells and rat liver cytosol

The mammalian enzymes responsible for reduction of the environmentally prevalent arsenate (AsV) to the much more toxic arsenite (AsIII) are unknown. In the previous paper (Nemeti and Gregus, 2005), we proposed that glyceraldehyde-3-phosphate dehydrogenase (GAPDH) and/or phosphoglycerate kinase (PGK) may catalyze reduction of AsV in human red blood cells (RBC), hemolysate, or rat liver cytosol. In testing this hypothesis, we show here that, if supplied with glutathione (GSH), NAD, and glycolytic substrate, the mixture of purified GAPDH and PGK indeed catalyzes the reduction of AsV. Further analysis revealed that GAPDH is endowed with AsV reductase activity, whereas PGK serves as an auxiliary enzyme, when 3-phosphoglycerate is the glycolytic substrate. The GAPDH-catalyzed AsV reduction required GSH, NAD, and glyceraldehyde-3-phosphate. ADP and ATP moderately, whereas NADH strongly inhibited the AsV reductase activity of the enzyme even in the presence of NAD. Koningic acid (KA), a specific and irreversible inhibitor of GAPDH, inhibited both the classical enzymatic and the AsV-reducing activities of the enzyme in a concentration-dependent fashion. To assess the contribution of GAPDH to the reduction of AsV carried out by hemolysate, rat liver cytosol, or intact erythrocytes, we determined the concentration-dependent effect of KA on AsV reduction by these cells and extracts. Inactivation of GAPDH by KA abolished AsV reduction in intact RBC as well as in the hemolysate and the liver cytosol, when GAPDH in the latter extracts was abundantly supplied with exogenous NAD and glycolytic substrate. However, despite complete inactivation of GAPDH by KA, the hepatic cytosol exhibited significant residual AsV-reducing activity in the absence of exogenous NAD and glycolytic substrate, suggesting that besides GAPDH, other cytosolic enzyme(s) may contribute to AsV reduction in the liver. In conclusion, the key glycolytic enzyme GAPDH can fortuitously catalyze the reduction of AsV to AsIII, if GSH, NAD, and glycolytic substrate are available. AsV reduction may take place during, or as a consequence of, the arsenolytic cleavage of the thioester bond formed between the enzyme's Cys149 and the 3-phosphoglyceroyl moiety of the substrate. Although GAPDH is exclusively responsible for reduction of AsV in human erythrocytes, its role in AsV reduction in vivo remains to be determined.

Characterization of phosphofructokinase-deficient canine erythrocytes

Dogs homozygously affected with muscle-type phosphofructokinase (PFK) deficiency had about 20% of normal erythrocyte PFK activity and exhibited a compensated haemolytic anaemia. Erythrocyte glucose-6-phosphate and fructose-6-phosphate concentrations were increased and dihydroxyacetone phosphate and 2,3-bisphosphoglycerate values were below normal in affected dogs. Other intermediates distal to the PFK step were not significantly below normal and fructose-1,6-bisphosphate was even above normal. Erythrocyte ATP was higher than normal in affected dogs owing to the reticulocytes present. Abnormal adenylate metabolism was demonstrated by low ATP/AMP and ADP/AMP ratios and the inability to maintain ATP content when affected erythrocytes were incubated with cyanide. Glucose-1,6-bisphosphate content was normal, and fructose-2,6-bisphosphate content in affected canine erythrocytes was higher than normal. Studies of erythrocyte PFK isozymes revealed altered enzyme kinetic properties in affected dogs which appeared to be due to the loss of the M-type subunit.

Enhancement of immune status by high levels of dietary vitamin B-6 without growth inhibition of human malignant melanoma in athymic nude mice

The effects of dietary vitamin B-6 supplementation on the development of human malignant melanoma (M21-HPB) xenografts and on in vitro responses of leukocytes were examined. Male athymic nude mice, five weeks old, were divided into two groups of 48 each and fed 20% casein diets containing pyridoxine (PN) at 4.1 (control diet) and 61.6 mg/kg diet for 10 weeks. After four weeks of dietary treatment, 20 animals from each dietary group were injected subcutaneously with 3 x 10(7) melanoma cells. After 4, 8, and 10 weeks of dietary regimen, animals from each group were killed and blood, liver, and spleen samples were obtained. Food consumption and mouse body weights were similar between groups, and no difference was noted in tumor incidence or volume. Noninjected and tumor-bearing mice given the PN 61.6 diet generally exhibited greater oxygen radical production by phagocytic cells from blood and spleen than did animals fed the PN 4.1 diet. Spleen and blood B lymphocyte proliferation in response to lipopolysaccharide (LPS) was enhanced (10 and 30%) in the noninjected animals given the PN 61.6 diet. In addition, tumor-bearing mice fed the PN 61.6 diet had significantly greater LPS-induced spleen cell proliferation at eight weeks when compared with mice consuming the PN 4.1 diet. Despite immune enhancement, tumor incidence and progression was not modified by a high level of dietary vitamin B-6. Therefore, it is tempting to speculate that tumor inhibition by high dietary vitamin B-6 may be mediated by T lymphocyte-dependent mechanisms that are lacking in these genetically immuno-deficient mice.

Studies on the pigeon red blood cell metabolism

1. Pigeon erythrocyte was found to depend on the glycolytic and pentose phosphate pathway for most of its energy production in the form of adenosine triphosphate and reducing potential, since there was no detectable activity of any of the citric acid cycle (TCA) cycle enzymes measured. 2. The absence of detectable amounts of 2,3-diphosphoglyceric acid (2-3-DPG) indicated that there is no direct relationship between the active glycolytic system and the function of these cells. 3. A comparison of the mass action ratios with the equilibrium constants of the glycolytic reactions showed that hexokinase, phosphofructokinase and pyruvate kinase reactions are displaced from equilibrium, implying that these are the key regulatory enzymes of glycolysis in pigeon erythrocytes. 4. The changes in the concentrations of the glycolytic metabolites under hypoxic conditions that stimulate the flux through the glycolytic pathway were found to be consistent with the above hypothesis. 5. Flux measurements of the pentose phosphate pathway showed that it metabolizes only 3.4% of the total glucose consumed by the resting erythrocyte. 6. Hypoxic conditions resulted in a stimulation of the pentose phosphate pathway by as much as four-fold, whilst the glycolytic pathway was not stimulated by more than about twice.

Studies on the energy metabolism of opossum (Didelphis virginiana) erythrocytes--VI. De novo purine nucleotide biosynthesis is limited to the final steps of the pathway in vitro

1. High pressure liquid radiochromatography was used to show the incorporation of [14C] formate with Z-compounds into ATP and GTP in opossum erythrocytes. 2. The use of Z-riboside with [14C] formate resulted in more extensive labeling of ATP than the Z-base/[14C] formate combination as substrates for nucleotide biosynthesis. 3. Substantial accumulation of ZMP and ZTP, but no ZDP was detected in the chromatograms. 4. ATP was unstable in red cells metabolizing in the presence of Z-compounds under an atmosphere of air as gas phase in these experiments.

Effects of fructose 2,6-bisphosphate and glucose 1,6-bisphosphate on phosphofructokinase from chicken erythrocytes

1. Phosphofructokinase (EC 2.7.1.11) from chicken erythrocytes is activated by fructose 2,6-bisphosphate, glucose 1,6-bisphosphate and AMP, and it is inhibited by 2,3-bisphosphoglycerate and inositol hexaphosphate. 2. The stimulatory effects produced by the two bisphosphorylated hexoses are additive and the effects produced by fructose 2,6-bisphosphate and by AMP are synergistic. 3. The activatory effect produced by fructose 2,6-bisphosphate is counteracted by fructose 1,6-bisphosphate. 4. The inhibition produced by both 2,3-bisphosphoglycerate and inositol hexaphosphate is released by fructose 2,6-bisphosphate. 5. It is concluded that, like phosphofructokinase from mammalian tissues, the enzyme from chicken erythrocytes can be modulated by the relative concentrations of those metabolites.

Levels of glycerate 2,3-P2, 2,3-bisphosphoglycerate synthase and 2,3-bisphosphoglycerate phosphatase activities in rat tissues. A method to quantify blood contamination of tissue extracts

The levels of glycerate 2,3-P2 and of 2,3-bisphosphoglycerate synthase and 2,3-bisphosphoglycerate phosphatase activities have been determined in isolated rat hepatocytes and adipocytes and in perfused rat tissues to discard blood contamination. The values obtained are much lower than those previously reported, ranging 0.50-40 nmol/g tissue. No relationship appears to exist between glycerate 2,3-P2 concentration and the levels of the enzymatic activities involved in glycerate 2,3-P2 metabolism. Assay of glycerate 2,3-P2 in tissue extracts constitute a very useful way to quantify blood contamination.

Regulatory properties of human erythrocyte hexokinase during cell ageing

Human red blood cell hexokinase exists in multiple molecular forms with different isoelectric points but similar kinetic and regulatory properties. All three major isoenzymes (HK Ia, Ib, and Ic) are inhibited competitively with respect to Mg.ATP by glucose 6-phosphate (Ki = 15 microM), glucose 1,6-diphosphate (Ki - 22 microM), 2,3-diphosphoglycerate (Ki = 4 mM), ATP (Ki = 1.5 mM), and reduced glutathione (Ki = 3 mM). All these compounds are present in the human erythrocyte at concentrations able to modify the hexokinase reaction velocity. However, the oxygenation state of hemoglobin significantly modifies their free concentrations and the formation of the Mg complexes. The calculated rate of glucose phosphorylation, in the presence of the mentioned compounds, is practically identical to the measured rate of glucose utilization by intact erythrocytes (1.43 +/- 0.15 mumol h-1 ml red blood cells-1). Hexokinase in young red blood cells is fivefold higher when compared with the old ones, but the concentration of many inhibitors of the enzyme is also cell age-dependent. Glucose 6-phosphate, glucose 1,6-diphosphate, 2,3-diphosphoglycerate, ATP, and Mg all decay during cell ageing but at different rates. The free concentrations and the hemoglobin and Mg complexes of both ATP and 2,3-diphosphoglycerate with hemoglobin in the oxy and deoxy forms have been calculated. This information was utilized in the calculation of glucose phosphorylation rate during cell ageing. The results obtained agree with the measured glycolytic rates and suggest that the decay of hexokinase during cell ageing could play a critical role in the process of cell senescence and destruction.

In vivo and in vitro inhibition of B16 melanoma growth by vitamin B6

The effect of vitamin B6 on the growth of B16 melanoma cells in vivo and in vitro was studied. B16 melanoma cells grown for three days in medium supplemented with 5.0 mM pyridoxine or 0.5 mM pyridoxal showed an 80% reduction in cell proliferation compared with control culture. Cells cultured for six hours in medium supplemented with 0.5 mM pyridoxal took up and incorporated 13 and 32% less [3H]thymidine, respectively, than did control cultures. A 17% reduction in [3H]glucose uptake was observed at this time point. When the incubation time was decreased to three hours, an inhibition of cellular uptake of [3H]thymidine (22%), [3H]uridine (14%), and [3H]glucose (15%) was observed; however, little or no inhibition in incorporation was detected. In in vivo studies, mice pretreated with pyridoxal for two weeks and then injected with B16 melanoma cells had a 62% reduction in tumor weight compared with controls at the end of a three-week period. If tumors were first established in mice and then treated with pyridoxal for six days, a 39% reduction in tumor growth was observed. There were no differences observed in body weights or liver weights in any of the animal groups. These results indicate that supraphysiological doses of vitamin B6 can inhibit the growth of B16 melanoma cells both in vitro and in vivo. The exact mechanism by which pyridoxal exerts its inhibitory effect was not ascertained, but experiments suggest that the vitamer may be acting on the plasma membrane to reduce precursor transport into the cell.

Regulatory properties of rabbit red blood cell hexokinase at conditions close to physiological

The true level of hexokinase in rabbit erythrocytes was determined by three different methods, including the spectrophotometric glucose-6-phosphate dehydrogenase coupled assay and a new radioisotopic assay. The value found at 37 degrees C (pH 7.2) was 10.23 +/- 1.90 mumol/h per ml red blood cells, which is lower than previously reported values. More than 40 cellular components of the rabbit erythrocytes were tested for their effects on the enzyme. Their intracellular concentrations were also determined. Several of these compounds were found to be competitive inhibitors of the enzyme with respect to Mg X ATP2-. Furthermore, reduced glutathione at a concentration of 1 mM was able to maintain hexokinase in the reduced state with full catalytic activity. The ability of orthophosphate to remove the inhibition of some phosphorylated compounds was examined under conditions similar to cellular (pH 7.2 and 50 microM of orthophosphate) and found to be of no practical interest. In contrast, the binding of ATP4- and 2,3-diphosphoglycerate to the rabbit hemoglobin significantly modifies their intracellular concentrations and the formation of the respective Mg complexes. The pH-dependence of the reaction velocity and of the kinetic properties of the enzyme in different buffer systems were also considered. This information was computerized, and the rate of glucose phosphorylation in the presence of the mentioned compounds was determined. The value obtained, 1.94 +/- 0.02 mumol/h per ml red blood cells, is practically identical to the measured rate of glucose utilization by intact rabbit erythrocytes (1.92 +/- 0.3 mumol/h per ml red blood cells). These results provide further evidence for the central role of hexokinase in the regulation of red blood cell glycolysis.

High-dose pyridoxal supplemented culture medium inhibits the growth of a human malignant melanoma cell line

The in vitro growth characteristics of a human malignant melanoma cell line cultured in 0.5 mM pyridoxal supplemented medium were studied. Experimentation revealed that the high-dose pyridoxal supplemented medium severely inhibited the growth of melanoma cells over a 72-hour growth period. Additional experimentation showed that cells cultured for 6 hours in the pyridoxal supplemented medium took up 25% less and incorporated 20% less [3H]uridine than control cultures. [3H]Glucose uptake was reduced by 23% at this time point. [3H]Thymidine uptake was inhibited by 12%, but no inhibition was detected in [3H]thymidine incorporation. When the vitamin B6 antagonist 4-deoxypyridoxine (which competes with pyridoxal for pyridoxal kinase) was added to the pyridoxal supplemented medium, the inhibition in [3H]uridine incorporation was reduced from 19% to 6%. However, 4-deoxypyridoxine did not reverse the inhibition of [3H]uridine uptake. These results indicate that pyridoxal and its metabolite, pyridoxal 5'-phosphate, may be involved in the growth regulation of a human malignant melanoma cell line.

Vitamin B6 kills hepatoma cells in culture

Data are presented which indicate that vitamin B6 (pyridoxine) can retard and eventually kill Fu5-5 rat hepatoma cells in culture. Additional studies indicate that the human kidney cell line 293-31, and a rat glial cell strain. C6, display growth characteristics similar to those of hepatoma cells when cultured in 5 mM pyridoxine-supplemented medium. However, the pyridoxine-supplemented culture medium had little effect on the growth of the human mammary cancer cell line MCF-7. The resistance of the MCF-7 cells to growth inhibition by vitamin B6 suggests that the vitamer pyridoxine must be metabolized by pyridoxal before it can act as a growth inhibitor. These findings suggest the potential use of vitamin B6 as an antineoplastic agent. Possible mechanisms by which vitamin B6 promotes this effect are presented.

Functional restoration of ACD-blood by pyrodoxal 5'-phosphate

Successful application of pyridoxal 5'-phosphate (PLP) to restore the oxygen transport function of ACD-stored blood is described. PLP is readily incorporated into ACD-erythrocytes by both carrier-mediated transport (in which ATP may participate) and passive diffusion. Plasma proteins (up to 2.5%) and inorganic phosphate (up to 40 mM) do not affect the incorporation of PLP, though more than 25 mM inorganic phosphate is necessary for the maintenance of ATP levels. Increasing the PLP concentration and/or decreasing the packed cell volume in the medium, increases the incorporation of PLP. Incubation of erythrocytes with PLP at pH 7.0 is most suitable for incorporation of PLP and the maintenance of ATP levels. PLP incorporated into erythrocytes restores the oxygen transport function of the ACD-erythrocytes, though decreased haem--haem interaction is observed. A procedure for the clinical application of PLP-loaded erythrocytes is suggested.

Structure and function of normal and variant human phosphoglycerate kinase

Complete amino acid sequence of normal human phosphoglycerate kinase (PGK) was determined. The enzyme consists of 417 amino acid residues with acetylserine at the NH2-terminal and isoleucine at the COOH-terminal. The structural abnormality of PGK-II, which is fairly common in Southern Pacific populations, is a single amino acid substitution from threonine in the normal enzyme to asparagine in the variant enzyme at the 352nd position. The substitution induced no change in the enzyme activity, but induced strong binding of the variant enzyme with citrate. The structural abnormality of PGK-München is a single amino acid substitution from aspartic acid in the normal enzyme to asparagine in the variant enzyme at the 267th position. PGK-München is associated with red cell enzyme deficiency (about 20% of normal), and substantial heat instability. Therefore, the negative charge of an aspartyl residue at the 267th position must play a role in maintaining the stability of the enzyme molecule. Possible mechanisms of hemolysis due to hereditary deficiency of PGK are discussed.

Functional changes associated with the sequential transformation of L'4 into L4 pyruvate kinase

The functional changes, associated with the sequential transformation of L'4 into L4 pyruvate kinase (ATP:pyruvate 2-O-phosphotransferase, EC 2.7.1.40) were studied. L'4 enzyme from human erythrocytes shows strong hysteretic behaviour: the initial rate of the enzyme preincubated with an unsaturating concentration of phosphoenolpyruvate is much higher than of the enzyme preincubated with ADP, at the same phosphoenolpyruvate concentration, although the "final activity" (the activity of the linear part of the reaction progress curve) was the same in both cases. This phenomenon was observed both in the presence and absence of fructose 1,6-diphosphate. High concentrations of both Mg2+free and MgATP2- diminish the difference in initial rate, between the ADP and phosphoenolpyruvate preincubated enzymes: Mg2+free by stabilizing the phosphoenolpyruvate-induced form; ATPMg2- by stabilizing the ADP-induced form. The magnitude of the difference in initial rates of the ADP-or phosphoenolpyruvate-preincubated enzyme is a function of both substrates. L4 pyruvate kinase (either from human liver or trypsin treated L'4 enzyme) does not, or to a very slight extent, show such behaviour. L'2L2 pyruvate kinase shows behaviour intermediate between L'4 and L4 enzymes. A model is proposed to describe the kinetic behaviour of L'4 and L4 enzymes.

Effect of pyridoxal 5'-phosphate on the oxygen affinity of human erythrocytes

Pyridoxal 5'-phosphate (PLP), an allosteric effector for the oxygenation of haemoglobin, was incorporated readily into erythrocytes and disappeared from them by simple passive diffusion. The disappearance of PLP from the cells was accelerated by the generation of 2,3-DPG in a medium of inosine, pyruvate and phosphate. The oxygen dissociation curve measured at an extracellular pH of 7.4 demonstrated that PLP incorporated into the cells also lowered the oxygen affinity and that PLP functionally compensated for a metabolically reduced 2,3-DPG. However, the dependency of the oxygen affinity on the intracellular PLP concentration showed a different pattern from the observed for 2,3-DPG. On the other hand, the lowering of intracellular pH by organic phosphates accumulated in the cells was much larger with PLP than with 2,3-DPG. The peculiar relationship between the oxygen affinity of erythrocytes and the intracellular PLP concentration is discussed in detail. The present study may offer a new prospect for the preservation of blood with a normal function.

Multifunctional enzyme, bisphosphoglyceromutase/2,3-bisphosphoglycerate phosphatase/phosphoglyceromutase, from human erythrocytes. Evidence for a common active site

Bisphosphoglyceromutase and 2,3-bisphosphoglycerate phosphatase activities responsible for 2,3-bisphosphoglycerate metabolsim in human red cells are displayed by the same enzyme protein which has phosphoglyceromutase activity [Sasaki, R., et al. (1975) Eur J. Biochem. 50, 581-593]. This enzyme was subjected to chemical modification by trinitrobenzenesulfonate. The three enzyme activities were inactivated by trinitrobenzenesulfonate at the same rate. The sulfhydryl content of the enzyme was unchanged during trinitrophenylation, indicating that derivatization was through the amino group. Trinitrophenylation of about one amino group per mole of the enzyme resulted in complete loss of the three activities. Both 2,3-bisphosphoglycerate and 1,3-bisphosphoglycerate inhibited trinitrophenylation and effectively protected the enzyme from inactivation. Although monophosphoglycerates did not show any protective effect at concentrations which should be adequate based upon their kinetic constants, they were protective at higher concentrations. Inactivation by trinitrophenylation was an apparent first-order reaction. The dissociation constant of the enzyme - 2,3-bisphosphoglycerate complex was determined by analyzing the first-order reaction on the assumption that the protective effect of 2,3-bisphosphoglycerate was due to competition with trinitrobenzenesulfonate. The dissociation constant was in good agreement with kinetic constants of 2,3-bisphosphoglycerate in the enzyme reactions, which indicated that 2,3-bisphosphoglycerate did indeed exert its protective effect through competition with trinitrobenzenesulfonate for an amino group of the enzyme. The protective effect of monophosphoglycerates could be rationalized with kinetic evidence that 2-phosphoglycerate at high concentrations interacts with the 2,3-bisphosphoglycerate binding site. These results indicate that the enzyme exhibits the three enzyme activities at a common active site at which one amino group essential for binding of bisphosphoglycerates is located. Based on the multifunctional properties of this enzyme, a possible mechanism was discussed for regulation of 2,3-bisphosphoglycerate metabolism in human red cells.

Glycolysis in human erythrocytes containing elevated concentrations of 2, 3-P2-glycerate

In studies on the mechanism of the inhibitory effect of 2, 3-diphosphoglycerate on glycolysis in human erythrocytes, the following results were obtained: 1) Glucose consumption and lactate production are reduced by 70 and 40 per cent relative to normal erythrocytes in red blood cells containing five times the normal amount of 2, 3, -P2-glycerate ("high-diphosphoglycerate" cells) at an extracellular pH of 7.4. The marked dependency of glycolysis on the extracellular pH observed in normal erythrocytes is almost completely lost in the "high-diphosphoglycerate" cells. 2) About 50 per cent of the inhibition of glycolysis in "high-diphosphoglycerate" cells can be accounted for by the 2, 3-P2-glycerate-induced decrease of the red-cell pH. This fall of the red-cell pH which occurs as a conswquence of the Donnan effect of the non-pentrating 2, 3-P2-glycerate anion leads to a reduction of the glycolytic rate due to the properties of the enzyme phosphofructokinase. 3) The remaining part of the inhibitory effect must be attributed to an inhibition by 2, 3-P2-glycerate of glycolytic enzymes. From measurements of glycolytic rates and of the concentrations of glycolytic intermediates in the absence and presence of methylene blue it is concluded that the hexokinase reaction is inhibited by an elevation of 2, 3-P2-glycerate concentration in "high-diphosphoglycerate" cells suggests that also the enzyme pyruvate kinase is inhibited by 2, 3-P2-glycerate. 4) The dependencies of net-change of 2, 3-P2-glycerate concentration on the red-cell pH are identical in normal and "high-diphosphoglycerate" cells indicating that the balance between formation and decomposition of 2, 3-P2-glycerate is the same in erythrocytes with normal and very high compositions of 2, 3-P2-glycerate.

Vitamin B6 metabolism in chronic alcohol abuse. Pyridoxal phosphate levels in plasma and the effects of acetaldehyde on pyridoxal phosphate synthesis and degradation in human erythrocytes

The plasma pyridoxal-5'-phosphate (PLP) level of alcoholic subjects has been compared with that of non-alcoholic individuals in order to ascertain the incidence of abnormal vitamin B(6) metabolism in chronic alcohol abuse. 66 alcoholic subjects were selected on the basis that they did not exhibit abnormal liver function tests and hematologic findings. 35 of them had plasma PLP concentrations less than 5 ng/ml, the lowest value encountered in 94 control subjects, indicating a high incidence of deranged PLP metabolism in alcoholic patients even when hepatic and hematologic abnormalities are absent. The biochemical basis for the altered PLP metabolism in chronic alcohol abuse was examined. Low plasma PLP levels in alcoholics were not accompanied by decreased pyridoxal kinase and pyridoxine phosphate oxidase activities in erythrocytes. Further studies with erythrocytes demonstrated that the cellular content of PLP is determined not only by the activities of these PLP-synthesizing enzymes but also by the activity of a phosphate-sensitive, membrane-associated, neutral phosphatase, which hydrolyzes phosphorylated B(6) compounds.Acetaldehyde, but not ethanol, impaired the net formation of PLP from pyridoxal, pyridoxine, and pyridoxine phosphate by erythrocytes. However, when the B(6)-phosphate phosphatase activity was inhibited by 80 mM phosphate, this effect of acetaldehyde was abolished. By the use of broken cell preparations, it was possible to demonstrate directly that the action of acetaldehyde is mediated by the phosphatase, resulting in an acceleration of the degradation of the phosphorylated B(6) compounds in erythrocytes.