Neil A. Lachant

Antioxidants in Sickle Cell Disease: The In Vitro Effects of Ascorbic Acid

The authors examined the ability of antioxidants to prevent in vitro oxidant damage to the sickle red blood cell (RBC). One millimolar ascorbic acid and α-mercaptopropionyl-glycine significantly (p < 0.005) protected against RBC Heinz body formation during incubation with acetylphenylhydrazine, while cysteine, cysteamine, and methionine did not. The effect of ascorbic acid was concentration dependent with concentrations as low as 0.1 mM having significant antioxidant effects. Ascorbic acid protected the RBC against hydrogen peroxide induced hemolysis as well (p < 0.05). Ascorbic acid had a significant stimulatory effect on the rate of glucose oxidation by the pentose phosphate shunt (PPS), especially in the sickle RBC. Ascorbic acid did not protect the RBC from a patient with chronic hemolytic anemia due to G6PDTorrance from Heinz body formation, suggesting that an intact PPS is necessary for ascorbic acid to express its antioxidant properties. These data suggest that clinical trials should be undertaken to examine the efficacy of ascorbic acid in the treatment of SCD.

Case Report: Vaso-occlusive Crisis-Associated Neutrophil Dysfunction in Patients with Sickle-Cell Disease

ABSTRACT Polymorphonuclear leukocyte (PMN) aggregation and chemotaxis were studied in 27 patients with sickle cell disease (SCD). Pain-free patients with SCD had a significantly impaired aggregation response to stimulation with n-formylmethionyl-leucyl-phenylalanine (FMLP) with or without cytochalasin B (CB), compared with normal volunteers (p

Inhibition of the pentose phosphate shunt by 2,3-diphosphoglycerate in erythrocyte pyruvate kinase deficiency

Summary. Pentose phosphate shunt activity was studied by the release of 14CO2 from 14C‐1‐glucose and 14C‐2‐glucose in the red cells of five patients with pyruvate kinase deficiency and found to be significantly decreased after new methylene blue stimulation when compared to high reticulocyte controls. Incubated Heinz body formation was increased and the ascorbate cyanide test was positive in blood from these patients. The activity of glucose‐6‐phosphate dehydrogenase (G6PD) as well as that of 6‐phosphogluconate dehydrogenase (6PGD) was inhibited to 20% of baseline in normal red cell haemolysate by 4 mM 2,3‐diphosphoglycerate at pH 7·1. 2,3‐Diphosphoglycerate was a competitive inhibitor with 6‐phosphogluconate (Ki= 1·05 mM) and a noncompetitive inhibitor with NADP (Ki= 3·3 mM) for 6PGD. Since the intracellular concentrations of glucose‐6‐phosphate, 6‐phosphogluconate and NADP are below their Kms for G6PD and 6PGD, the kinetic data suggest that increased concentrations of 2,3‐diphosphoglycerate in pyruvate kinase deficient red cells are sufficiently high to suppress pentose phosphate shunt activity. This suppression may be an additional factor contributing to the haemolytic anaemia of pyruvate kinase deficiency, particularly during periods of infection or metabolic stress.

Hemolytic anemia in hereditary pyrimidine 5'-nucleotidase deficiency. II. Effect of pyrimidine nucleotides and their derivatives on glycolytic and pentose phosphate shunt enzyme activity.

We evaluated the glycolytic intermediate concentrations from the erythrocytes of a patient with hereditary pyrimidine 5-nucleotidase (P5N) deficiency. Conclusive evidence for a metabolic block was not found. We evaluated the effects of the pyrimidine (cytidine and uridine) tri- and diphosphate nucleotides (CTP, CDP, UTP, UDP) and the choline and ethanolamine derivatives of CDP (CDP-choline, CDP-ethanolamine) on the activities of key enzymes of the Embden-Meyerhof pathway. CTP and UTP inhibited fructose-6-phosphate competitively for phosphofructokinase and phosphoenolpyruvate competitively for pyruvate kinase. In both cases, the Ki of the pyrimidine nucleotide and Km of the glycolytic substrate were above their intraerythrocytic concentrations. CTP was a competitive inhibitor of ADP for pyruvate kinase with a Ki near its intraerythrocytic concentration. CDP-choline and CDP-ethanolamine had no effect on the activities of Embden-Meyerhof or pentose phosphate shunt enzymes. Thus, the nature of the hemolytic anemia in hereditary P5N deficiency remains enigmatic.

Case Report: Dapsone-Associated Heinz Body Hemolytic Anemia In a Cambodian Woman with Hemoglobin E Trait

A Cambodian woman with hemoglobin E trait (AE) and leprosy developed a Heinz body hemolytic anemia while taking a dose of dapsone (50 mg/day) not usually associated with clinical hemolysis. Her red blood cells (RBCs) had increased incubated Heinz body formation, decreased reduced glutathione (GSH), and decreased GSH stability. The pentose phosphate shunt activity of the dapsone-exposed AE RBCs was increased compared to normal RBCs. Although the AE RBCs from an individual not taking dapsone had increased incubated Heinz body formation, the GSH content and GSH stability were normal. The pentose phosphate shunt activity of the non-dapsone-exposed AE RBCs was decreased compared to normal RBCs. Thus, AE RBCs appear to have an increased sensitivity to oxidant stress both in vitro and in vivo, since dapsone does not cause hemolytic anemia at this dose in hematologically normal individuals. Given the influx of Southeast Asians into the United States, oxidant medications should be used with caution, especially if an infection is present, in individuals of ethnic backgrounds that have an increased prevalence of hemoglobin E.

Pyrimidine nucleoside monophosphate kinase hyperactivity in hereditary erythrocyte pyrimidine 5'-nucleotidase deficiency

The pyrimidine nucleoside triphosphates (CTP, UTP) increase in the pyrimidine 5′‐nucleotidase (P5N) deficient red blood cell (RBC) to a greater degree than do the pyrimidine nucleoside monophosphates (CMP, UMP). Pyrimidine nucleoside monophosphate (PNMP) kinase phosphorylates CMP and UMP to their respective phosphodiesters. We tested the hypothesis that increased PNMP kinase activity contributes to the disproportionate increase in CTP and UTP in the P5N deficient RBC. CMP and UMP kinase activities were increased in high reticulocyte (4.4±2.1 and 8.5±3.3 μmol/ml RBC per minute) compared to normal RBC (2.8±1.0 and 6.0±2.5 μmol/ml RBC per minute). P5N deficient RBC (n= 2) had significantly increased CMP and UMP kinase activities (14.0 and 26.5 ±mol/ml RBC per minute). UMP and CDP‐ethanolamine were able to increase the activity of CMP kinase in crude haemolysate and the activity of partially purified enzyme. Since the Km for CMP of CMP kinase was 33 μmol/1 in P5N deficient RBC and since the CMP concentration is 25‐90 μmol/1 in the P5N deficient RBC, the enzyme should be nearly saturated with CMP in the P5N deficient RBC. Thus, PNMP kinase hyperactivity appears to contribute to the disproportionate increase in CTP and UTP in the P5N deficient RBC.

Relationship between the nicotinamide adenine dinucleotide redox potential and the 2,3‐diphosphoglycerate content in the erythrocyte in sickle cell disease

Summary. The percentage of nicotinamide adenine dinucleotide (NAD) in the oxidized form [NAD+/(NAD+ and NADH); i.e. the NAD+/NADT ratio] is increased in the red cell (RBC) in sickle cell disease. We tested the hypothesis that the increased NAD+/NADT ratio was a determinant of the increased 2,3‐diphosphoglycerate (DPG) content of the SCD RBC. Using normal subjects and individuals with sickle cell disease or autoimmune haemolytic anaemia (AIHA), we observed an inverse relationship between the packed cell volume (PCV) and the RBC DPG concentration (r=−0·69) and a direct relationship between the RBC NAD+/NADT ratio and the DPG concentration (r= 0·74). When the effect of the PCV on DPG was removed using analysis of covariance [DPGady(PCV)], the NAD+/NADT ratio had a significant relationship with the DPGadj(PCV) (r= 0·50, P < 0·001). In in vitro incubation studies, increasing the NAD+/NADT ratio significantly increased the DPG content of both normal and AIHA RBC. Conversely, decreasing the NAD+/NADT ratio decreased the DPG content of normal, AIHA and SCD RBC. Thus, the increased DPG content in the SCD RBC appears to be due, in part, to the increased NAD+/NADT ratio and is not purely a physiologic response to decreased oxygen carrying capacity.

Pyrimidine nucleotides impair phosphoribosylpyrophosphate (PRPP) synthetase subunit aggregation by sequestering magnesium. A mechanism for the decreased PRPP synthetase activity in hereditary erythrocyte pyrimidine 5'-nucleotidase deficiency.

The activity of phosphoribosylpyrophosphate (PRPP) synthetase (ATP: D-ribose-5-phosphate pyrophosphotransferase, EC 2.7.6.1) is decreased in the erythrocyte in hereditary pyrimidine 5-nucleotidase (P5N) deficiency. Given the increased pyrimidine nucleotide content of the P5N-deficient erythrocyte, we evaluated the effects of prototypic pyrimidine nucleotides on the activity of PRPP synthetase. In normal hemolysate a 1.0 mM combination of cytidine tri-, di- and monophosphate (CTP/CDP/CMP) inhibited PRPP synthetase activity and changed the ribose 5-phosphate (R5P) saturation curve from a hyperbola to a biphasic shape. Untreated crude hemolysate from P5N-deficient erythrocytes showed a biphasic R5P kinetic curve. Since the activity of PRPP synthetase is dependent on its state of subunit aggregation, we examined PRPP synthetase subunit aggregation using gel permeation chromatography. P5N-deficient erythrocytes had a decreased absolute amount of aggregated PRPP synthetase and almost a total loss of disaggregated PRPP synthetase. Using normal hemolysate, 1 mM CTP/CDP/CMP interfered with the ability of 1.0 mM ATP and 2.0 mM MgCl2 to promote PRPP synthetase subunit aggregation. Increasing the MgCl2 to 6.0 mM overcame the inhibitory effect of CTP/CDP/CMP. Thus, the decreased PRPP synthetase activity of the P5N-deficient erythrocyte is due, at least in part, to the ability of the accumulated pyrimidine nucleotides to sequester magnesium and to interfere with the subunit aggregation of PRPP synthetase.

Red cell metabolism in hereditary pyrimidine 5′‐nucleotidase deficiency: effect of magnesium

Since pyrimidine nucleotides avidly bind magnesium, we tested the hypothesis that the haemolytic anaemia in hereditary pyrimidine 5′‐nucleoti∼ dase (P5N) deficiency is due to a state of functional magnesium depletion in the red cell (RBC). In haemolysates from normal subjects, cytidine triphos‐phate (CTP) inhibited the activity of pyruvate kinase in a competitive manner for magnesium. The CTP Ki was 0.4 mmol/1. CTP inhibited the activity of hexokinase in a competitive manner for ATP (Mg‐ATP2‐) with a Ki of 4 mmol/1. The inhibitory effect of CTP on both enzymes was overcome by increasing the magnesium content of the test system. Since CTP appeared to inhibit enzymes which required magnesium as a cofactor or Mg‐ATP2‐ as a substrate, we tested the effect of exogenous magnesium on the metabolism of P5N deficient RBC. The autohaemolysis test, the incubated Heinz body assay and the rate of glucose oxidation by the pentose phosphate shunt were abnormal in the intact RBC from a patient with hereditary P5N deficiency. The addition of MgCl2 (6–10 mmol/1) did not improve these abnormal in vitro measures of metabolism in the P5N deficient RBC. This lack of effect of exogenous magnesium may be due to the slow uptake of magnesium by the human RBC.

Inhibitory effect of mannose on erythrocyte defense against oxidants

The erythrocyte can phosphorylate a variety of hexoses. Since it can consume mannose and glucose equivalently in the hereditary deficiencies of hexokinase and phosphoglucose isomerase and since erythrocyte defense against oxidants is impaired in a variety of hereditary hemolytic anemias, we tested the hypothesis that mannose may be a significant alternative to glucose as a fuel for this defense system. Unexpectedly, mannose inhibited defense against oxidants as manifested by increased Heinz body formation when both normal and high-reticulocyte erythrocytes were incubated with acetylphenylhydrazine (APH). Using APH as the oxidant, mannose-incubated erythrocytes had decreased reduced glutathione stability and impaired hexose oxidation by the pentose shunt compared to glucose-incubated erythrocytes. After incubation with mannose and APH, normal erythrocytes showed a decrease in ATP content. Approximately 25% of the consumed mannose accumulated in the erythrocytes as mannose 6-phosphate. Erythrocytes incubated with mannose and APH displayed a significant loss of redox potential as manifested by decreased NADH/(NADH + NAD+) and NADPH/(NADPH + NADP+) ratios. Since phosphomannose isomerase is the rate-limiting step for mannose metabolism, our results suggest that mannose impairs erythrocyte defense against oxidants by causing ATP depletion and by impairing the regeneration of reduced pyridine nucleotides by the Embden-Meyerhof and pentose phosphate pathways.