Effect of haemolysis on the hexose monophosphate pathway in normal and in glucose-6-phosphate dehydrogenase-deficient erythrocytes

Bound and unbound pyridine dinucleotides in normal and glucose-6-phosphate dehydrogenase-deficient erythrocytes

We have measured, by a sensitive cycling assay, the concentration of bound and unbound dinucleotides in normal and glucose-6-phosphate dehydrogenase (G6PD)-deficient erythrocytes. Measurement of free NADP in ultrafiltrates confirms that in normal erythrocytes almost all NADP is bound to cytosolic proteins. In glucose-6-phosphate dehydrogenase-deficient erythrocytes unbound NADP is significantly higher than in normal red cells and the NADP+/NADPH ratio is largely in favor of the oxidized form. In normal and glucose-6-phosphate dehydrogenase-deficient erythrocytes essentially all NAD (bound and unbound) is in the oxidized state. About 50% of the total amount of NAD (NAD+ + NADH) is free in the cytosol, with a NAD+/NADH ratio greater than 100.

Regulation of the human-erythrocyte hexose-monophosphate shunt under conditions of oxidative stress. A study using NMR spectroscopy, a kinetic isotope effect, a reconstituted system and computer simulation

The regulation of the hexose monophosphate shunt of human erythrocytes under conditions of oxidative stress has been investigated by monitoring the reduction of oxidised glutathione (GSSG) to reduced glutathione (GSH) in erythrocytes containing high levels of GSSG; 1H NMR and a biochemical assay were used to measure the changes. A reconstituted metabolic system prepared with the purified erythrocyte enzymes was used in conjunction with studies of intact cells and haemolysates to determine the dependence of the rate of GSH production on the activities of hexokinase and glucose-6-phosphate dehydrogenase. Both of these enzymes have previously been claimed to be the rate-limiting step of oxidatively stimulated flux through the hexose monophosphate shunt. The absence of a kinetic isotope effect on the rate of GSH production in these systems, when [1-2H]glucose replaced glucose as the source of reducing equivalents, showed that glucose-6-phosphate dehydrogenase activity was not a strong determinant of the flux. The dependence of the rate of GSH production on the concentration of the hexokinase inhibitors glucose 1,6-bisphosphate and glycerate 2,3-bisphosphate showed that, under conditions of oxidative stress, hexokinase was the principal determinant of flux through the shunt. Glucose 1,6-bisphosphate at the concentration present in vivo appears to be more important in limiting hexokinase activity, and thus the rate of glucose utilisation, than was previously assumed. A detailed computer model of the system was developed based on the reported kinetic parameters of the enzymes involved. A sensitivity analysis of this model predicted that the hexokinase reaction would have a sensitivity coefficient of 0.995 with respect to the maximal rate of GSH production.

Mediterranean glucose 6-phosphate dehydrogenase (G6PD) deficiency--near normal decay of the mutant enzyme protein in circulating erythrocytes

Complete removal of leucocytes and platelets from erythrocytes and the development of a sensitized procedure for the assay of G6PD activity allowed the biochemical mechanisms of the Mediterranean variety of G6PD deficiency to be re-evaluated. Activity in the young erythrocytes from 9 G6PD-deficient subjects averaged 0.1% of the levels observed in the corresponding erythrocyte fraction from normal individuals: moreover, the decline of activity during aging of the G6PD-deficient erythrocytes was comparable with that observed for the normal enzyme. Mutant G6PD purified from granulocytes of a G6PD-deficient subject and entrapped within the corresponding erythrocytes was remarkably stable. Exposure of native erythrocytes to an oxidative stress (divicine plus ascorbate) resulted in a decrease of G6PD activity that was significantly more rapid and extensive in control than in G6PD-deficient cells. These results seem to exclude enhanced intracellular breakdown of the mutant protein within the circulating erythrocytes.

Genetic heterogeneity of glucose 6-phosphate dehydrogenase deficiency in Sardinia

Glucose 6-phosphate dehydrogenase (G6PD) activity was assayed quantitatively in red cells from 100 consecutive G6PD-deficient newborn male babies born in a city hospital in Sassari, Sardinia. In four cases G6PD activity was between 30% and 45% of normal: these appeared on electrophoresis to be identical with G6PD Seattle-like. In 65 cases G6PD activity ranged from 2% to 14% of normal, while in the remaining 31 samples no activity could be detected in crude hemolysates. G6PD was partiall purified from 39 samples having activity below 14% of normal (including 20 with zero activity). G6PD activity could now be determined in all, and it was fully characterized in nine samples. These were shown to belong to two distinct classes on grounds of the Michaelis constant for glucose 6-phosphate and the elution profile from DEAE-Sephadex columns. These properties were compared with those of G6PD-deficient samples from Greece and from Israel. We conclude that there are at least three polymorphic G6PD-deficient variants in Northern Sardinia: G6PD Seattle-like, G6PD Mediterranean, and a new variant, which we designate G6PD Sassari. The GdMediterranean and GdSassari genes have been shown to breed true in family studies. We also produce evidence that the definition of G6PD Mediterranean must be carefully reassessed.

A contradiction between in vivo and in vitro activities of normal and variant glucose 6-phosphate dehydrogenase

The presumed "physiologic activity" of normal glucose 6-phosphate dehydrogenase (G6PD), i.e. activity assayed in the presence of physiologic concentrations of ATP, 2,3-diphosphoglycerate, glucose 6-phosphate, NADP and NADPH in the normal red cells, is comparable to shunt pathway activity of intact normal red cells. In contrast, the presumed "physiologic activity" of G6PD variants associated with enzyme deficiency (Gd A- and Gd Mediterranean) is one order of magnitude higher than actual shunt pathway activity of the variant red cells. The difference of kinetic parameters of the enzyme in artificial buffer media and presumed physiologic medium in red cells cannot explain the discrepancy. The apparent discrepancy could be attributed either to (a) over-estimation of NADP and under-estimation of NADPH concentrations in the G6PD deficient red cells due to rapid oxidation of NADPH to NADP in preparation of hemolysate, or to (b) a large portion of NADP which accumulated in the G6PD deficient red cells binds with cellular components of intact red cells and is not available as substrate for G6PD.

Abnormal erythrocyte metabolism in hepatic disease: effect of NADP repletion

Erythrocytes from ten patients with severe liver disease displayed low methylene blue-stimulated hexose monophosphate (HMP) shunt activity and glucose recycling despite elevated total glucose consumption when compared to controls. Heinz body formation was increased and reduced glutathione concentration significantly decreased. After hemolysis, no differences in methylene-blue estimulated HMP shunt activity or glucose recycling could be demonstrated between patients and controls. The addition of 2- and 4-mM NADP to the hemolysates produced significantly greater HMP shunt activity and glucose recycling in the patients' hemolysates. The addition of NADPH to the incubation mixture produced no significant stimulation of either HMP shunt activity or glucose recycling, unless methylene blue was also added. Omission of NAD or phosphate from the incubation mixture produced no change in shunt metabolism. The absence of supplemental ATP resulted in extremely low shunt metabolism and refractoriness to NADP stimulation in both patients and controls. In the absence of additional magnesium, a reduction of shunt metabolism was noted. These data suggest that the defect in stimulated shunt metabolism in the intact erythrocytes of patients with hepatic disease does not result from an absolute enzyme deficiency, but rather from an unavailability of NADP or other cofactor.