Transport of vitamin B6 in human erythrocytes

Vitamin B6 status of children with sickle cell disease

PURPOSE

In vitro, vitamin B(6) has antisickling properties, but the effect of vitamin B status on the health of children with sickle cell disease-SS (SCD-SS) is not well described. The purpose of this study was to assess vitamin B(6) status of children with SCD-SS ages 3 to 20 years and determine its relationship to growth, dietary intake, and disease severity.

PATIENTS AND METHODS

Vitamin B(6) status was assessed by serum pyridoxal 5-phosphate (PLP) concentration in subjects with SCD-SS and by urinary 4-pyridoxic acid (4-PA) concentration in other subjects with SCD-SS and healthy control children. Concentration of PLP was compared with anthropometric measures of growth and nutritional status, dietary intake, hematologic indices, and frequency of SCD-related illness.

RESULTS

The PLP concentration of subjects with SCD-SS was 15.6 +/- 15.2 nmol/L. Seventy-seven percent had a PLP concentration below the deficiency criterion (20 nmol/L) suggested by the Dietary Reference Intakes (1998). Controlling for alkaline phosphatase, age, and gender, PLP concentration was associated positively with weight, body mass index, and arm circumference -scores and negatively with reticulocyte count. Urinary 4-PA was lower in children with SCD-SS versus controls, although 4-PA/creatinine values did not differ between groups.

CONCLUSIONS

Children with SCD-SS had apparently low serum PLP concentrations in the absence of excess vitamin B(6) excretion, suggesting low vitamin B(6) status. Low serum PLP concentration was associated with indicators of poor nutritional status and may be related to increased hemolysis in children with SCD-SS.

Asymmetric transport of a fluorescent glucose analogue by human erythrocytes

A fluorescent glucose analogue, 6-deoxy-N-(7-nitrobenz-2-oxa-1,3-diazol-4-yl)-aminoglucose (NBDG), was synthesized and its interactions with the hexose transport system of the human red blood cell were investigated. NBDG entry is inhibited by increasing concentrations of D-glucose (Ki = 2 mM). However, NBDG exit is unaffected by D-glucose in red blood cells. Cytochalasin B was found to inhibit both NBDG entry and exit. NBDG accumulates in the red blood cell above the theoretical equilibrium concentration. Accumulation of NBDG is temperature-sensitive and is due to the binding of NBDG to some intracellular substance. The binding of NBDG to purified hemoglobin suggests that accumulation of NBDG by erythrocytes is due to the intracellular binding of NBDG to hemoglobin. NBDG does not accumulate in pink erythrocyte ghosts, while its rate of uptake is still inhibited by D-glucose and cytochalasin B. Although there was no apparent D-glucose inhibition of NBDG exit by intact red blood cells, D-glucose was able to inhibit NBDG exit by pink erythrocyte ghosts. The differing properties of NBDG influx and efflux support the interpretation that the hexose transport system of the human red blood cell appears asymmetric although it may be intrinsically symmetric.

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.

Restoration of the poor oxygen transport function of ACD-stored blood by pyridoxal 5'-phosphate

Pyridoxal 5'-phosphate is easily incorporated into ACD-stored erythrocytes without decrease of ATP, and restores the poor oxygen transport function with a similar effect to 2,3'-diphosphoglycerate.

Inhibition of erythrocyte sickling in vitro by pyridoxal

To test the antisickling activity of pyridoxal, we compared the oxygen affinity and the percent sickling at low PO2 of untreated erythrocytes with values for cells from the same blood sample incubated with pyridoxal, glyceraldehyde, or pyridoxine. Pyridoxal increased oxygen affinity much more than glyceraldehyde. 20 mM pyridoxal and glyceraldehyde had equivalent antisickling activity. At PO2 levels above 20 mm Hg, both agents reduced sickling to less than 2%. In samples examined by electron microscopy, pyridoxal reduced the percent sickled cells and the percent cells that contain hemoglobin S fibers by the same amount (from 74 to 3%). Pyridoxine had no effect on oxygen affinity or sockling. Pyridoxal reacts with intracellular hemoglobin to increase oxygen affinity, which inhibits hemoglobin S polymerization and sickling.

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.

Conversion of vitamin B 6 compounds to active forms in the red blood cell

In studies with pyridoxine and other B(6) compounds in blood, the active forms pyridoxal and pyridoxal phosphate were measured by differential assays using Lactobacillus casei. Red cell uptake of tritiated pyridoxine was also measured. A new metabolic pathway for conversion of pyridoxine to active forms was demonstrated in red cells. In vivo studies in normal subjects suggested that pyridoxine was taken up by red cells where it was converted to pyridoxal phosphate and then pyridoxal, followed by gradual release of a proportion of pyridoxal into plasma. In vitro incubation of pyridoxine with blood confirmed this observation. Increasing amounts of pyridoxine were taken up and converted as the amount added to blood was increased, and only very small numbers of red cells were needed to convert appreciable amounts. Conversion was markedly inhibited at temperatures lower than 37 degrees C, and stopped altogether at - 20 degrees C.Release of pyridoxal into plasma was always directly proportional to the amount of pyridoxal formed and to the volume of plasma present. That pyridoxal phosphate was not released into plasma was demonstrated in stored blood, for pyridoxine was converted mainly only as far as pyridoxal phosphate, probably due to inactivation of the phosphatase. Pyridoxal phosphate remained in the red cells. Pyridoxine was converted when incubated with washed red cells in saline or phosphate buffer suspension (0.08 M). In saline suspension, pyridoxal formed but was not released in the absence of plasma. In phosphate buffer suspension, pyridoxal phosphate was formed but was not changed to pyridoxal, probably due to inactivation of phosphatase by excess phosphate. Pyridoxamine was converted to active forms in red cells less efficiently. Pyridoxal entered red cells rapidly, equilibrating between plasma and cells within 1 min in the same ratio as pyridoxal formed inside red cells. Pyridoxal phosphate did not enter red cells in whole blood but did so readily in washed cells in saline.