Conversion of vitamin B6 compounds to active forms in the red blood cell

Abstract

A B S T R A C T In studies with pyridoxine and other B6 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 370C, and stopped altogether at -20'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.This work was presented in part at the 8th International

Plasma content of B6 vitamers and its relationship to hepatic vitamin B6 metabolism.

“…The propensity of these B6 vitamers to cireculate in plasma is most likely due to protein binding: pyridoxal-P is known to bind very avidly to albtumin, and pyridoxal, less so (9,22). On the other hand, both pyridoxal-P and pyridoxal are also cleared rapidly from plasma in vivo (18,19). Sinee pyridoxal-P is not excreted at all and pyridoxal only minimally in the urine (15), physiological mechanisms must exist for their transfer into tissiues.…”

Vitamin B6 Metabolism in Chronic Alcohol Abuse

“…Studies on the metabolismn of PLP in normal human erythrocytes. Pyridoxal kinase, pyridoxine phosphate oxidase, and Be-phosphate phosphatase activities have been described in human erythrocytes (17,18). The first two enzymes catalyze the synthesis of PLP whereas the third enzyme catalyzes the hydrolysis of phosphorylated vitamin B. compounds.…”

“…The action of the Be-phosphate phosphatase is relevant to red cell metabolism since high concentrations of PLP have been shown to interfere with the activities of many glycolytic and pentose phosphate shunt enzymes (27). Anderson and asso-ciates have also suggested that the phosphatase enzyme in erythrocytes may play an important role in converting pyridoxine to pyridoxal which is then transported from erythrocytes to other tissues (17).…”

“…Since ethanol itself had no demonstrable effect, the observed phenomenon and mechanism therefore represent a unique action of acetaldehyde. The (17,29). Furthermore, little is known of the role of the different organs in the interconversion of Be compounds in vivo (30)(31)(32).…”

“…Moreover, since the liver is the major site of ethanol oxidation, it appears that the concentration of acetaldehyde in situt may be higher in this tissue than in other organs. Anderson, Fulford-Jones, Child, Beard, and Bateman (17) have recently suggested that the role of erythrocytes in vitamin Be metabolism in vivo is to convert pyridoxine to pyridoxal. They speculate that pyridoxal may be the physiologically important vitamer, more readily utilized by the cells of other tissues.…”