Considerations in the Use of B6 Vitamers in Hematologic Disorders: I. Red Cell Transport and Metabolism of Pyridoxal

Abstract

AbstractIn order to assess the potential usefulness of pyridoxal (PL) in sickle cell anemia and other hematologic disorders, the in vitro characteristics of the transport and metabolism of pharmacologic levels of this B6 vitamer by normal human red cells were defined. Red cells suspended in phosphate-buffered NaCl rapidly accumulated PL against a concentration gradient until intracellular levels reached 14 µmole/ml. This corresponded to the PL concentration that maximally increased red cell hemoglobin–oxygen affinity. PL uptake was not affected by adenosine triphosphate (ATP) depletion, sulfhydryl reagents, inhibition of the anion transport channel or the addition of pyridoxal 5′-phosphate (PLP). However, when plasma or human serum albumin (5g/dl) was present in the medium, uptake of PL was decreased by 45%. Moreover, when red cells previously loaded with PL were incubated for 1 hr in the presence of plasma or albumin, 75% of their PL content was released and hemoglobin–oxygen affinity returned to normal. Red cell metabolism of PL was also limited, with formation of sufficient PLP to activate the endogenous aspartate amino transferase (GOT) apoenzyme requiring long incubations and high PL concentrations. It is concluded that (1) red cells transport PL primarily by passive diffusion; (2) net red cell accumulation of PL depends on the binding of PL to hemoglobin; (3) PL is not readily metabolized by mature red cells; and (4) plasma proteins, particularly albumin, retard PL uptake and increase its release from hemoglobin binding sites. Thus, the distribution of PL between red cells and plasma reflects the relative binding affinities of hemoglobin (Hb) and albumin for PL. These findings demonstrate that protein binding modifies both the physiologic and pharmacologic effects of PL and suggest that in vivo, PL may have only a transient effect on hemoglobin oxygen affinity and red cell sickling.