Mechanism of vitamin B12 uptake by erythocytes.

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Abstract
Serum-mediated B12 -57Co uptake by reticulocyte-rich erythrocytes appeared to represent rapid adsorptior to the red cell surface; ionic Ca or Mg was essential for this reaction, but Sr could partially replace these cations. In the test system used, B12 -57Co uptake was maximal after 20 min. incubation, with near maximal adsorption during the 1st 5 min. Uptake increased with a rising reticulocyte count, but mature erythrocytes could also adsorb small amounts of B12 -57Co. Trypsin and papain reduced B12 uptake, but metabolic poisons had no effect. Na2 EDTA and trypsin could elute virtually all B12 -57Co previously adsorbed to erythrocytes; elution was much less complete with serum and saline. B12 -57Co taken up from a saline medium was less than from serum, did not concentrate in red cell stroma (unlike B12 -57Co from serum), did not show Ca or reticulocyte dependence, and could not be eluted by Na2 EDTA. It is suggested that 2 mechanisms exist for B12 uptake by erythrocytes analogous to the duel mechanisms for B12 transport across the intestinal mucosa: Ca or Mg- (or both) dependent, carrier glycOprotein-mediated transfer to receptors on the cell surface, operative primarily in the presence of physiologic quantities of B12 and simple diffusion independent of receptor sites (primarily operative in the presence of excess unbound B12).