Exogenous free vitamin B12 or B12 bound to human transcobalamin II (TCII) accumulated in the near-term rat visceral yolk sac. The rates of their uptakes in vitro and in vivo increased rapidly with time then reached a plateau, which supports a saturable transport/binding process as the rate-limiting step for the uptake of free and TCII complexed B12. Both uptakes were significantly decreased by trypan blue, colchicine, and low temperature but not by ouabain. Such inhibition suggests that the absorption of free and bound B12 is via an endocytosis process dependent upon energy but not the magnesium-dependent sodium/potassium-activated ATPase. Thus, the role of the visceral yolk sac in vitamin transfer to the conceptus and the alterations in yolk sac function associated with birth defects and diminished growth can be integrally related.
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