FREE VITAMIN-B-12 AND TRANSCOBALAMIN-II VITAMIN-B-12 COMPLEX UPTAKE BY THE VISCERAL YOLK-SAC OF THE SPRAGUE-DAWLEY RAT - EFFECT OF INHIBITORS

Exogenous free vitamin B-12 or B-12 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 r eached a plateau, which supports a saturable transport/binding process as the rate-limiting step for the uptake of free and TCII complexed B -12. Both uptakes were significantly decreased by trypan blue, colchic ine, and low temperature but not by ouabain, Such inhibition suggests that the absorption of free and bound B-12 is via an endocytosis proce ss dependent upon energy but not the magnesium-dependent sodium/potass ium-activated ATPase. Thus, the role of the visceral yolk sac in vitam in transfer to the conceptus and the alterations in yolk sac function associated with birth defects and diminished growth can be integrally related. (C) 1997 Elsevier Science Inc.