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Am J Physiol Gastrointest Liver Physiol 257: G791-G797, 1989;
0193-1857/89 $5.00
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AJP - Gastrointestinal and Liver Physiology, Vol 257, Issue 5 791-G797, Copyright © 1989 by American Physiological Society

ARTICLES

Cobalamin release from intrinsic factor and transfer to transcobalamin II within the rat enterocyte

M. Ramasamy, D. H. Alpers, C. Tiruppathi and B. Seetharam
Department of Medicine, School of Medicine, Washington University, St. Louis, Missouri 63110.

To ascertain the mechanism of release of cobalamin (Cbl) from intrinsic factor (IF) and subsequent formation of transcobalamin II (TC-II)-Cbl complex, we studied the intracellular distribution of 57Co-labeled Cbl after its uptake in suckling and adult rats. The amount of Cbl bound to IF, to the IF-Cbl receptor via IF, and to TC-II was determined by immunoprecipitation with monospecific antisera raised to these proteins. IF-Cbl receptor activity was found to be very low in suckling rats up to 12 days after birth. Oral administration of leupeptin in amounts known to alter protein turnover had no effect on the release of Cbl from IF nor did it inhibit the formation of the TC-II-Cbl complex in either adult or suckling animals. However, oral administration of chloroquine resulted in a transient increase in the intestinal concentration of Cbl in both adult and suckling rats and in total inhibition of Cbl released from IF in adults rats. Chloroquine prevented completely the transfer of Cbl to TC-II in adult rats and inhibited the transfer by 50% in suckling rats. These data demonstrate that in adult mucosa utilizing receptor-mediated endocytosis, Cbl is transferred from IF to TC-II. This transfer does not require the IF-Cbl receptor, as it occurs in suckling rats. Finally, transfer of Cbl to TC-II is decreased by a drug that alters vesicular pH. Because Cbl can be released at acid pH from IF, it is proposed that release of Cbl from IF and its transfer to TC-II occurs in an acidic vesicle.


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