Inactivation of Vitamin B12 by a Binder in Rat Intestine and the Role of Intrinsic Factor

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Abstract
Using the rat intestinal loop technique, it has been demonstrated that in the absence of intrinsic factor, small amounts of 57Co-B12 placed in the loop are either adsorbed onto the mucosa or bound to a macromolecular substance in the lumen to become unabsorbable. Only a large excess of intrinsic factor with respect to B12 binding capacity makes such complexed B12 available for absorption. The mucosa of rat small intestine has a considerable reserve of B12 binding capacity, probably on the surface, and is constantly releasing a non-intrinsic factor B12 binder into the lumen. The surface bound free B12 is eventually shed with mucous flocculi to be expelled. Pronase counteracted this ‘inactivating’ reaction in the loop, and kept small doses of B12 in its free form for a long period of time, effecting an absorption that was slow but almost as large as that obtained with intrinsic factor. Indication was obtained by the analyses of intestinal contents following oral administration of 57C-B12, that this inactivating reaction is normally operative in the small intestine counter to the protection of B12 molecules by intrinsic factor and possibly by pancreatic proteases.

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