SEQUENTIAL MECHANISMS IN THE ENHANCED ABSORPTION OF VITAMIN B12 BY INTRINSIC FACTOR IN THE RAT*

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Abstract
In dialysis experiments human gastric juice competitively removed native vitamin B12 or added Co60-B12 from pork or beef muscle preparations, especially at low pH. Human gastric juice lost none of its native vitamin B12 when dialyzed at pH 6.0 or 1.5. Paired loops of rat ileum with intact circulation perfused intraluminally with rat or human intrinsic factor in the presence of Krebs-Ringer phosphate solution showed a linear increase with time of the Co60-B12 adsorbed. As in the gastrectomized rat, rat stomach extract was more active than human gastric juice, and a purified hog intrinsic factor was inert. This action of rat or human intrinsic factor was not affected by sodium azide, cyanide, or dinitrophenol. It was prevented by occluding the blood supply or by sodium ethylene-diaminetetraacetate (EDTA) during the intraluminal perfusion, but not be calcium EDTA. Mn or Mg partially neutralized the inhibitory effect of sodium EDTA. After the perfusion, a portion of the Co60-b12 adsorbed in the presence of human gastric juice but none of that adsorbed in the presence of rat stomach extract was removed by rinsing with sodium EDTA. In further dialysis experiments, a crude, aqueous extract of rat intestine was able to free Co60-B12 bound to rat stomach extract, less of that bound to human gastric juice and none of that bound to the hog intrinsic factor preparation. Boiling partially inhibited the intestinal extract as did low pH, mercuric chloride and sodium arsenite. The authors suggest that 3-phases can be recognized in the action of gastric intrinsic factor in the rat: Phase I, a non-species-related competitive binding by intrinsic factor of dietary vitamin B12; Phase II, a non-energy-requiring adsorption of intrinsic factor-bound vitamin B12 by the intestinal mucosa involving a bivalent cationic dependent bond; and Phase III, a species-related conversion of a Ca++ dependent bond and release of free vitamin B12 at the surface of or within the intestinal wall, possibly as a result of intestinal enzymic action.