Studies on the Absorption by Guinea Pig Intestine of Cyanocobalamin Incubated with Intrinsic

The absorption of cyanocobalamin from the gastrointestinal tract requires the mediation of the gastric intrinsic factor (1). Although recent studies suggest that the cobamide coenzymes may be the naturally occurring form of vitamin B12 (2), these coenzymes also require intrinsic factor for absorption from the gastrointestinal tract (3, 4). The demonstration by Wilson and Strauss (5) that everted sacs of guinea pig intestine in vitro take up cyanocobalamin in the presence of human gastric juice, has provided a tool for studying the action of human intrinsic factor in vitro. We have utilized this system to study the absorption of cyanocobalamin from the gastrointestinal tract, in conjunction with studies of cyanocobalamin transport into normal and malignant cells (6). Previous studies with perfused isolated rat intestinal loops in vivo have indicated that the absorption of cyanocobalamin is not affected by metabolic poisons (7). The difficulties associated with studies with metabolic inhibitors in vivo have prompted us to repeat these studies in the isolated sac system. Herbert (8) has demonstrated that calcium ions are required for the uptake of hog intrinsic factor by rat intestine segments at 40 C in vitro, and that most of this radioactivity could be removed by subsequent rinsing with ethylenediamine tetraacetate (EDTA). This calcium dependence of intrinsic factor has been observed in vivo in the rat (7, 9) and in man (10). Although calcium-binding materials interfere with the in-