Development of a Possible in vitro Assay for Intrinsic Factor.

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Abstract
When rat liver slices are incubated in hog intrinsic factor concentrate (HIFC) and Co60-labeled vitamin B12 simultaneously, there is an optimal concentration of HIFC producing maximal Co60-B12 uptake by the liver slices. Any deviation from this optimal level results in a decreased enhancement of Co60-B12 uptake. Slices incubated in high concentration of HIFC, washed, and then incubated in Co60-B12 take up much more Co60-B12 than slices incubated in these materials together. The hypothesis is presented that receptors for intrinsic factor exist on rat liver slices, and that in the presence of calcium these receptors can "take up" either free intrinsic factor or intrinsic factor to which Co60-B12 is attached. Sequential incubation of rat liver slices, first in intrinsic-factor-containing material and then in Co60-B12, resulted in Co60-B12 uptakes by the slices which reflected the relative in vivo potencies of seven HIFC preparations and also the presence or absence of intrinsic factor in human gastric juice.