Ileal Mucosa in Familial Selective Vitamin B12Malabsorption

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Abstract
Three brothers with selective vitamin B12 malabsorption and proteinuria were studied to characterize the absorptive defect in this disorder. Gastric juice from all three brothers contained adequate quantities of biologically active intrinsic factor. Serum concentrations of transcobalamin II were normal, and the patients had no demonstrable serum antibodies against intrinsic factor or against ileal receptors for intrinsic-factor-vitamin B12 complex (IF-B12). Mucosal biopsies from the terminal ileum of one brother were normal on light and electron microscopy. Intrinsic factor markedly stimulated uptake of cyanocobalamin by homogenates of these biopsies. Thus, selective vitamin B12 malabsorption is not associated with a morphologically identifiable lesion, and the absorptive defect does not seem to result from lack of ileal receptors for IF-B12. Rather, the defect appears at a stage of vitamin B12 absorption that occurs after IF-B12 attaches to the surface of the ileal cell and before the absorbed vitamin binds to transcobalamin II.

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