Iain L. Mackenzie M.B.¹, Robert M. Donaldson Jr. M.D.¹, William L. Kopp M.D.¹, and Jerry S. Trier M.D.¹

¹ From The Department of Medicine, University of Wisconsin, the Veterans Administration Hospital, Madison, Wisconsin 53705, and the Departments of Medicine, Boston University, Boston, Massachusetts 02110, and the University of New Mexico, Albuquerque, New Mexico 87106

Microvillous membranes isolated from the distal, but not proximal, half of hamster small bowel induced in rabbits the formation of antisera which inhibited intrinsic factor-mediated uptake of vitamin B₁₂ by hamster brush borders. The extent of inhibition was directly proportional to the concentration of antiserum, and an excess of IF-bound vitamin B₁₂ could overcome the inhibitory effect. The inhibitory factor was absorbed from antisera by brush borders isolated from the distal, but not proximal, half of the hamster intestine. Fractionation of antisera by gel filtration and DEAE-cellulose chromatography established that immunoglobulin G contained the inhibitory factor. Antisera capable of completely blocking uptake of IF-bound vitamin B₁₂ did not react with hamster IF or with the IF-vitamin B₁₂ complex, did not inhibit brush border disaccharidase activity and did not impair glucose transport by everted sacs of hamster intestine. These results demonstrate that an antibody to distal microvillous membranes competes with the IF-vitamin B₁₂ complex for a specific binding site or receptor located on the surface of distal hamster intestine.

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