Intrinsic Factor Studies VI. Competition for Vit. B12 Binding Sites Offered by Analogues of the Vitamin.

Abstract

Vitamin B12 bound to a protein is nondialyzable. Using radioactive cyanocobalamin as a marker, studies were made of the competition offered by several vitamin B12 analogs for the B12 binding sites in human gastric juice, serum, saliva, colostrum, kidney and liver, and an intrinsic factor prepared from hog stomach. Human gastric juice was the most selective of these substances as regards preference for cyanocobalamin. B12 analogs studied included sulfato-, chloro-, and nitro-cobalamins; desdimethyl-, trifluoromethyl-, 5-hydroxy-, and 5-amino-benziminizole B12''s; lactam, lactone, and pseudo B12''s. The first three analogs were as effective as cyanocobalamin in competing for the B12 binding sites. The majority of cyanocobalamin molecules bound by gastric juice did not dissociate in the presence of excess vitamin added subsequently.