Intrinsic Factor Studies VI. Competition for Vit. B 12 Binding Sites Offered by Analogues of the Vitamin. ∗

Abstract

Summary and Conclusions

1) This article reports some observations on the general nature of vit B₁₂ binding by gastric juice and certain other biologic substances. The majority of vit B₁₂ molecules bound by gastric juice did not dissociate in the presence of excess vitamin molecules added subsequently. 2) Our data show competition offered by certain vit B₁₂ analogues for the cyanocobalamin binding sites of gastric juice, intrinsic factor concentrate, saliva, colostrum, and serum. Of these substances, gastric juice exhibited the greatest preference for cyanocobalamin in the presence of excess analogue. The intrinsic factor concentrate tested was much less fastidious than gastric juice. Saliva, colostrum, and serum exhibited little selectivity in their B₁₂ binding mechanisms. 3) Occurrence of a sulfate, nitrate, or chloride ion in lieu of the cyanide ion in the cobalamin did not diminish competition for cyanocobalamin binding sites of gastric juice.