Identification of a New Vitamin B₁₂ Binder (Transcobalamin III) in Normal Human Serum

Abstract

Summary. Fractionation of vitamin B 12 binders in a normal human serum pool showed a distribution of 92% Transcobalamin II (TCII) and 8% Transcobalamin I (TCI) on DEAE‐cellulose chromatography. Separation of the same serum pool using gel filtration gave 74% TCII and 26% TCI. This 18% discrepancy could be due either to massive contamination of the DEAE‐cellulose TCII peak with TCI, or to the presence of a high molecular weight binder which elutes with TCII on DEAE‐cellulose, and with TCI on gel filtration. Refractionation of the TCII peak showed it to be relatively uncontaminated with TCI. Removal of the low molecular weight binders with charcoal consistently gave a peak in the TCII area on DEAE, which subsequent gel filtration analysis showed to be of high molecular weight. This new binder is further distinguished from TCI by the observation that its vitamin B 12 binding capacity does not increase to the same extent as TCI on fasting. Since this binder could be demonstrated in 50 different normal sera, it is clearly not a pathological binder, and should be called Transcobalamin III (TCIII).