Transcobalamin from cow milk: isolation and physico-chemical properties

• Published in: Biochimica et biophysica acta 1996-01, Vol.1292 (1), p.113-119
• Main Authors: Fedosov, Sergey N., Petersen, Torben E., Nexø, Ebba
• Format: Article
• Language: English
• Subjects: Amino Acid Sequence; Animals; Apoproteins - analysis; Cattle; Chromatography, Affinity; Chromatography, Gel; Cobalamin; Cow milk; dairy products; Electrophoresis, Polyacrylamide Gel; food composition; food quality; Humans; Isolation; Milk - chemistry; Molecular Sequence Data; Molecular Weight; Protein Binding; Rats; Receptors, Cell Surface - metabolism; Sequence; Sequence Homology, Amino Acid; Spectrophotometry; Transcobalamin; Transcobalamins - analysis; Transcobalamins - chemistry; Transcobalamins - isolation & purification; Transcobalamins - metabolism; Urea - pharmacology; Vitamin B 12 - metabolism
• ISSN: 0167-4838; 0006-3002; 1879-2588; 1878-2434
• DOI: 10.1016/0167-4838(95)00173-5

The concentration of endogenous cobalamin (Cbl) in cow milk was 3.3 nM while the Cbl-binding capacity was 0.05 nM. Both endogenous and newly added Cbl showed similar quantitative distribution between a 280 kDa protein complex (45%) and a 43 kDa Cbl-binder (55%). Long time incubation, as well as urea treatment, was accompanied by a slow release of the 43 kDa Cbl-binder from the 280 kDa fraction. No other Cbl-binding proteins appeared after these procedures. The 43 kDa binder from cow milk, depleted of the ligand by urea treatment, reacted with Cbl even in the presence of a B 12 analogue cobinamide (Cbi) at the ratio Cbl:Cbi = 1:40. The Stokes radius of the binder changed from 2.7 nm for the Cbl-free protein to 2.5 nm for the Cbl-saturated form and the Cbl-saturated binder was able to displaze human transcobalamin (TC) from the TC-receptor. The interaction between the protein and Cbl was significantly suppressed at pH 2.0. The N-terminal sequence of the purified 43 kDa Cbl-binder revealed homology with TC from human and rabbit plasma. In conclusion we have shown that TC is the main Cbl-binding protein in cow milk. This is surprising, since previous studies on human and rat milk have shown another Cbl-binder, apo-haptocorrin, to be the dominating Cbl-binding protein.