TRANSCOBALAMIN FROM COW MILK - ISOLATION AND PHYSICOCHEMICAL PROPERTIES

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 incubat ion, as well as urea treatment, was accompanied by a slow release of t he 43 kDa Cbl-binder from the 280 kDa fraction. No other Cbl-binding p roteins appeared after these procedures. The 43 kDa binder from cow mi lk, 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 C bl-free protein to 2.5 nm for the Cbl-saturated form and the Cbl-satur ated binder was able to displace human transcobalamin (TC) from the TC -receptor. The interaction between the protein and Cbl was significant ly suppressed at pH 2.0. The N-terminal sequence of the purified 43 kD a Cbl-binder revealed homology with TC from human and rabbit plasma. I n 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 dominat ing Cbl-binding protein.