Sn. Fedosov et al., TRANSCOBALAMIN FROM COW MILK - ISOLATION AND PHYSICOCHEMICAL PROPERTIES, Biochimica et biophysica acta. Protein structure and molecular enzymology, 1292(1), 1996, pp. 113-119
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.