Binding of aquocobalamin to bovine casein and its peptides via coordination to histidine residues

Vitamin B12 (cobalamin, Cbl) is an essential nutrient of bovine milk, bound to the casein fraction and the Cbl-specific protein transcobalamin at a ratio of 50:50. This study aimed to elucidate the mechanism of interaction between Cbl and the caseins. It was found that the isolated caseins bind aquocobalamin (HOCbl) via histidine-[Co3+]Cbl coordination. The casein-Cbl complex slowly dissociated in the presence of KCN due to formation of CNCbl. The “active” His groups (5.7 mm measured in 41.6 mg mL−1 casein) accumulate ≤ 3 mm HOCbl at a high rate (t½ ≈ 10 min, 20 °C) and with a high affinity (Kd = 0.1 mm). Low pH hinders the binding, but does not accelerate a very slow dissociation (t½ ≈ 8 h). Increased temperature and/or the presence of the specific Cbl-binding proteins accelerate the dissociation. The consequences of casein-Cbl interactions for the intestinal uptake of Cbl remain unclear.