Vitamin B and its binding proteins in milk from cow and buffalo in relation to bioavailability of B.

Milk is an important source of highly bioavailable vitamin B (cobalamin) in human nutrition. In most animal products, vitamin B is strongly bound to various specific protein carriers. The 2 vitamin B-specific proteins, predominantly transcobalamin (TC) and haptocorrin (HC), were earlier found in milk from Holstein Friesian cows and in human or sow milk, respectively. As the type of vitamin B binders may influence bioavailability of the vitamin, we examined vitamin B carriers in pooled milk specimens derived from European and Indian cow and buffalo herds. The total endogenous vitamin B concentration was comparable in all milk pools (≈3 nM), but the vitamin carriers varied considerably: TC + caseins in Danish cows, TC + HC in Indian cows and buffaloes, and mainly HC in Italian buffaloes. Danish cow milk contained half as much TC as vitamin B, and the surplus vitamin was all attached via a single coordination bond to abundantly available histidine residues of casein. The specific binding proteins in Indian cow milk (TC + HC) approximately matched the molar content of vitamin B. Milk from the 2 buffalo breeds contained more specific binders than vitamin B, and the surplus proteins included the unsaturated TC ≈ 3 nM (Indian stock), or both TC ≈ 4 nM and HC ≈ 23 nM (Italian stock). The abundant HC of the latter sample bound nearly all endogenous vitamin B. We tested (in vitro) the transfer of vitamin B from milk proteins to human carriers, involved in the intestinal uptake. The bovine TC-vitamin B complex rapidly dissociated at pH 2 (time of half reaction, τ < 1 min, 37°C) and was susceptible to digestion with trypsin + chymotrypsin (pH 7.5). Transfer of vitamin B from the precipitated bovine casein (pH 2) to human carriers proceeded with τ ≈ 7 min (37°C) and τ ≈ 35 min (20°C). Liberation of vitamin B from buffalo HC was hampered because of its pH stability and slow proteolysis. Nutritional availability of vitamin B is expected to be high in cow milk (with TC-vitamin B and casein-vitamin B complexes) but potentially constrained in buffalo milk (with HC-vitamin B). This especially concerns the Italian buffalo milk, where a high excess of HC was found. We speculate whether the isolated stock of Italian buffalo maintained the ancestral secretion of carriers (HC ≫ vitamin B, TC ≈ 0), whereas intensive crossbreeding of cows and buffaloes from other regions caused a change to TC ≤ vitamin B, with low or absent HC. The substitution of HC by less sturdy carriers is apparently more beneficial to human consumers as far as vitamin B bioavailability is concerned.