Vitamin B12 and its binding proteins in milk from cow and buffalo in relation to bioavailability of B12

Milk is an important source of highly bioavailable vitamin B-12 (cobalamin) in human nutrition. In most animal products, vitamin B-12 is strongly bound to various specific protein carriers. The 2 vitamin B-12-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-12 binders may influence bioavailability of the vitamin, we examined vitamin B-12 carriers in pooled milk specimens derived from European and Indian cow and buffalo herds. The total endogenous vitamin B-12 concentration was comparable in all milk pools (approximate to 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-12 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-12. Milk from the 2 buffalo breeds contained more specific binders than vitamin B-12, and the surplus proteins included the unsaturated TC approximate to 3 nM (Indian stock), or both TC approximate to 4 nM and HC approximate to 23 nM (Italian stock). The abundant HC of the latter sample bound nearly all endogenous vitamin B-12. We tested (in vitro) the transfer of vitamin B-12 from milk proteins to human carriers, involved in the intestinal uptake. The bovine TC-vitamin B-12 complex rapidly dissociated at pH 2 (time of half reaction, tau(1/2) < 1 min, 37 degrees C) and was susceptible to digestion with trypsin chymotrypsin (pH 7.5). Transfer of vitamin B 1 , from the precipitated bovine casein (pH 2) to human carriers proceeded with tau(1/2 )approximate to 7 min (37 degrees C) and tau(1/2 )approximate to 35 min (20 degrees C). Liberation of vitamin B-12 from buffalo HC was hampered because of its pH stability and slow proteolysis. Nutritional availability of vitamin B-12 is expected to be high in cow milk (with TC-vitamin B-12 and casein-vitamin B 1 , complexes) but potentially constrained in buffalo milk (with HC-vitamin B 12 ). 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-12, TC approximate to 0), whereas intensive crossbreeding of cows and buffaloes from other regions caused a change to TC <= vitamin B-12, 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-12 bioavailability is concerned.