Department of Molecular Biology and Genetics, Aarhus University, Science Park, Gustav Wieds Vej 10C 8000, Aarhus C, Denmark.
Department of Clinical Biochemistry, Aarhus University Hospital, Palle Juul-Jensens Boulevard 99, 8200 Aarhus N, Denmark.
J Dairy Sci. 2019 Jun;102(6):4891-4905. doi: 10.3168/jds.2018-15016. Epub 2019 Mar 28.
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.
牛奶是人类营养中生物可利用性很高的维生素 B（钴胺素）的重要来源。在大多数动物产品中，维生素 B 与各种特定的蛋白质载体紧密结合。两种维生素 B 特异性蛋白，主要是转钴胺素（TC）和触珠蛋白（HC），分别在荷斯坦弗里生奶牛的牛奶中和人乳或猪乳中被发现。由于维生素结合蛋白的类型可能会影响维生素的生物利用度，我们检测了来自欧洲和印度牛群和水牛群的混合牛奶样本中的维生素 B 载体。所有牛奶池中的内源性维生素 B 总浓度相当（≈3 nM），但维生素载体差异很大：丹麦奶牛中的 TC+酪蛋白、印度奶牛和水牛中的 TC+HC、以及意大利水牛中的主要 HC。丹麦奶牛的 TC 含量是维生素 B 的一半，剩余的维生素通过单一配位键附着在酪蛋白丰富的组氨酸残基上。印度奶牛乳中的特异性结合蛋白（TC+HC）与维生素 B 的摩尔含量大致匹配。两种水牛品种的牛奶中含有的特异性结合蛋白多于维生素 B，并且多余的蛋白质包括不饱和 TC≈3 nM（印度品种），或 TC≈4 nM 和 HC≈23 nM（意大利品种）。后者样本中的丰富 HC 几乎结合了所有内源性维生素 B。我们测试了（体外）维生素 B 从牛奶蛋白转移到人肠道吸收所涉及的载体的情况。在 pH 2 下（半衰期 τ<1 分钟，37°C），牛 TC-维生素 B 复合物迅速解离，并且容易被胰蛋白酶+糜蛋白酶消化（pH 7.5）。在 pH 2 沉淀的牛酪蛋白中，维生素 B 向人载体的转移以 τ≈7 分钟（37°C）和 τ≈35 分钟（20°C）进行。水牛 HC 中维生素 B 的释放受到其 pH 稳定性和缓慢蛋白水解的阻碍。由于存在 TC-维生素 B 和酪蛋白-维生素 B 复合物，牛乳中的维生素 B 营养利用率预计很高，但在水牛乳中（HC-维生素 B）可能受到限制。这尤其涉及到意大利水牛乳，其中发现了大量 HC 的过剩。我们推测，意大利水牛的孤立种群是否保持了载体的原始分泌（HC>维生素 B，TC≈0），而来自其他地区的牛和水牛的密集杂交导致了 TC≤维生素 B 的变化，而 HC 含量低或不存在。就维生素 B 的生物利用度而言，用不太稳定的载体替代 HC 显然对人类消费者更有益。
