
<ArticleTitle>High affinity binding of the transcobalamin II-cobalamin complex and mRNA expression of <prot>haptocorrin</prot> by human mammary epithelial cells.</ArticleTitle>
<AbstractText>Little is known about the acquisition of cobalamin by the mammary gland and its secretion into milk. Human milk and plasma contain at least two types of cobalamin binding proteins: <prot>transcobalamin II</prot> (<prot>TC</prot>) and <prot>haptocorrin</prot> (<prot>HC</prot>). In plasma, <prot>TC</prot> is responsible for the transport of cobalamin to tissues and cells; however, cobalamin in milk is present exclusively bound to <prot>HC</prot>. We show that human mammary epithelial cells (HMEC) exhibit high affinity for <prot>TC</prot>; Scatchard analysis revealed a single class of binding sites for the <prot>TC</prot>-[(57)Co]cyanocobalamin complex with a dissociation constant (K(d)) of 4.9 x 10(-11) M. Uptake of the <prot>TC</prot>-[(57)Co]cyanocobalamin complex at 37 degrees C was saturable by 24 h. Binding of free [(57)Co]cyanocobalamin to HMEC was not saturable and very limited binding of the <prot>HC</prot>-[(57)Co]cyanocobalamin complex was observed. Expression of the haptocorrin gene by HMEC was confirmed by Northern blot and PCR analysis. Thus, a specific cell surface receptor for the <prot>TC</prot>-cobalamin complex exists in the mammary gland and once cobalamin is internalized, it may be transferred to HC and subsequently secreted into milk as a <prot>HC</prot>-cobalamin complex.</AbstractText>
