High affinity binding of the transcobalamin II-cobalamin complex and mRNA expression of haptocorrin by human mammary epithelial cells

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: transcobalamin II (TC) and haptocorrin (HC). In plasma, TC is responsible for the transport of cobalamin to tissues and cells; however, cobalamin in milk is present exclusively bound to HC. We show that human mammary epithelial cells (HMEC) exhibit high affinity for TC; Scatchard analysis revealed a single class of binding sites for the TC-(57)Co]cyanocobalamin complex with a dissociation constant (K(d)) of 4.9 x 10(-11) M. Uptake of the TC-(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 HC-(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 TC-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 HC-cobalamin complex.