title: High affinity binding of the transcobalamin II-cobalamin complex and mRNA expression of haptocorrin by human mammary epithelial cells.
abstract: 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.
authors.affiliation: Department of Nutrition, University of California, One Shields Ave, Davis, CA 95616, USA.
authors.forename: Y
authors.initials: Y
authors.lastname: Adkins
authors.forename: B
authors.initials: B
authors.lastname: Lönnerdal
mesh_terms: D001692:Biological Transport; D001940:Breast / Q000378:metabolism*; D002478:Cells, Cultured; D003037:Cobalt Radioisotopes; D004847:Epithelial Cells / Q000378:metabolism; D006801:Humans; D008895:Milk, Human / Q000737:chemistry / Q000378:metabolism; D012333:RNA, Messenger / Q000096:biosynthesis; D014155:Transcobalamins / Q000096:biosynthesis* / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D014805:Vitamin B 12 / Q000737:chemistry / Q000378:metabolism*
chemical_list: D003037:Cobalt Radioisotopes; D012333:RNA, Messenger; D014155:Transcobalamins; D014805:Vitamin B 12