THE COMPETITION BETWEEN TRANSFERRINS LABELED WITH FE-59, ZN-65, AND MN-54 FOR THE BINDING-SITES ON LACTATING MOUSE MAMMARY-GLAND CELLS

Using a homologous competition of Mn-54-transferrin with Mn-transferri n and Zn-65-transferrin with Zn-transferrin, it was found that on the plasma membrane of lactating mouse mammary gland cells there are recep tor binding Mn-transferrin and Zn-transferrin. The heterologous compet ition between labeled and nonlabeled Fe-transferrin, Mn-transferrin an d Zn-transferrin, as well as almost equal affinity constants of cellul ar receptors toward the three metals by competition of Fe-transferrin suggests that one and the same receptor accepts all three metals from the transferrin molecule. The cell receptors therefore possess a polym etal binding function. A model and a mechanism for regulation of the t ransport metal flow toward the mammary gland cell acting Like ''automa ted switching over'' are proposed.