Iron-reducing activity of plasma membranes.

Human placental trophoblast plasma membranes were prepared by differential centrifugation and solubilized in nonionic detergent. Transferrin receptors were isolated from the solubilized membranes by affinity chromatography on diferric transferrin-coupled Sepharose 4B. The trophoblast plasma membrane vesicles demonstrated NADH-ferricyanide oxidoreductive activity. However, NADH-Fe(III) oxidoreductive activity was very weak when Fe(III)-ammonium citrate or diferric transferrin was used as electron acceptor in the presence of bathophenanthroline disulfonate as an indicator of the reaction. After solubilization, only NADH-ferricyanide oxidoreduction was recovered. Affinity chromatography-purified transferrin receptors did not exhibit any measurable oxidoreductase activity. Thus, when these receptors are present in plasma membranes they mediate redox reactions, but biochemically isolated receptors do not mediate such reactions. These observation suggest that transferrin receptors in plasma membranes bind diferric transferrin, and, in an undetermined way, facilitate Fe(III) release so that iron reduction can occur.