Binding of lactoferrin and transferrin to the human promonocytic cell line U937. Effect on iron uptake and release

We have compared the ability of lactoferrin and transferrin to interact with and donate iron to the monocytic cell line U937. About 10 times more lactoferrin was bound than transferrin, but most lactoferrin bound nonspecifically, and the degree of specific binding was similar for both proteins (2-3 x 10(6) sites/cell). The binding affinity for lactoferrin (83 nM) was about 4-fold lower than for transferrin (21 nM). Lactoferrin did not inhibit binding of transferrin, or vice versa. Binding of lactoferrin was not inhibited by 30 mM glucose or fucose nor by incubating the cells with heparinase. Transferrin, but not lactoferrin, was internalized, and 3 mM primaquine caused intracellular accumulation of transferrin but not lactoferrin. The cells rapidly acquired iron from transferrin, but uptake from lactoferrin was 10-fold slower and probably resulted from transfer of 59Fe from lactoferrin to unlabeled transferrin during culture. Lactoferrin, but not transferrin, released iron to the extracellular medium when bound to U937 cells. Lactoferrin inhibited cellular uptake of iron from Fe-nitrilotriacetate but not from transferrin. It is concluded that transferrin, but not lactoferrin, acts as an iron donor to U937 cells. Lactoferrin may regulate uptake of potentially toxic non-transferrin-bound iron.