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Acquisition of iron from lactoferrin and transferrin by a parasitic protozoon Tritrichomonas foetus has been studied in vitro. Specific, time-dependent, and saturable binding of iodinated ligands to the outer membrane of T. foetus at 4 degrees C was demonstrated for 125I-labeled lactoferrin only. About 1.7 x 10(5) binding sites of a single class with Kd approximately equal to 3.6 microM was estimated by means of Scatchard analysis. Internalization of the bound lactoferrin was observed at 37 degrees C. The cell-associated radioactivity after 30 min incubation of the parasite with 125I-lactoferrin at 37 degrees C was about 3.5-fold higher than the amount bound at 4 degrees C. The majority of internalized 125I-lactoferrin was released within 15 min of cell reincubation at 37 degrees C in the presence of a 100-fold excess of nonlabeled lactoferrin. Released lactoferrin displayed unchanged mobility on autoradiography. In contrast to lactoferrin, binding of 125I-transferrin was nonspecific and did not display saturable kinetics. The growth of T. foetus in iron-restricted media was stimulated by both lactoferrin and transferrin. The ability of the cells to remove and accumulate iron from both proteins was therefore examined using 59Fe-saturated lactoferrin and transferrin. It was found that trichomonads acquired a comparable amount of iron from both lactoferrin and transferrin during 60 min incubation at 37 degrees C (495 and 577 pmole Fe/mg of protein, respectively). The pH of the assay medium (PBS) decreased from pH 7.4 to 5.6 after incubation with trichomonads. At this pH, marked release of iron from transferrin (up to 47%) but not from lactoferrin (4%) was determined in cell-free media. These results indicate that T. foetus is able to utilize both lactoferrin and transferrin to cover its iron requirements. However, mechanisms of iron acquisition from these host proteins appear to be different. Specific binding and internalization of lactoferrin suggests the possible involvement of receptor-mediated endocytosis in the acquisition of lactoferrin-bound iron, while retrieval of iron from transferrin may depend on the extracellular release of iron from this ligand.
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