Acquisition of iron from lactoferrin and transferrin by a parasitic protozoan 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°C was demonstrated for125I-labeled lactoferrin only. About 1.7 x 10 binding sites of a single class with K(d) ≃ 3.6 μM was estimated by means of Scatchard analysis. Internationalization of the bound lactoferrin was observed at 37°C. The cell-associated radioactivity after 30 min incubation of the parasite with125I-lactoferrin at 37°C was about 3.5-fold higher than the amount of 4°C. The majority of internalized125I-lactoferrin was released within 15 min of cell reincubation at 37°C in the presence of a 100-fold excess of nonlabeled lactoferrin. Released lactoferrin displayed unchanged mobility on autoradiography. In contrast to lactoferrin, binding of125I-lactoferrin 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 using89Fe-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°C (495 and 577 pmole Fe/mg of protein, respectively). The pH of the assay medium (PBS) decreased from pH7.4 to 5.6 after incubation with trichomonads. At ths 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, mechanism of iron acquisition from these host proteins appear to be different. Specific binding and internalization of lactoferrin suggest he 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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