Trypanosoma cruzi receptors for human transferrin and their role

Trypanosoma cruzi amastigotes present receptors for human transferrin as indicated by the saturable binding of 125I-transferrin to this form of the parasite. Computerized Scatchard analysis revealed one class of receptors present at 8.1 X 10(4) receptors per amastigote with a Kd of 2.82 microM. Immunofluorescence studies indicate that more than 90% of amastigotes bind human transferrin, whereas trypomastigotes do not. Iron is required for amastigote growth in cell-free medium since deferoxamine, an iron chelator, inhibits amastigote growth. Amastigote growth is restored when deferoxamine is removed from the medium. 59Fe-transferrin, which bound to amastigotes at 4 degrees C for 1 h, was readily dissociated from the parasite surface upon treatment with acid. However, this treatment did not disrupt binding that occurred at 37 degrees C for 1 h. Amastigote growth in cell-free medium is inhibited in ferrotransferrin-depleted serum, and addition of ferrotransferrin but not apotransferrin restores parasite growth. Western blots of solubilized amastigote membranes probed with anti-human transferrin receptor antibody recognize a protein of 200 kDa. This protein is present on the amastigote cell surface; therefore, human transferrin seems to interact with a 200-kDa surface amastigote protein receptor. Iron, which is essential for amastigote growth, thus appears to be delivered to T. cruzi amastigotes by transferrin receptor-mediated endocytosis.