Specific Binding of Ferrilactoferrin and Ferritransferrin in the Protozoan Leishmania chagasi

Summary

Previous work from our laboratory demonstrated that the promastigote form of the parasite Leishmania chagasi, the cause of South American visceral leishmaniasis, can grow in media containing Fe in the form of hemin, or Fe bound to lactoferrin or transferrin. Additionally, promastigotes were shown to take up ⁵⁹Fe from lactoferrin or transferrin, but uptake from lactoferrin was more rapid. Iron acquisition varied with the growth stage of the organism (log > stationary). The inability to detect any L. chagasi-derived siderophores or evidence of lactoferrin or transferrin cleavage led us to investigate the presence of specific promastigote lactoferrin and/or transferrin receptors. We now report evidence for specific and saturable binding of lactoferrin to L. chagasi promastigotes. Binding of [¹²⁵I]-labeled lactoferrin was inhibited by cold lactoferrin, but to a much lesser extent by cold transferrin. Binding kinetics for human apolactoferrin, human diferriclactoferrin, and bovine apolactoferrin were similar, as was lactoferrin binding to log and stationary-phase promastigotes. In contrast, preliminary studies suggest that saturable binding of [¹²⁵I]-labeled transferrin is inhibited by both cold transferrin and cold lactoferrin. Preliminary Scatchard data suggest that the promastigote lactoferrin receptor has a K_d of approx 4 x 10^–7 M with 2.5 × 10⁴ receptors/cell. In summary, lactoferrin binding to L. chagasi promastigotes is specific and saturable. Whether transferrin binds to the same or different receptor remains to be elucidated.