Heme binding to Leishmania mexicana amazonensis.

Leishmania mexicana amazonensis is a pathogenic parasite whose growth, due to a biosynthetic deficiency, is dependent on a supply of exogenous heme. Utilizing [55Fe]hemin, we have demonstrated that heme binding to non-dividing cultured promastigotes of L. m. amazonensis at 4 degrees C reaches equilibrium within 6 h, is 95% dissociable by 28 h and is elevated approximately 5-fold by decreasing the pH of the binding buffer to 5.4. Metalloporphyrins substituted either at the central metal atom or in the porphyrin ring all displaced [55Fe]hemin binding to varying extents. Scatchard analysis revealed the affinity of the interaction to be 0.03 nM-1 and the number of binding sites to be 400 per promastigote. These findings are remarkably similar to those demonstrated in murine erythroleukemia cells and are characteristic of a receptor-ligand interaction. During logarithmic growth, promastigote heme binding was increased approximately 10-fold compared to stationary phase organisms. The increase was caused by a 4-fold greater number of binding sites per promastigote with no significant change in affinity. These findings demonstrate not only that L. m. amazonensis promastigotes bind heme specifically, but that the binding may be regulated by the growth phase of the parasite.