Characterization of the myo‐inositol transport system in Trypanosoma cruzi

myo‐Inositol is a growth factor for mammalian cells as well as for the pathogenic protozoa Trypanosoma cruzi. Most of the cell surface molecules in this organism rely on myo‐inositol as the biosynthetic precursor for phosphoinositides and glycosylated phosphatidylinositols. The aim of this work was to investigate the process of myo‐inositol translocation across the parasite cell membrane. myo‐Inositol uptake was concentration‐dependent in the concentration range 0.1–10 µm with maximal transport obtained at 8 µm. Using sodium‐free buffers, where Na+ was replaced by choline or K+, myo‐inositol uptake was inhibited by 50%. Furosemide, an inhibitor of the ouabain‐insensitive Na+‐ATPase, inhibited the Na+‐dependent and Na+‐independent myo‐inositol uptake by 68 and 33%, respectively. In contrast, ouabain, an (Na++/K+) ATPase inhibitor, did not affect transport. Part of the myo‐inositol uptake is mediated by active transport as it was inhibited when energy metabolism inhibitors such as carbonyl cyanide p‐(trifluoromethoxy)‐phenylhydrazone (34%), 2,4‐dinitrophenol (50%), KCN (71%) and NaN3 (69%) were added to the medium, or the temperature of the medium was lowered to 4 °C. The addition of glucose (5–50 mm) or mannose (10 mm) did not change the myo‐inositol uptake, whereas the addition of 10 mm nonlabeled myo‐inositol totally inhibited this transport, indicating that the transporter is specific for myo‐inositol. Phloretin (0.3 mm) and phoridzin (5 mm), but not cytochalasin B, were efficient inhibitors of myo‐inositol uptake. A portion of the accumulated myo‐inositol is converted to inositol phosphates and phosphoinositides. These data show that myo‐inositol transport in T. cruzi epimastigotes is mediated by at least two specific transporters — one Na+‐dependent and the other Na+‐independent.