Characterization of the myo-inositol transport system in Trypanosoma cruzi

myo-Inositol is a growth factor for mammalian cells as well as for the path ogenic protozoa Trypanosoma cruzi. Most of the cell surface molecules in th is organism rely on myo-inositol as the biosynthetic precursor for phosphoi nositides and glycosylated phosphatidylinositols. The aim of this work was to investigate the process of myo-inositol translocation across the parasit e cell membrane. myo-Inositol uptake was concentration-dependent in the con centration range 0.1-10 mu M with maximal transport obtained at 8 mu M Usin g sodium-free buffers, where Na+ was replaced by choline or K+, myo-inosito l uptake was inhibited by 50%. Furosemide, an inhibitor of the ouabain-inse nsitive Na+-ATPase, inhibited the Na+-dependent and Na+-independent myo-ino sitol uptake by 68 and 33%, respectively. In contrast, ouabain, an (Na++/K) ATPase inhibitor, did not affect transport. Part of the myo-inositol upta ke is mediated by active transport as it was inhibited when energy metaboli sm 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 degrees C. The ad dition of glucose (5-50 mM) or mannose (10 mM) did not change the myo-inosi tol uptake, whereas the addition of 10 mM nonlabeled myo-inositol totally i nhibited this transport, indicating that the transporter is specific for my o-inositol. Phloretin (0.3 mM) and phoridzin (5 mM), but not cytochalasin B , were efficient inhibitors of myo-inositol uptake. A portion of the accumu lated myo-inositol is converted to inositol phosphates and phosphoinositide s. 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.