OUABAIN-SENSITIVE NA-ATPASE IN THE PLASMA-MEMBRANE OF LEISHMANIA-MEXICANA(,K+)

The mechanism responsible for the regulation of intracellular Na+ and K+ concentrations in trypanosomatids is unknown. In higher eukaryotes a ouabain-sensitive Na+,K+-ATPase located in the plasma membrane is th e main mechanism for the regulation of the intracellular concentration s of Na+ and K+, while in trypanosomatids there are conflicting eviden ces about the existence of this type of ATPase. By the use of a highly enriched plasma membrane fraction, we showed that an ouabain-sensitiv e Na+,K+-ATPase is present in L. mexicana. The affinity of the enzyme for Na+ and K+ is similar to that reported for the mammalian Na+,K+-AT Pase, showing also the same kinetic parameters regarding the relative concentration of those cations that give the optimal activity. Vanadat e (10 mu M) fully inhibits the ATPase activity, suggesting that the en zyme belongs to the P-type family of ionic pumps. The enzyme is sensit ive to ouabain and other cardiac glycosides. These cardiac glycosides do not show any appreciable effect on the higher Mg2+-ATPase activity present in the same preparation. By the use of [H-3]ouabain, we also s how in this report that the binding of the inhibitor to the enzyme was specific, Taken together, these results demonstrate that an ouabain-s ensitive Na+,K+-ATPase is present in the plasma membrane of Leishmania mexicana. Therefore, this Na+,K+-ATPase should participate in the int racellular regulation of these cations in Leishmania.