Intracellular calcium homeostasis in Leishmania mexicana. Identification and characterization of a plasma membrane calmodulin-dependent Ca(2+)-ATPase

The intracellular Ca2+ concentration in different trypanosomatids is about 50 nanomolar, which concentration in different trypanosomatids is about 50 nanomolar, which is 4 orders of magnitude lower than in the extracellular milieu. This fact implies the existence of well developed mechanisms for the maintenance of such a high calcium gradient. In higher eukaryotics a number of different structures have been implicated in this function. Some of them are located in intracellular organelles, and others in the plasma membrane. Since intracellular organelles are limited by their storage capacity, long-term Ca2+ homeostasis resides solely in the plasma membrane. In higher eukaryotics, a calcium pump or Ca(2+)-ATPase located in the plasma membrane, because of its high Ca2+ affinity, has been proposed as the structure responsible for the maintenance of the cytoplasmic Ca2+ concentration at the submicromolar level. The presence of a Ca(2+)-ATPase in trypanosomatids has been debated. While some groups have reported its absence, others have reported the existence of an enzyme which is Mg(2+)-independent or even inhibited by Mg2+. On the other hand, in none of these reports any correlation was shown between the Ca(2+)-ATPase activity observed and the Ca2+ transport function attributed to this enzyme. We have previously shown that a calmodulin-stimulated Mg(2+)-dependent Ca(2+)-ATPase is present in the plasma membrane of Leishmania braziliensis and of Trypanosoma cruzi. Plasma membrane vesicles from these parasites are able to accumulate Ca2+ in the presence of the ATP-Mg complex. The similarities found between the kinetics parameters and other properties of the Ca(2+)-ATPase and the Ca2+ transport activity strongly suggest a common molecular entity. The stoichiometry calculated from these parameters approaches the 1:1 stoichiometry for Ca2+ and ATP, as reported for the Ca2+ pump from higher eukaryotic cells. In this report we show that plasma membrane vesicles from Leishmania mexicana possess a Ca(2+)-ATPase with characteristics which are similar to that reported by us for other trypanosomatids. Thus, the enzyme has a high Ca2+ affinity which is further increased upon addition of calmodulin. The maximal velocity is also increased by calmodulin...

Tegumental Ca-stimulated adenosine triphosphatase activity in adult Schistosoma mansoni worms

A atividade ATPse (pH 9.5) estimulada por ions de Ca associados a uma fraçäo enriquecida de membranas do tegumento (fraçäo EMT) de vermes adultos de Schistosoma mansoni, foi inibida pro NAP-taurina ou por concentraçöes crescentes de clorpromacina. Foi encontrada calmodulina enfogena associada principlamente a esta fraçäo. Em vermes adultos fixados com glutaraldeido se detectou histoquimicamente uma atividade ATPase similar (pH 8.6) na face citoplasmática da dupla membrana de superfície e da membrana por 1 µM de clorpromacina e foi também observada na face interna de vesículas de dupla membrana presentes na fraçäo EMT. Näo se pôde detectar atividade ATpase em pH alcalino na presença de ions de Mg ou Na/K. A adiçäo externa de Ca, sem ATP, aos vermes fixados induz ao aparecimento de precipitados nos corpos discóides do tegumento; esta reaçäo foi inibida. Os resultados säo discutidos em relaçäo a uma possível regulaçäo intrategumentária de Ca pelos sistemas descritos e o possível uso de fenotiacinas contra os esquistossomas

Comparasion between plasma membrane Ca2 + and Na, K-ATPases: short review

This paper is a short review of the comparative biochemistry of the Na, K-ATPase and Ca2+ -ATPase of plasma membranes. The two ATPases share the same biphasic activation by ATP. Ca2+ - ATPase activation by ATP is strongly affected by calmodulin. The possibility of Mg2+ occlusion is proposed in connection with low-affinity activation by ATP. Both ATPase are activated by alkaline earth metal ions and display phosphatase activity toward p-nitrophenylphosphat for which Ca2 + - ATPase is strongly dependent on K + and regulated by calmodulin. The requirements for ligands of the phosphatase activity of both ATPase are strikingly similar except for the effect of calmodulin. Both ATPases are inhibited by vanadat and for both the effect of vanadate is modulated by Mg2+ and K + in the same way. These similarities indicate that, although Ca2 + - ATPase and Na, K-ATPase are different enzymes, their mechanisms of action may have more feactures in common than previously thought