A proton pumping pyrophosphatase in acidocalcisomes of Herpetomonas sp

Acidocalcisomes are acidic calcium storage organelles found in several microorganisms. They are characterized by their acidic nature, high electron density, high content of polyphosphates and several cations. Electron microscopy contrast tuned images of Herpetomonas sp. showed the presence of several electron dense organelles ranging from 100 to 300 nm in size. In addition, X-ray element mapping associated with energy-filtering transmission electron microscopy showed that most of the cations, namely Na, Mg, P, K, Fe and Zn, are located in their matrix. Using acridine orange as an indicator dye, a pyrophosphate-driven H⁺ uptake was measured in cells permeabilized by digitonin. This uptake has an optimal pH of 6.5–6.7 and was inhibited by sodium fluoride (NaF) and imidodiphosphate (IDP), two H⁺-pyrophosphatase inhibitors. H⁺ uptake was not promoted by ATP. Addition of 50 μM Ca²⁺ induced the release of H⁺, suggesting the presence of a Ca²⁺/H⁺ countertransport system in the membranes of the acidic compartments. Na⁺ was unable to release protons from the organelles. The pyrophosphate-dependent H⁺ uptake was dependent of ion K⁺ and inhibited by Na⁺ Herpetomonas sp. immunolabeled with monoclonal antibodies raised against a Trypanosoma cruzi V-H⁺-pyrophosphatase shows intense fluorescence in cytoplasmatic organelles of size and distribution similar to the electron-dense vacuoles.

Together, these results suggest that the electron dense organelles found in Herpetomonas sp. are homologous to the acidocalcisomes described in other trypanosomatids. They possess a vacuolar H⁺-pyrophosphatase and a Ca²⁺/H⁺ antiport. However, in contrast to the other trypanosomatids so far studied, we were not able to measure any ATP promoted H⁺ transport in the acidocalcisomes of this parasite.