Enzymes Involved in Pyrophosphate and Calcium Metabolism as Targets for Anti‐scuticociliate Chemotherapy

Inorganic pyrophosphate (PPi) is a key metabolite in cellular bioenergetics under chronic stress conditions in prokaryotes, protists and plants. Inorganic pyrophosphatases (PPases) are essential enzymes controlling the cellular concentration of PPi and mediating intracellular pH and Ca²⁺ homeostasis. We report the effects of the antimalarial drugs chloroquine (CQ) and artemisinin (ART) on the in vitro growth of Philasterides dicentrarchi, a scuticociliate parasite of turbot; we also evaluated the action of these drugs on soluble (sPPases) and vacuolar H+‐PPases (H+‐PPases). CQ and ART inhibited the in vitro growth of ciliates with IC50 values of respectively 74 ± 9 μM and 80 ± 8 μM. CQ inhibits the H+ translocation (with an IC50 of 13.4 ± 0.2 μM), while ART increased translocation of H+ and acidification. However, both drugs caused a decrease in gene expression of H+‐PPases. CQ significantly inhibited the enzymatic activity of sPPases, decreasing the consumption of intracellular PPi. ART inhibited intracellular accumulation of Ca²⁺ induced by ATP, indicating an effect on the Ca²⁺‐ATPase. The results suggest that CQ and ART deregulate enzymes associated with PPi and Ca²⁺ metabolism, altering the intracellular pH homeostasis vital for parasite survival and providing a target for the development of new drugs against scuticociliatosis.