Evidence of an alternative oxidase pathway for mitochondrial respiration in the scuticociliate Philasterides dicentrarchi.

The presence of an alternative oxidase (AOX) in the mitochondria of the scuticociliate P. dicentrarchi was investigated. The mitochondrial oxygen consumption was measured in the presence of KCN, an inhibitor of cytochrome pathway (CP) respiration and salicylhydroxamic acid (SHAM), a specific inhibitor of alternative pathway (AP) respiration. AOX expression was monitored by western blotting with an AOX polyclonal antibody. The results showed that P. dicentrarchi possesses a branched mitochondrial electron transport chain with both cyanide-sensitive and -insensitive oxygen consumption. Mitochondrial respiration was partially inhibited by cyanide and completely inhibited by the combination of cyanide and SHAM, which is direct evidence for the existence of an AP in this ciliate. SHAM significantly inhibited in vitro growth of trophozoites both under normoxic and hypoxic conditions. AOX is a 42kD monomeric protein inducible by hypoxic conditions in experimental infections and by CP inhibitors such as cyanide and antimycin A, or by AP inhibitors such as SHAM. CP respiration was greatly stimulated during the exponential growth phase, while AP respiration increased during the stationary phase, in which AOX expression is induced. As the host does not possess AOX, and because during infection P. dicentrarchi respires via AP, it may be possible to develop inhibitors targeting the AP as a novel anti-scuticociliate therapy.