The concomitant expression and availability of conventional and alternative, cyanide-insensitive, respiratory...

The opportunistic oral pathogen Candida albicans expresses a cyanide-insensitive alternative oxidase (AOX) upon exposure to respiratory inhibitors that act downstream from coenzyme Q, and upon ageing of cells. To investigate whether the conventional pathway is retained when the alternative pathway is induced, cells were grown in the presence of sodium cyanide, a reversible inhibitor of cytochrome oxidase. AOX expression was monitored by Western blotting and the presence of cytochromes associated with complexes III and IV of the conventional pathway was monitored by recording spectra between 500 and 650 nm at 77K. The activities of complexes III and IV were determined in polarographic and enzyme-kinetic experiments using specific respiratory substrates and inhibitors. Results indicated that complexes III and IV are constitutively expressed and are functional in cells expressing AOX. Furthermore, the enzymatic activities of complexes III and IV were similar in mitochondrial preparations from cells grown with or without cyanide. We next investigated whether both pathways are simultaneously available for electron transfer from the Q pool to molecular oxygen. Respiration was virtually completely inhibited by the combination of cyanide and salicyl hydroxamic acid (SHAM) or antimycin A and SHAM, but only partly inhibited by either of these inhibitors alone. This indicates that electrons can in principle flow either through the conventional or the alternative respiratory pathway. The availability of two electron pathways in C. albicans and the potential use of either pathway endows this pleomorphic fungus with another level at which it can rapidly adjust to altered environmental conditions.