Characterization of the mitochondrial respiratory pathways in Candida albicans

Candida albicans is an opportunistic oral pathogen. The flexibility of this microorganism in response to environmental changes includes the expression of a cyanide-resistant alternative respiratory pathway. In the present study, we characterized both conventional and alternative respiratory pathways and determined their ADP/O ratios, inhibitor sensitivity profiles and the impact of the utilization of either pathway on susceptibility to commonly used antimycotics. Oxygen consumption by isolated mitochondria using NADH or malate/pyruvate as respiratory substrates indicated that C albicans cells express both cytoplasmic and matrix NADH-ubiquinone oxidoreductase activities. The ADP/O ratio was higher for malate/pyruvate (2.2 +/- 0.1), which generate NADH in the matrix, than for externally added NADH (1.4 +/- 0.2). In addition, malate/pyruvate respiration was rotenone-sensitive, and an enzyme activity assay further confirmed that C albicans cells express Complex I activity. Cells gown in the presence of antimycin A expressed the cyanide-insensitive respiratory pathway. Determination of the respiratory control ratio (RCR) and ADP/O ratios of mitochondria from these cells indicated that electron transport from ubiquinone to oxygen via the alternative respiratory pathway was not coupled to ATP production; however, an ADP/O ratio of 0.8 was found for substrates that donate electrons at Complex I. Comparison of antifungal susceptibility of C albicans cells respiring via the conventional or alternative respiratory pathways showed that respiration via the alternative pathway does not reduce the susceptibility of cells to a series of clinically employed antimycotics (using Fungitest(R)), or to the naturally occurring human salivary antifungal peptide, histatin 5. (C) 2002 Elsevier Science B.V. All rights reserved.