High-affinity myo-inositol transport in Candida albicans: substrate specificity and pharmacology

Abstract

Inositol is considered a growth factor in yeast cells and it plays an important role inCandidaas an essential precursor for phospholipomannan, a glycophosphatidylinositol (GPI)-anchored glycolipid on the cell surface ofCandidawhich is involved in the pathogenicity of this opportunistic fungus and which binds to and stimulates human macrophages. In addition, inositol plays an essential role in the phosphatidylinositol signal transduction pathway, which controls many cell cycle events. Here, high-affinitymyo-inositol uptake inCandida albicanshas been characterized, with an apparentKmvalue of 240±15 μM, which appears to be active and energy-dependent as revealed by inhibition with azide and protonophores (FCCP, dinitrophenol).Candida myo-inositol transport was sodium-independent but proton-coupled with an apparentKmvalue of 11·0±1·1 nM for H+, equal pH 7·96±0·05, suggesting that theC. albicansmyo-inositol–H+transporter is fully activated at physiological pH.C. albicansinositol transport was not affected by cytochalasin B, phloretin or phlorizin, an inhibitor of mammalian sodium-dependent inositol transport. Furthermore,myo-inositol transport showed high substrate specificity for inositol and was not significantly affected by hexose or pentose sugars as competitors, despite their structural similarity. Transport kinetics in the presence of eight different inositol isomers as competitors revealed that proton bonds between the C-2, C-3 and C-4 hydroxyl groups ofmyo-inositol and the transporter protein play a critical role for substrate recognition and binding. It is concluded thatC. albicansmyo-inositol–H+transport differs kinetically and pharmacologically from the human sodium-dependentmyo-inositol transport system and constitutes an attractive target for delivery of cytotoxic inositol analogues in this pathogenic fungus.