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 in Candida as an essential precursor for phospholipomannan, a glycophosphatidylinositol (GPI)-anchored glycolipid on the cell surface of Candida which 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-affinity myo-inositol uptake in Candida albicans has been characterized, with an apparent K m value 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 apparent K m value of 11·0±1·1 nM for H+, equal pH 7·96±0·05, suggesting that the C. albicans myo-inositol–H+ transporter is fully activated at physiological pH. C. albicans inositol 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 of myo-inositol and the transporter protein play a critical role for substrate recognition and binding. It is concluded that C. albicans myo-inositol–H+ transport differs kinetically and pharmacologically from the human sodium-dependent myo-inositol transport system and constitutes an attractive target for delivery of cytotoxic inositol analogues in this pathogenic fungus.

myo-Inositol Uptake Is Essential for Bulk Inositol Phospholipid but Not Glycosylphosphatidylinositol Synthesis in Trypanosoma brucei

“…This classification is consistent with the close phylogenetic relationship of TbHMIT to plant and mammalian HMITs and more distant relationship to SMITs. In addition, the observed specificity of TbHMIT for myo-inositol, and certain inositol derivatives, is in line with previous studies on HMITs from L. donovani, T. cruzi, A. thaliana, and C. albicans (29,30,52,53), showing that other polyols and hexoses are not transported by HMITs, in contrast to SMITs, which readily transport glucose (54 -56). Our observation that scyllo-inositol and myo-inositol phosphate inhibited FIGURE 5.…”

Arabidopsis INOSITOL TRANSPORTER2 Mediates H+ Symport of Different Inositol Epimers and Derivatives across the Plasma Membrane

“…Similarly, it was shown for human HepG2 liver cells that myoinositol and D-chiroinositol are transported by the same protein (Ostlund et al, 1996). Interestingly, however, in the yeast Candida albicans (Jin and Seyfang, 2003) and in the protozoan parasite Leishmania donovani (Mongan et al, 2004) transporters were identified that are highly specific for myoinositol, because no or only marginal competition by scyllo-or D-chiroinositol was seen. Moreover, for both inositol transport systems a 50% inhibition of myoinositol transport by phytate was observed.…”

Candida albicans Uses Multiple Mechanisms To Acquire the Essential Metabolite Inositol during Infection

“…The inositol uptake assay protocol was adapted in part from a protocol of Jin and Seyfang (15). The wild-type, itr1⌬/ITR1, itr1⌬/itr1⌬, and itr1⌬/itr1⌬::ITR1 strains were grown in YPD liquid cultures overnight at 30°C.…”

“…In addition, it has been shown previously that C. albicans has potent inositol transporter activity (15). It was demonstrated that the inositol transporter in C. albicans has an apparent K m of 240 Ϯ 15 M, and the transport system appears to be active and energy dependent (15). C. albicans inositol transport activity differs in substrate and cotransporter specificities from the human inositol-Na ϩ transporter.…”

“…C. albicans INO1 (CaINO1) shows 64% identity to ScINO1 at the amino acid level (18). In addition, it has been shown previously that C. albicans has potent inositol transporter activity (15). It was demonstrated that the inositol transporter in C. albicans has an apparent K m of 240 Ϯ 15 M, and the transport system appears to be active and energy dependent (15).…”