Inhibition of inositol phosphorylceramide synthase by aureobasidin A in Candida and Aspergillus species

Inositol phosphorylceramide (IPC) synthase is an enzyme common to fungi and plants that catalyzes the transfer of phosphoinositol from phosphatidylino sitol to ceramide to form IPC, The reaction is a key step in fungal sphingo lipid biosynthesis and the target of the antibiotics galbonolide A, aureoba sidin A, and khafrefungin. As a first step toward understanding the antifun gal spectrum of IPC synthase inhibitors, we examined the sensitivity of IPC synthase to aureobasidin A in membrane preparations of Candida species (Ca ndida albicans, C. glabrata, C. tropicalis, C, parapsilosis, and C, krusei) and Aspergillus species (Asperigillus fumigatus, A. flavus, A, niger, and A. terreus). As expected, preparations from the five Candida species, all e xquisitely susceptible to aureobasidin A (MICs, <2 mu g/ml), had IPC syntha se activity (specific activity, 50 to 400 pmol/min/mg of protein) sensitive to aureobasidin A (50% inhibitory concentrations [IC(50)s], 2 to 4 ng/ml), Surprisingly, preparations from the four Aspergillus species, including A. fumigatus and A. flavus, which are intrinsically resistant to aureobasidin A (MICs, >50 mu g/ml), had IPC synthase activity (specific activity, 1 to 3 pmol/min/mg of protein) also sensitive to aureobasidin A (IC(50)s, 3 to 5 ng/ml), The mammalian multidrug resistance modulators verapamil, chlorprom azine, and trifluoperazine lowered the MIC of aureobasidin A for A. fumigat us from >50 mu g/ml to 2 to 3 mu g/ml, suggesting that the resistance of th is major fungal pathogen is the result of increased efflux.