The mycocidal, membrane-active complex of Cryptococcus humicola is a new type of cellobiose lipid with detergent...

The chemical composition of the mycocidal complex (formerly known as microcin) secreted by Cryptococcus humicola was investigated by chemical, mass spectrometric and nuclear magnetic resonance methods. The results indicate that the mycocidal complex is composed of glycolipids with a highly acetylated (up to five acetyl groups) cellobiose backbone [beta-D-Glcp-(1'-->4)-beta-D-Glcp] linked to the omega-hydroxyl group of alpha,omega-dihydroxy palmitate [16:0-alpha,omega-di-OH] with an unsubstituted carboxyl group. The acyl chain forming aglycon can be replaced by [18:0-(alpha,omega-di-OH)], [18:0-(alpha,omega-1,omega-tri-OH)], and [18:0-(alpha,omega-2,omega-tri-OH)]. The complex has a comparatively high surface activity; 0.5 mg/ml of it reduced the surface tension of 0.1 M NaHCO(3) from 71 mN/m to 37 mN/m and interfacial tension against n-hexadecane from 39 mN/m to 10 mN/m. The critical micelle concentration of the complex at pH 4.0, determined by the fluorometric method with N-phenyl-1-naphthylamine as fluorescent probe and by the De Nouy ring method, was 2 x 10(-5) M (taking the average molecular mass of the complex to be 750); it did not depend on the presence of 100 mM KCl and was an order of magnitude higher at pH 7.0. By fluorescence resonance energy transfer spectroscopy with N-(7-nitro-2,1,3-benzoxadiazol-4-yl)-phosphatidylethanolamine as energy donor and N-(rhodamine B sulfonyl)-phosphatidylethanolamine as energy acceptor the complex was shown to intercalate into the liposomal lipid matrix. Primary lesions caused by the complex in planar lipid bilayers were revealed as short-living current fluctuations of a broad spectrum of amplitudes. The mycocidal effect of the complex is suggested to be associated with its detergent-like properties.