Demonstration of the presence of alpha-1,6-branched side chains in the mannan of Candida stellatoidea.

A mild acetolysis of the mannans of Candida stellatoidea was performed after acetylation to yielded an alpha-1,6-branched mannohexaose, the presence of which had been predicted from the appearance of a specific H1-H2-correlated cross-peak in two-dimensional homonuclear Hartmann-Hahn spectroscopy. In this study, we found that the de-O-acetylation of a 4-O-acetyl group at the branching point, the 3,6-di-O-substituted mannose unit, of an acetylated oligosaccharide by sodium methoxide is significantly slower than that of other acetyl groups. We could separate the 4-O-acetylated branching oligosaccharide from linear isomer using high-performance liquid chromatography. Before and after the de-O-acetylation of the purified branching oligosaccharide, their 1H-NMR signals were sequentially assigned by means of the nuclear Overhauser effect. In the sequential NMR assignment study, we showed that the alpha-1,6-linked mannose unit is attached to the 3-O-substituted unit based on the presence of NOE cross-peak between H1 of the branching mannose unit and H6 of the 3-O-substituted mannose unit. An enzyme-linked immunosorbent inhibition assay of the reactivity of factor 4 serum to C. stellatoidea mannan by several oligosaccharides indicated that the alpha-1,6-branched oligosaccharide and the beta-1,2 linkage-containing oligosaccharides showed inhibitory activity. This result indicates that factor 4 serum, as well as factor 5 and 6 sera, contains antibodies against beta-1,2-linked mannose units which have been reported to participate in pathogenicity via cytokine production and/or adherence. From the assignment results of H1-H2-correlated cross-peaks of oligosaccharides and mannans, the molar ratio of the mannan side chains was proposed. In this study, we demonstrated that the epitope structure of the C. stellatoidea type I strains was the same as that of the C. albicans NIH B-792 (serotype B) strain.