2-O-methyl-D-mannose residues are immunodominant in extracellular polysaccharides of Mucor racemosus and related molds.

In this study, the structure of the immunodominant carbohydrate epitope of the extracellular polysaccharides from mold species belonging to the order Mucorales reactive with rabbit IgG antibodies was elucidated. An exo-alpha-D-mannanase which was able to abolish the antigenicity of these polysaccharides completely was purified and characterized, and the activity was compared with that of an alpha-D-mannosidase. Analysis of the monomeric reaction products after enzymatic treatment revealed the presence of 2-O-methyl-D-mannose residues. This compound is a constituent of the polysaccharides from the mold genera Mucor, Rhizopus, Rhizomucor, Absidia, Syncephalastrum, and Thamnidium, and its occurrence in fungi has not been reported until now. Two mannan fractions which are highly reactive with rabbit IgG were isolated from the extracellular polysaccharides of Mucor racemosus and characterized with ethylation analysis. The role of the newly found 2-O-methyl-D-mannose residues in the immunoreactivity was assessed by specific degradation of these mannans with the exo-alpha-D-mannanase and subsequent ethylation analysis. It was concluded that the immunodominant carbohydrates reactive with rabbit IgG are chains composed of a single terminal non-reducing 2-O-methyl-D-mannose residue, alpha (1-2)-linked to a short sequence of alpha(1-2)-linked D-mannose residues.