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Abstract. Pyruvic acid is an immunological determinant of the quite rigorously type-specific capsular polysaccharide of pneumococcal type IV (S IV). Removal of pyruvic acid by mild hydrolysis converts the capsular polysaccharide of type IV into an analog of the pneumococcal group-specific C-substance. Depyruvylated S IV resembles C-substance so closely immunologically that it not only precipitates a high proportion of the anti-C in antipneumococcal sera, regardless of their immunological types, but also, like C, precipitates human C-reactive protein in the presence of calcium ions. Apparently, the removal of pyruvic acid ketal rings from adjacent sugars unmasks N-acetylgalactosamine residues which must be linked and spaced much as are those in C-substance. Groupings reactive with suitably linked N-acetylgalactosamine, therefore, appear to be located on the surfaces of molecules of human C-reactive protein.Pyruvic acid has been found, from time to time, in bacterial polysaccharides but its immunological importance has only recently been realized.1 Linked to two hydroxyls of a sugar residue as a ketal2'3 it becomes a factor in the serological reactions of microorganisms as widely disparate as Rhizobia, pneumococci, and Klebsiella.4 We have therefore resumed the study of the capsular polysaccharide (S IV) of pneumococcal type IV,5 which contains pyruvic acid. Its components are, in addition, D-galactose and three amino sugars, galactosamine, mannosamine, and another, probably fucosamine, presumably present as their N-acetyl derivatives.The present report covers the removal of pyruvic acid from S IV and two unusual immunological consequences of its cleavage.

1. In addition to the previously identified components, d-glucose, N-acetyl-d-glucosamine and d-glucuronic acid, the immunologically specific capsular polysaccharide of pneumococcal type IX (polysaccharide S IX) is now found to contain mannosamine and galactosamine, probably as their N-acetyl derivatives. 2. Partial hydrolysis yields a complex mixture of oligosaccharides, several of which have been separated. 3. One of these is alpha-d-glucuronopyranosyl-(1-->3)-d-glucose. It is an inhibitor of the precipitation of antipneumococcal type IX serum by polysaccharide S IX. 4. Two others, d-glucuronopyranosyl-(1-->3)-d-glucosamine and d-glucuronopyranosyl-(1-->3)-d-galactosamine have been identified and a trisaccharide and pentasaccharide partially characterized.

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