The cross-reactivity of ketha gum and pneumococcal type I--short cut to a constituent of a polysaccharide.

Although ketha gum has been reported to contain arabinose, galactose, xylose, an unidentified neutral sugar and glucuronic acid, its marked cross-reaction with Type I antipneumococcal horse sera led to the conclusion that D-galacturonic acid must be an additional component. The capsular polysaccharide of Type I pneumococcus, the determinant of immunological specificity, consists of more than 50 per cent of D-galacturonic acid. Chromatography of hydrolysates of ketha gum confirmed the presence of arabinose, galactose and xylose, and furnished spots with the mobilities of galacturonic and 4-O-methylglucuronic acids. Traces of glucuronic acid appeared only under conditions such that demethylation of the 4-O-methyl acid might have occurred. Hydrolysis of larger amounts furnished D-galacturonic acid, as yet syrupy, [α]+32° instead of +51°, and 4-O-methyl-D-glucuronic acid, [α]+20° instead of +35°. The latter, especially if present as non-reducing end-groups, would account for the strong precipitation of ketha gum in Type II antipneumococcal horse sera. When ketha gum was esterified, reduced and hydrolysed, the two acids disappeared almost entirely, a component corresponding to 4-O-methylglucose was observed, and the hexose content was increased. Oxidation of the gum with periodate, followed by reduction and hydrolysis, caused the disappearance of the 4-O-methylglucuronic acid and most of the galacturonic acid. Comparison of analyses and chromatograms of the original gum with those of samples recovered from the specific precipitates with Types I and II antisera showed little or no fractionation.