D-Arabino-D-galactan, the main antigenic polysaccharide of Mycobacterium species, was isolated from the cell walls of M. tuberculosis strain H₃₇R_v, H₃₇R_a and Aoyama B, M. bovis BCG, M. phlei and M. smegmatis, and also atypical Mycobacterium P1. All arabinogalactan preparations had similar chemical properties ([α]D+23.5-28°; ratio of D-arabinose to D-galactose, 5:2), and were immunologically identical. They have ramified structures, with repeating units of 11-16 sugar residues, consisting of α-(1→5)- and (1→2) (minor)-D-arabinofuranosidic linkages, and, β-(1→4)-D-galactopyranosidic (or (1→5)-furanosidic) linkages; the side chains are terminated with arabinofuranose residues and are attached to the main chains at C-3 of arabinose and probably at C-6 of galactose residues, respectively.
The action of bacterial M-2 enzyme on the arabinogalactan of M. phlei resulted in the release of a series of D-arabino-oligosaccharides, which included α-(1→5)-linked arabinobiose, -triose, and -tetraose, leaving a degraded polysaccharide (ratio of arabinose to galactose, 2:5.4) which may represent a back-bone chain. The inhibitory activities of these oligosaccharides in the precipitin reaction between the arabinogalactan and the anti-cell-wall sera indicate that the arabinofuranosyl side chains are responsible for the serological activity of the cell-wall polysaccharides. The arabinogalactan preparations reacted not only with antisera against mycobacterial cell walls but also with those against Corynebacterium diphtheriae and Nocardia asteroides, suggesting that the arabinogalactan is a common antigen of these bacterial groups.