Studies on the chemical structure and antitumor activity of an exopolysaccharide from Rhizobium sp. N613.

The molecular structure of the rhizobium exopolysaccharide (REPS) was analyzed by enzymolysis, periodate oxidation, and Smith degradation, and by IR and NMR spectroscopy. The results indicated that REPS was a beta-glucan with a backbone of beta-D-(1-->4)-linked glucose residues and branches of beta-D-(1-->6)-linked glucose residues. The branch was attached to the main chain at the 6-O-position. The molar ratio of 1-->4 and 1-->6 was 2:1. The terminal C3 of the (1-->6)-Glc branch had an O-acetyl group. The molecular weight was estimated to be 35 kDa by Sephadex G-100 column chromatography. The antitumor activity of REPS was evaluated in mice bearing sarcoma 180, hepatoma 22, and Ehrlich ascites carcinoma tumor, respectively. At doses of 10-60 mg/kg, it was observed that tumor formation decreased significantly (P <0.01), but the relative spleen and thymus weight, the phagocytic function of monocytes, lymphocyte proliferation, and serum hemolysis antibody increased significantly (P <0.05). Results of these studies demonstrated that the REPS polysaccharide possessed antitumor activity.