Activation of limulus coagulation factor G by several (1-->3)-beta-D-glucans: comparison of the potency of glucans with identical degree of polymerization but different conformations.

It has been demonstrated that both linear and branched (6-O-beta-D-glucosyl) (1-->3)-beta-D-glucans taking a single helical conformation are more effective than those taking a triple helical conformation for the activation of factor G from horseshoe crab amebocytes, as revealed by high-resolution solid-state 13C-NMR spectroscopy [Saitô, H. et al. (1991) Carbohydr. Res. 217, 181-190]. Annealing the linear glucan at 180 degrees C was essential to convert the conformation from the single helix to the triple helix. We found that heating of the glucan at such a high temperature resulted in depolymerization of the sample to molecular weight smaller than 10,000, which may influence the conformation and the above-mentioned biological activity. To eliminate ambiguity arising from the depolymerization of the glucan during annealing, we aimed to relate the biological activity to the conformation of samples whose chain lengths are identical, because the potency is known to depend on the molecular weight of the glucans. This molecular weight dependency of the potency, however, was found to be not the dominant factor, provided that the molecular weight is large enough to allow formation of the single helix conformation. Therefore, the single helical conformation of (1-->3)-beta-D-glucans is clearly demonstrated to be the dominant contributor to the activation of limulus coagulation factor G.