Pestalotia sp. 815, a newly isolated fungus, produces extracellularly a highly (1----6)-branched (1----3)-beta-D-glucan in high yield when grown in a
D-glucose-containing medium. This extracellular
glucan, designated "
Pestalotan", has [alpha]25D-0.1 degree (c 0.5, M NaOH) and a molecular weight greater than 2 X 10(6). Chemical and enzymic studies indicated that
pestalotan has a very highly branched structure containing a back-bone chain of beta-D-(1----3)-linked D-glucosyl residues, and three out of five D-glucosyl residues are substituted at O-6, mostly with single D-glucosyl groups, and a very few with short beta-(1----6)-linked
oligosaccharide units. This
D-glucan becomes water-insoluble after isolation from the culture filtrate followed by
dehydration, and shows moderate growth-inhibitory activities against mouse-implanted
tumors. However, when the D-glucosyl groups of the side chains were modified by
periodate oxidation and
borohydride reduction, the resulting, still water-insoluble,
D-glucan polyol exhibited potent antitumor activities, confirming that the attachment of many polyhydroxy groups to the beta-D-(1----3)-linked
D-glucan back-bone gives a remarkable enhancement effect on the antitumor activity of the branched
D-glucan. Prolonged treatment of the
D-glucan polyol by ultrasonic irradiation afforded a low-molecular-weight
D-glucan polyol (SD-
pestalotan polyol), without alteration of its chemical structure. The water-soluble, SD-
pestalotan polyol, having a molecular weight of 4.7 X 10(5), exhibited remarkable antitumor activities against both allogeneic and syngeneic, mouse-implanted
tumors, at small dosages (1-5 mg/kg for 10 days) by intraperitoneal administration. A comparison of values of the molecular weight of SD-
pestalotan polyol, estimated by 3-MPa l.c. for the aqueous
solution and the
dimethyl sulfoxide solution, strongly suggested that the
D-glucan polyol must form a triple-chain conformation in aqueous
solution.