Botryosphaeran – A Fungal Exopolysaccharide of the (1→3)(1→6)-β-D-Glucan Kind: Structure and Biological Functions

A fungal isolate (MAMB-05) from a stem canker on a eucalypt tree, molecularly classified as Botryosphaeria rhodina, produced an exopolysaccharide (EPS) of the β-D-glucan type when cultivated on glucose by submerged fermentation. Isolation and elucidation of the chemical structure demonstrated that the β-glucan was of the (1→3)(1→6)-linked type and was named botryosphaeran. When cultivated on different carbohydrates, B. rhodina MAMB-05 produced a family of structurally related botryosphaerans sharing the same backbone chain of (1→3)-linked D-glucosidic residues, but varied in the degree of branching on C-6 with glucose and gentiobiose. A branch point occurred on every fifth glucose residue along the backbone chain for the botryosphaeran produced on glucose (EPSGLC) and sucrose (EPSSUC) and one in every three glucose residues when produced on fructose (EPSFRU). Details of the chemical structures were resolved by methylation, Smith degradation, FT-IR, and 13C NMR. Membrane-modifying agents (soybean oil, Tween 80) enhanced botryosphaeran production without altering its chemical structure. Sulfonylation of botryosphaeran increased water solubility and conferred new functions, anticoagulation and antiviral activity. Through extensive screening, botryosphaeran was found to be non-mutagenic and non-genotoxic and is regarded as having Generally Recommended As Safe (GRAS) status. The biopolymer exhibited strong antimutagenic/anticlastogenic activity being chemoprotective against several cancer chemotherapy drugs. Botryosphaeran demonstrated hypoglycemia in diabetes-induced rats, and hypocholesterolemia in mice and rats conditioned on a high-lipid ration. Antiproliferative activity was demonstrated in breast cancer MCF-7 cells, was associated with apoptosis, necrosis, and oxidative stress, and mediated by AMP-activated protein kinase and forkhead transcription factor, FOXO3a. This chapter addresses the studies carried out with botryosphaeran.