Chemical characterization of the immunomodulating polysaccharide of Aloe vera L. Original Research Article

The polysaccharide isolated by alcohol precipitation of Aloe vera mucilaginous gel was found to have a Man:Glc:Gal:GalA:Fuc:Ara:Xyl ratio of 120:9:6:3:2:2:1 with traces of Rha and GlcA. Linkage analysis of the endo-(1→4)-β-d-mannanase-treated sample yielded Manp-(1→ (∼26%), 4-Manp (∼53%), 2,4-Manp (∼3%), 3,4-Manp (∼1%), 4,6-Manp (∼1%), 4-Glcp (∼5%), 4-Xylp (∼1%), Xylp-(1→ (∼2%), Galp-(1→ (∼5%), and traces of 4,6-Galp and 3,6-Galp. Hydrolysis with strong acids produced a mixture of short oligosaccharides and an acid-resistant fraction containing greater relative fractions of Manp-(1→, Araf-(1→, Xylp-(1→, and 4-Xylp than the bulk polysaccharide. NMR analysis of oligosaccharides generated by endo-(1→4)-β-d-mannanase and acid hydrolysis showed the presence of di-, tri-, and tetrasaccharides of 4-β-Manp, β-Glcp-(1→4)-Man, β-Glcp-(1→4)-β-Manp-(1→4)-Man, and β-Manp-(1→4)-[α-Galp-(1→6)]-Man, consistent with a backbone containing alternating →4)-β-Manp-(1→ and →4)-β-Glcp-(1→ residues in a ∼15:1 ratio. Analysis of the sample treated sequentially with endo-(1→4)-β-d-mannanase and α-d-galactosidase showed that the majority of α-Galp-(1→ residues were linked to O-2, O-3, or O-6 of →4)-β-Manp-(1→ residues, with ∼16 →4)-β-Manp-(1→ residues between side chains. Our data provide direct evidence of a previously proposed glucomannan backbone, but draw into question previously proposed side-chain structures.