Chemical characterization of the immunomodulating polysaccharide of Aloe vera L.

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)-beta-D-mannanase-treated sample yielded Manp-(1 ->(similar to 26%), 4-Manp(similar to 53%), 2,4-Manp (similar to 3%), 3,4-Manp (similar to 1%), 4,6-Manp (similar to 1%), 4-Glcp (similar to 5%), 4-Xylp (similar to 1%), Xylp-(1 ->(similar to 2%), Galp-(1 ->(similar to 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)-beta-D-mannanase and acid hydrolysis showed the presence of di-, tri-, and tetrasaccharides of 4-beta-Manp, beta-Glcp-(1 -> 4)-Man, beta-Glcp-(1 -> 4)-beta-Manp-(1 -> 4)-Man, and beta-Manp-(1 -> 4)-[alpha-Galp-(1 -> 6)]-Man, consistent with a backbone containing alternating and -> 4)-beta-Manp-(1 -> and -> 4)-beta- Glcp(1 -> residues in a similar to 15:1 ratio. Analysis of the sample treated sequentially with endo-(1 -> 4)-beta-D-mannanase and alpha-D-galactosidase showed that the majority of alpha-Galp-(1 -> residues were linked to O-2, O-3, or O-6 of -> 4)-beta-Manp-(1 -> residues, with similar to 16 -> 4)-beta-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. (c) 2005 Elsevier Ltd. All rights reserved.