Mass spectrometry characterization of an Aloe vera mannan presenting immunostimulatory activity

Abstract

Aloe vera acemannan is a polysaccharide composed by a backbone of β-(1→4)-linked D-mannose residues interspersed by few glucose residues, acetylated in O-2, O-3, and O-6 containing side chains constituted by O-6-linked single α-D-galactose and α-L-arabinose residues. This structural features are rather similar to mannans from other sources, namely coffee and locust bean gum. However, Aloe vera acemannan and coffee mannans present immunostimulatory activity but locust bean gum does not. In order to know more about the structural features of a commercial preparation of Aloe vera presenting comparable immunostimulatory activity to that observed for coffee mannans, this preparation was submitted to sugar and methylation analysis. To gain further insight to the structural details of the mannans, focusing in the study of acetylation pattern, a specific hydrolysis with an endo-β-(1→4)-D-mannanase was performed and the resulting oligosaccharides (OS) were fractionated by size exclusion chromatography and characterized by ESI-MS, ESI-MS/MS and MALDI-MS. The majority of the OS obtained for acemannan had a ratio of two acetyl groups per sugar residue. The observation of OS highly acetylated as well as non-acetylated OS, allowed to infer a non-homogeneous distribution of the acetyl groups. Also, it was observed OS presenting fully acetylated arabinose residues. The occurrence of a high abundance of acetylated residues shows that this polysaccharide contains odd acetylation content. These unusual features are reinforced by the presence of acetylated side chains, only previously observed in chemically acetylated mannans with immunostimulatory activity prepared from coffee residue. The comparison with other galactomannans allowed to infer that lower branching, shorter chains, and higher acetylation seems to promote the immunostimulatory activity attributed to these polysaccharides.