Characteristics of xyloglucan after attack by hydroxyl radicals

• Published in: Carbohydrate research Vol. 332; no. 4; pp. 389 - 403
• Main Authors: Miller, Janice G., Fry, Stephen C.
• Format: Journal Article
• Language: English
• Published: Netherlands Elsevier Ltd 15.06.2001
• Subjects: acid hydrolysis; alkaline hydrolysis; analysis; aniline; Ascorbate; Ascorbic Acid; Borohydrides; Carbohydrate Conformation; Carbohydrate Sequence; cell walls; chemistry; digestion; endo-1,4-beta-glucanase; Fabaceae; Fabaceae - chemistry; galactose; Galactose - analysis; Glucans; glucose; Glucose - analysis; Glycosulose residues; Hemicellulose; Hydrogen Peroxide; Hydroxyl Radical; hydroxyl radicals; mannose; Mannose - analysis; Molecular Sequence Data; Non-enzymic scission; Oligodeoxyribonucleotides; Oligodeoxyribonucleotides - chemistry; oxygen; Plant cell wall; Plants, Medicinal; Polysaccharides; Polysaccharides - chemistry; ribose; Ribose - analysis; Seeds; Seeds - chemistry; staining; Tritium; viscosity; Xylans; Xyloglucan; xyloglucans; xylose; Xylose - analysis; Xyloglucan; Hemicellulose; Ascorbate; Plant cell wall; Non-enzymic scission; Glycosulose residues
• ISSN: 0008-6215; 1873-426X
• DOI: 10.1016/S0008-6215(01)00110-0

It has been proposed that plant cell-wall polysaccharides are subject in vivo to non-enzymic scission mediated by hydroxyl radicals ( OH). In the present study, xyloglucan was subjected in vitro to partial, non-enzymic scission by treatment with ascorbate plus H 2O 2, which together generate OH. The partially degraded xyloglucan appeared to contain ester bonds within the backbone, as indicated by an irreversible decrease in viscosity upon alkaline hydrolysis. Aldehyde and/or ketone groups were also introduced into the polysaccharide by OH-attack, as indicated by staining with aniline hydrogen-phthalate and by reaction with NaB 3H 4. The introduction of ester and oxo groups supports the proposed sequence of reactions: (a) OH-mediated H-abstraction to produce a carbon-centred carbohydrate radical; (b) reaction of the latter with O 2; and (c) elimination of a hydroperoxyl radical (HO 2 ). When the partially degraded xyloglucan was reduced with NaB 3H 4 followed by acid hydrolysis, several 3H-aldoses were detected ([ 3H]galactose, [ 3H]xylose, [ 3H]glucose, [ 3H]ribose and probably [ 3H]mannose), in addition to unidentified 3H-products (probably including anhydroaldoses). 3H-Alditols were undetectable, showing that few or no conventional reducing termini were introduced. Digestion of the NaB 3H 4-reduced, partially degraded xyloglucan with Driselase released 25 times more [ 3H]Xyl-α-(1→6)-Glc than Xyl-α-(1→6)-[ 3H]Glc, suggesting that the xylose side-chains of the xyloglucan had been more heavily attacked by OH than the glucose residues of the backbone. The radioactive xyloglucan was readily digested by cellulase, yielding 3H-products in the hepta- to nonasaccharide range. A fingerprinting strategy for identifying OH-attacked xyloglucan in plant cell walls is proposed. OH radicals, generated by ascorbate+O 2, cause non-enzymic scission of the plant cell-wall polysaccharide, xyloglucan. A sensitive method is proposed for ‘fingerprinting’ OH-attacked xyloglucan by 3H-labelling.