Towards understanding vacuolar antioxidant mechanisms: a role for fructans?

Recent in vitro, in vivo, and theoretical experiments strongly suggest that sugar-(like) molecules counteract oxidative stress by acting as genuine reactive oxygen species (ROS) scavengers. A concept was proposed to include the vacuole as a part of the cellular antioxidant network. According to this view, sugars and sugar-like vacuolar compounds work in concert with vacuolar phenolic compounds and the ‘classic’ cytosolic antioxidant mechanisms. Among the biologically relevant ROS (H2O2, O2•–, and •OH), hydroxyl radicals are the most reactive and dangerous species since there are no enzymatic systems known to neutralize them in any living beings. Therefore, it is important to study in more detail the radical reactions between •OH and different biomolecules, including sugars. Here, Fenton reactions were used to compare the •OH-scavenging capacities of a range of natural vacuolar compounds to establish relationships between antioxidant capacity and chemical structure and to unravel the mechanisms of •OH–carbohydrate reactions. The in vitro work on the •OH-scavenging capacity of sugars and phenolic compounds revealed a correlation between structure and •OH-scavenging capacity. The number and position of the C=C type of linkages in phenolic compounds greatly influence antioxidant properties. Importantly, the splitting of disaccharides and oligosaccharides emerged as a predominant outcome of the •OH–carbohydrate interaction. Moreover, non-enzymatic synthesis of new fructan oligosaccharides was found starting from 1-kestotriose. Based on these and previous findings, a working model is proposed describing the putative radical reactions involving fructans and secondary metabolites at the inner side of the tonoplast and in the vacuolar lumen.