Screening of antioxidant activities of polysaccharides extracts from endemic plants in Gabon

The usage of plants as food, cosmetics or medicinal products is common throughout the world, particularly in the traditional medicines of Africa, Asia, and South America. Despite this long-standing use, many properties of plant constituents, including their capacity to inactivate reactive oxygen species, remain unknown today. In this context, polysaccharides were isolated from barks, leaves or stems of endemic plants of Gabon by sequential extractions of crude cell walls with oxalate and KOH. Analysis of fractions indicated that pectic extracts are mainly composed of HG and RG-I in various proportions whereas hemicellulose fractions are composed of both XXXG-type xyloglucans and (1,4)-xylans substituted by 4-O-Me GlcUA residues. Antioxidant activities of these plant extracts were evaluated by monitoring their free radical scavenging activity of 2-2-diphenyl, 1-picrylhydrazyl, iron reducing capacity and metal chelating activity. Pectin from four extracts presented antioxidant activity in the DPPH assay compared to ascorbic acid (AA). These extracts also exhibited high ferric iron reducing powers compared to AA but low ferrous ion-chelating properties compared to EDTA. These activities were concentration-dependent. In contrast, commercially available pectins, xyloglucan and arabinoxylan were found to be inactive in both assays. The free scavenging activity of these molecules disappeared after enzymatic digestion and saponification, whereas the iron reducing power partially remains after these two treatments. The fact that all pectins and hemicelluloses from the three plants reacted differently suggests that specific structural motifs are involved for generation of antioxidant activity