Effect of molecular weight on the antioxidant property of low molecular weight alginate from Laminaria japonica

In this study, three alginate fractions with different molecular weights and ratios of mannuronic acid (M) to guluronic acid (G) were prepared by enzymatic hydrolysis and ultrafiltration to assess the antioxidant property of alginates from Laminaria japonica with molecular weight below 10 kDa. The antioxidant properties of different molecular weight alginates were evaluated by determining the scavenging abilities on superoxide, hydroxyl, and hypochlorous acid and inhibitory effect on Fe²⁺-induced lipid peroxidation in yolk homogenate. The results showed that low molecular weight alginates exhibited high scavenging capacities on superoxide, hydroxyl, and hypochlorous acid radicals and good inhibition of Fe²⁺-induced lipid peroxidation in yolk. By comparison, alginate A1 with molecular weight below 1 kDa and M/G of 1.84 had better scavenging activity on superoxide, hydroxyl, and hypochlorous acid radicals in vitro than A2 (1–6 kDa), A3 (6–10 kDa), ascorbic acid, and carnosine. With similar M/G ratio, A2 exhibited better antioxidant activity on superoxide and hypochlorous acid radicals than A3. However, fraction A3 with molecular weight of 6–10 kDa exhibited higher inhibitory ability on lipid peroxidation in yolk in vitro than A1 and A2. The results indicated that molecular weight played a more important role than M/G ratio on alginate to determine the antioxidant ability. By comparison, low molecular weight alginates composed of guluronic acid and mannuronic acid exhibited better antioxidant ability on oxygen free radicals than sulfated polysaccharides from L. japonica in our previous study and represent a good source of marine polysaccharide with potential application as natural antioxidant.