The ability of Laminaria digitata, Himanthalia elongata, Fucus vesiculosus, Fucus serratus and Ascophyllum nodosum to scavenge peroxyl radicals was investigated by kinetic studies in a model system. The thermal initiated oxidation of methyl linoleate was performed at 60°C in heptanol, with or without antioxidants. When they reached 1% of the substrate, seaweed extracts exhibited antioxidant activities by extending the induction period, but they did not suppress the rate of oxygen uptake as did vitamin E.
A synergistic effect occurred when both algal extracts (1.5 g L-1) and vitamin E (0.4 mmol L-1) were present, and the effectiveness of the combined antioxidants during the whole induction period was vitamin E effectiveness. The synergistic effect of L. digitata, however, was subject to seasonal variations: samples collected in summer were effective synergists, whereas samples collected in winter displayed a marked negative synergism.
The phospholipid fractions of F. vesiculosus, F. serratus and A. nodosum, including pigments, accounted for only 6% of the total lipid fraction, and did not exhibit a large synergistic effect. The main phospholipid was not phosphatidyl ethanolamine as usually related, but phosphatidyl inositol. Fucoxanthin had some antioxidant activity per se under our experimental conditions, but did not act as a synergist of vitamin E. The most potent synergists were recognized as chlorophyll a and related compounds by the application of liquid-liquid partition and chromatography for the identification of active components.