In this study, we evaluated the antioxidant activity (diphenylpicrylhydrazyl (DPPH)-radical scavenging activity) of the low-molecular weight fraction of water-soluble Susabinori (Porphyra yezoensis) extract (LMF). DPPH-radical scavenging activity of LMF (65.5 μmol-Trolox eq/ml) was enhanced at temperatures over 100 °C (1.4, 2.0 and 2.4 times higher at 100, 110 and 120 °C, respectively). As a result of HPLC separations, a newly produced peak was observed in heat treated LMF, while a marked decrease of another peak was found after heat treatment of LMF. The decreased peak was identified as porphyra-334, which is a typical mycosporine-like amino acid, by ESI/MSn analysis (protonated molecule [M + H]+ at m/z 347.1 and its fragment ion 303.1). The newly produced compound with [M + H]+ at m/z 329.1 was found to be a dehydrated compound of porhyra-334. The compound exhibited an extremely high-IC50 value of 10.1 μg/ml in DPPH-radical scavenging activity, compared to porphyra-334 (>1000 μg/ml).
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