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Antioxidant activities of minor pigments of sea urchins (1–5) and synthetic naphthazarins (7–13) were evaluated and compared with echinochrome A (6) using 2,2-diphenyl-1-picrylhydrazyl (DPPH) and 2,2′-azinobis(3-ethylbenzothiazoline-6-sulfonate) (ABTS) scavenging assays. Structure-activity relationships showed that the antioxidant activities of the tested compounds depended on the number and positions of hydroxyl groups. Compounds bearing 3 or 2 hydroxyl groups on a naphthazarin core (5,8-dihydroxy-1,4-naphthoquinone) were the most active in both assays. Echinochrome A (6) (IC 50  7.0 μM) and its monomethyl ethers 7 (IC 50  15.0 μM) and 8 (IC 50  15.0 μM) displayed stronger activities than Trolox (IC 50  16.0 μM) in the DPPH and ABTS assays (TE = 3.41, 2.35, and 2.35 mM, respectively). Compounds with either one or without hydroxyl groups on a naphthazarin core displayed activities significantly lower than Trolox in both assays. These results suggest that hydroxylated naphthazarin pigments of sea urchins have a potential use as natural antioxidants.

Free Radical Scavenging Activities of Naturally Occurring and Synthetic Analogues of Sea Urchin Naphthazarin Pigments