Radical Scavenging and Singlet Oxygen Quenching Activity of Marine Carotenoid Fucoxanthin and Its Metabolites

Antioxidant activity of carotenoids is suggested to be one of the factors for their disease preventing
effects. Marine carotenoids fucoxanthin and its two metabolites, fucoxanthinol and halocynthiaxanthin,
have been shown to exhibit several biological effects. The antioxidant activities of these three
carotenoids were assessed in vitro with respect to radical scavenging and singlet oxygen quenching
abilities. The 1,1-diphenyl-2-picrylhydrazyl radical scavenging activity of fucoxanthin and fucoxanthinol
was higher than that of halocynthiaxanthin, with the effective concentration for 50% scavenging (EC50)
being 164.60, 153.78, and 826.39 µM, respectively. 2,2′-Azinobis-3-ethylbenzo thizoline-6-sulphonate
radical scavenging activity of fucoxanthinol (EC50, 2.49 µM) was stronger than that of fucoxanthin
(EC50, 8.94 µM). Hydroxyl radical scavenging activity as measured by the chemiluminescence
technique showed that the scavenging activity of fucoxanthin was 7.9 times higher than that by
fucoxanthinol, 16.3 times higher than that by halocynthiaxanthin, and 13.5 times higher than that by
R-tocopherol. A similar trend was observed when the hydroxyl radical scavenging was assessed
by the electron spin resonance (ESR) technique. ESR analysis of the superoxide radical scavenging
activity also showed the superiority of fucoxanthin over the other two carotenoids tested. Singlet
oxygen quenching ability of the three carotenoids was lower than that of �-carotene, with quenching
rate constants (kQ, ×1010 M-1 s-1) being 1.19, 1.81, 0.80, and 12.78 for fucoxanthin, fucoxanthinol,
halocynthiaxanthin, and �-carotene, respectively. The higher radical scavenging activity of fucoxanthin
and fucoxanthinol compared with halocynthiaxanthin is assumed to be due to presence of the allenic
bond.