Characterization and antioxidant activity of selenium-containing phycocyanin isolated from Spirulina platensis

Selenium-containing phycocyanin (Se–PC) and its different aggregates – monomer (αβ), trimer (αβ)3 and hexamer (αβ)6, which are represented as Se–PC1, Se–PC2 and Se–PC3, as well as their α and β subunits, were prepared and purified from selenium-rich Spirulina platensis. The Se speciation in Se–PC, its aggregates and subunits, as well as the antioxidant activities of these compounds against free radicals of superoxide, hydrogen peroxide and 2,2-diphenyl-1-picryl-hydrazyl (DPPH) were investigated. By gel filtration on SephadexG-200 column and SDS-PAGE, the molecular weight of Se–PC1, 2, and 3 were determined to be 38.5, 116 and 247 kDa, respectively, and these of α and β subunits were 17.8 and 19.5 kDa, respectively. Se concentrations of Se–PC1, 2 and 3 were of 197, 289 and 422 μg/g protein, respectively, and those of α and β subunits were 126 and 187 μg/g protein, respectively, which suggested that the Se content of Se–PC was decreased in the dissociation process of Se–PC and highly associated with its aggregation state. The antioxidant activities of Se–PC, its aggregates and subunits against different free radicals were found to be variable. Compared with phycocyanin without bound Se, although there were no differences among the DPPH-scavenging activities of Se–PC, its α and β subunits and the pure phycocyanin, the superoxide and hydrogen peroxide radical-scavenging activities of Se–PC, as well as Se–PC2 and Se–PC3, were significantly higher and proved to be positively correlated with their Se contents.