Assay of total antioxidant capacity: comparison of four methods as applied to human blood plasma

Four spectrophotometric methods of determination of antioxidant capacity: a method based on the scavenging of the 1,1-diphenyl-2-picrylhydrazyl (DPPH) free radical, the "ferric-reducing ability of plasma" (FRAP), a method based on reduction of the 2,2'-azinobis (3-ethylbenzthiazolinesulfonate) free radical (ABTS(.+)) and a kinetic method based on the oxidation of dihydro-2,7-dichlorofluorescein by 2,2'-azobis(2-amidopropane) (ABAP) were compared with respect to standard antioxidants (ascorbate, glutathione, Trolox and urate) and human blood plasma. Various reactivities of standard antioxidants in different tests were found, glutathione showing a low reactivity in the FRAP assay. Kinetic measurements show that the reduction of indicators, especially by blood plasma, may not be complete at recommended times of the assays and the time of measurement is an important parameter when comparing the results.