Protection of Cells against Oxidative Stress by Nanomolar Levels of Hydroxyflavones Indicates a New Type of Intracellular Antioxidant Mechanism
Figure 10
Antioxidant activity measured by EPR spectroscopy.
Upper panel: EPR spectra showing the reaction between galvinoxyl and antioxidants after 5 min incubation. (a) 10 µM galvinoxyl in ethanol; (b) with the addition of 2 µM 5,6-dihydroxyflavone; (c) with the addition of 1.0 µM quercetin. For each spectrum 4 scans were accumulated [18]. The reduced product, galvinol, is not a radical, therefore it does not show an EPR spectrum. In theory it should be possible to see also the radical of the oxidized antioxidant, but almost all radicals of flavonoids and most other polyphenolic compounds are too reactive to be detected in this assay; their EPR spectra are not visible when the antioxidants are used in the micromolar range. The capacity of a compound to eliminate the galvinoxyl radical gives a relative measure of its general radical scavenging activity, although the intracellular antioxidant effect may depend on other factors and mechanisms. Lower panel: Kinetics of antioxidant activities of the four flavones measured by the EPR technique. The data show the concentration of galvinoxyl remaining in the samples at different times after addition of flavones, and are given as mean ± SD of 3 experiments. Control 10 µM galvinoxyl in ethanol (▪), and after addition of mosloflavone 10 µM (♦), negletein 1 µM (•), 5,6-dihydroxyflavone 1 µM (▴), baicalein 1 µM (▾), or baicalein 0.1 µM (▵). In samples containing 10 µM of either negletein, 5,6-dihydroxyflavone or baicalein the galvinoxyl signal disappeared completely within 30 s.