Antioxidant activity: what do we measure?

Inhibition of oxidation of 2,2 '-azinobis(3-ethylbenzothiazoline-6-sulfonic acid) (ABTS) by free radicals generated by decomposition of 2,2 '-azobis(2-amidopropane) (ABAP) by antioxidants and biological material was studied. A correlation was found between the ability of various substances to delay the onset of ABTS oxidation and their rapid reduction of the ABTS(+.) cation radical, and between the ability to reduce the maximal rate of ABTS oxidation and slow reduction of ABTS(+.). The length of the lag period of ABTS oxidation was found to be independent of ABTS concentration. Similar decrease of peroxynitrite-induced ABTS(+.) formation by antioxidants was observed when the antioxidants were added before and after peroxynitrite. All these findings indicate that the main effect of antioxidants in this system is reduction of ABTS(+.) and not prevention of its formation. Reduction of oxidation products rather than inhibition of their formation may be the predominant mode of action of antioxidants in various assays of antioxidant activity.