Structure-activity relationships of flavonoids in the cellular antioxidant activity assay.

Chemical antioxidant activity assays are used extensively to evaluate the potential bioactivity of plant foods and their phytochemical constituents, but they do not mimic the complexity of biological systems. The cellular antioxidant activity (CAA) activity assay was developed to be a more biologically relevant model to measure antioxidant activity. Structure-activity relationships of flavonoids have been determined in many chemistry antioxidant activity assays, and they vary with the protocols. The objective of this study was to determine structure-activity relationships of selected flavonoids in the CAA assay. The structures that conferred flavonoids with the most antioxidant activity in the CAA assay were a 3',4'- o-dihydroxyl group in the B-ring, a 2,3-double bond combined with a 4-keto group in the C-ring, and a 3-hydroxyl group. Isoflavones had no cellular antioxidant activity. Flavanols with a galloyl moiety had higher antioxidant activity than those without, and a B-ring 3',4',5'-trihydroxyl group further improved their efficacy. ORAC values for flavonoids were not related to their CAA values. Knowledge of structure-activity relationships in the CAA assay may be helpful in assessing potential in vivo antioxidant activity of flavonoids.