Oat avenanthramides exhibit antioxidant activities in vitro

Avenanthramides are a group of alkaloids that consist of an anthranilic acid derivative linked to a hydroxycinnamic acid derivative by a pseudo peptide bond, which are constituents of oat (Avena sativa L.) grain. The three most abundant avenanthramides, N-(4′-hydroxy-3′-methoxycinnamoyl)-5-hydroxyanthranilic acid (Bf), N-(4′-hydroxycinnamoyl)-5-hydroxyanthranilic acid (Bp), and N-(3′,4′-dihydroxycinnamoyl)-5-hydroxyanthranilic acid (Bc), were synthesized and purified. These were tested for antioxidant activity using two in vitro systems: inhibition of β-carotene bleaching and reaction with the free radical 2,2-diphenyl-1-picrylhydrazyl (DPPH). Each avenanthramide displayed antioxidant activity in both systems. Bc had greater activity than Bp and Bf. Bc was nearly as active as the standard synthetic antioxidant, butylated hydroxytoluene (BHT) in the β-carotene system. In the DPPH system, Bc and Bf were more active than 6-hydroxy-2,5,7,8-tetramethylchromane-2-carboxylic acid (Trolox®). The relative activities of the avenanthramides corresponded to those determined for their component hydroxycinnamic acid moieties using an aqueous DPPH system or in a lipophilic system that measured the autoxidation of methyl linoleate.