Kinetic study of the scavenging reaction of the aroxyl radical by seven kinds of rice bran extracts in ethanol solution. Development of an aroxyl radical absorption capacity (ARAC) assay method.

Recently, a new assay method that can quantify the aroxyl radical (ArO center dot) absorption capacity (ARAC) of antioxidants (AOHs) was proposed. In the present work, the second-order rate constants (k(s)(Extract)) and ARAC values for the reaction of ArO with seven kinds of rice bran extracts 1-7, which contain different concentrations of alpha-, beta-, gamma-, and delta-tocopherols and -tocotrienols (alpha-, beta-, gamma-, and delta-Tocs and -Toc-3s) and gamma-oryzanol, were measured in ethanol at 25 degrees C using stopped-flow spectrophotometry. The k(s)(Extract)) value (1.26 x 10(-2) M-1 s(-1)) of Nipponbare (extract 1) with the highest activity was 1.5 times larger than that (8.29 x 10(-3)) of Milyang-23 (extract 7) with the lowest activity. The concentrations (in mg/100 g) of alpha-, beta-, gamma-, and delta-Tocs and -Toc-3s and gamma-oryzanol found in the seven extracts 1-7 were determined using HPLC-MS/MS and UV-vis absorption spectroscopy, respectively. From the results, it has been clarified that the ArO-scavenging rates (k(s)(Extract)) (that is, the relative ARAC value) obtained for the seven extracts 1-7 may be approximately explained as the sum of the product {Sigma k(s)(AOH-i) [AOH-i]/10(5)} of the rate constant (k(s)(AOH-i)) and the concentration ([AOH-i]/10(5)) of AOH-i (Tocs, Toc-3s, and gamma-oryzanol) included in rice bran extracts. The contribution of gamma-oryzanol to the k(s)(Extract)) value was estimated to be between 3.0-4.7% for each extract. Taken together, these results suggest that the ARAC assay method is applicable to general food extracts.