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

Abstract

Recently, a new assay method that can quantify the aroxyl radical (ArO•) absorption capacity (ARAC) of antioxidants (AOHs) was proposed. In the present work, the second-order rate constants (<i>k</i>_s^Extract) and ARAC values for the reaction of ArO• with seven kinds of rice bran extracts 1–7, which contain different concentrations of α-, β-, γ-, and δ-tocopherols and -tocotrienols (α-, β-, γ-, and δ-Tocs and -Toc-3s) and γ-oryzanol, were measured in ethanol at 25 °C using stopped-flow spectrophotometry. The <i>k</i>_s^Extract value (1.26 × 10^–2 M^–1 s^–1) of Nipponbare (extract 1) with the highest activity was 1.5 times larger than that (8.29 × 10^–3) of Milyang-23 (extract 7) with the lowest activity. The concentrations (in mg/100 g) of α-, β-, γ-, and δ-Tocs and -Toc-3s and γ-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 (<i>k</i>_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 {Σ <i>k</i>_s^{AOH‑<i>i</i>} [AOH-<i>i</i>]/10⁵} of the rate constant (<i>k</i>_s^{AOH‑<i>i</i>}) and the concentration ([AOH-<i>i</i>]/10⁵) of AOH-<i>i</i> (Tocs, Toc-3s, and γ-oryzanol) included in rice bran extracts. The contribution of γ-oryzanol to the <i>k</i>_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