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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

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journal contribution
posted on 10.12.2014, 00:00 by Kazuo Mukai, Aya Ouchi, Takumi Abe, Kazumasa Murata, Kiyotaka Nakagawa, Teruo Miyazawa
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 (ksExtract) 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 ksExtract 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 (ksExtract) (that is, the relative ARAC value) obtained for the seven extracts 1–7 may be approximately explained as the sum of the product {Σ ksAOH‑i [AOH-i]/105} of the rate constant (ksAOH‑i) and the concentration ([AOH-i]/105) of AOH-i (Tocs, Toc-3s, and γ-oryzanol) included in rice bran extracts. The contribution of γ-oryzanol to the ksExtract 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.