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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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><sub>s</sub><sup>Extract</sup>) 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><sub>s</sub><sup>Extract</sup> value (1.26 × 10<sup>–2</sup> M<sup>–1</sup> s<sup>–1</sup>) of Nipponbare
(extract 1) with the highest activity was 1.5 times larger than that
(8.29 × 10<sup>–3</sup>) 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><sub>s</sub><sup>Extract</sup>) (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><sub>s</sub><sup>AOH‑<i>i</i></sup> [AOH-<i>i</i>]/10<sup>5</sup>} of the rate constant (<i>k</i><sub>s</sub><sup>AOH‑<i>i</i></sup>) and the concentration ([AOH-<i>i</i>]/10<sup>5</sup>) 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><sub>s</sub><sup>Extract</sup> 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