Zielinska Danuta, Wiczkowski Wieslaw, Piskula Mariusz Konrad
University of Warmia and Mazury in Olsztyn, Plac Lodzki 4, 10-957 Olsztyn, Poland.
J Agric Food Chem. 2008 May 28;56(10):3524-31. doi: 10.1021/jf073521f. Epub 2008 May 3.
This paper describes the use of cyclic voltammetry (CV), spectrophotometric methods [Trolox equivalent antioxidant capacity (TEAC), peroxyl radical trapping capacity (PRTC), DPPH radical scavenging activity (RSA), and Folin-Ciocalteu reagent (FCR) reducing capacity], and photochemiluminescence (PCL) for the measurement of the antioxidant capacity of onion var. Sochaczewska and var. Szalotka. The antioxidant and reducing activity of the dominant onion flavonoids quercetin (Q), quercetin-3- O-beta-glucoside (Q3G), quercetin-4'- O-beta-glucoside (Q4'G), and quercetin-3,4'-di- O-beta-glucoside (Q3,4'G) were determined by spectrophotometric (TEAC and PRTC) and CV methods, respectively. The contribution of quercetin and its glucosides to the antioxidant capacity of onion was calculated in consequence of the qualitative and quantitative analysis of onion flavonoids by high-performance liquid chromatography-ultraviolet-mass spectrometry. The dominant forms of quercetin in the onion var. Sochaczewska and Szalotka included Q4'G (61 and 54%), Q3,4'G (37 and 44%), Q3G (1.4 and 1.1%), and free quercetin (1.1 and 0.7%), respectively. The CV experiment showed the highest reducing activity of Q while Q3G, Q4'G, and Q3,4'G exhibited about 68, 51, and 30% of the reducing power noted for Q. The order of the reducing activity of onion flavonoids was confirmed by their free radical scavenging activity and evaluated by TEAC and PRTC assays as follows: Q > Q3G > Q4'G > Q3,4'G. The Q4'G and Q3,4'G showed poor antioxidant activity under both applied spectrophotometric assays but still exhibited reducing activity based on CV experiments. The reducing capacity of onions determined by CV method was twice higher than the antioxidant capacity formed by water-soluble compounds (ACW) evaluated by PCL, and it was about 50% higher than PRTC and DPPH RSA results and the converted FCR reducing capacity. In contrast, the reducing capacity of onions determined by the CV method was 3-fold and about four times lower when compared to the antioxidant capacity evaluated by the TEAC method and that formed by lipid-soluble compounds (ACL) provided by PCL, respectively. The highest antioxidant capacity of onion was found under cumulative consideration of PCL (ACW + ACL) and TEAC assays. The relative contribution of Q and its glucosides to the antioxidant capacity of onions showed a low contribution of Q, Q3G, and Q3,4'G derived from CV, TEAC, and PRTC assays while the highest contribution to the antioxidant capacity of onions was provided by Q4'G.
本文描述了使用循环伏安法（CV）、分光光度法[Trolox等效抗氧化能力（TEAC）、过氧自由基捕获能力（PRTC）、DPPH自由基清除活性（RSA）和福林-西奥尔特试剂（FCR）还原能力]以及光化学发光法（PCL）来测定Sochaczewska品种和Szalotka品种洋葱的抗氧化能力。分别通过分光光度法（TEAC和PRTC）和循环伏安法测定了主要洋葱类黄酮槲皮素（Q）、槲皮素-3-O-β-葡萄糖苷（Q3G）、槲皮素-4'-O-β-葡萄糖苷（Q4'G）和槲皮素-3,4'-二-O-β-葡萄糖苷（Q3,4'G）的抗氧化和还原活性。通过高效液相色谱-紫外-质谱联用对洋葱类黄酮进行定性和定量分析，进而计算出槲皮素及其糖苷对洋葱抗氧化能力的贡献。Sochaczewska品种和Szalotka品种洋葱中槲皮素的主要形式分别为Q4'G（61%和54%）、Q3,4'G（37%和44%）、Q3G（1.4%和1.1%）以及游离槲皮素（1.1%和0.7%）。循环伏安实验表明，Q的还原活性最高，而Q3G、Q4'G和Q3,4'G的还原能力分别约为Q的68%、51%和30%。洋葱类黄酮的还原活性顺序通过其自由基清除活性得到证实，并通过TEAC和PRTC测定评估如下：Q＞Q3G＞Q4'G＞Q3,4'G。在两种应用的分光光度法测定中，Q4'G和Q3,4'G的抗氧化活性较差，但基于循环伏安实验仍表现出还原活性。通过循环伏安法测定的洋葱还原能力是通过PCL评估的水溶性化合物形成的抗氧化能力（ACW）的两倍，并且比PRTC和DPPH RSA结果以及转化后的FCR还原能力高约50%。相比之下，通过循环伏安法测定的洋葱还原能力与通过TEAC法评估的抗氧化能力以及PCL提供的脂溶性化合物形成的抗氧化能力（ACL）相比，分别低3倍和约4倍。在综合考虑PCL（ACW + ACL）和TEAC测定时，发现洋葱具有最高的抗氧化能力。Q及其糖苷对洋葱抗氧化能力的相对贡献表明，来自循环伏安法、TEAC和PRTC测定的Q、Q3G和Q3,4'G的贡献较低，而对洋葱抗氧化能力贡献最高的是Q4'G。
