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Mukai, K. et al.

Mukai, Tokunaga, Itoh, Kanesaki, Ohara, Nagaoka, Abe,

Structure-Activity Relationship of the Free-Radical-Scavenging Reaction by Vitamin E (alpha-, beta-, gamma-, delta-Tocopherols) and Ubiquinol-10: pH Dependence of the Reaction Rates.

The reaction rates (ks) of vitamin E (alpha-, beta-, gamma-, delta-tocopherols, TocH), ubiquinol-10, and related antioxidants (tocol, ubiquinol-0, and hydroquinone) with aroxyl (ArO.) radical have been measured in micellar solution by stopped-flow spectrophotometer. The ks values increased in the order of hydroquinone < tocol < delta-TocH < ubiquinol-0 < gamma-TocH approximately beta-TocH < ubiquinol-10 < alpha-TocH at pH 4 approximately 8. The antioxidants which have lower oxidation potentials showed higher reactivities. The ks values of alpha-, beta-, gamma-, delta-tocopherol, and tocol remained constant between pH 4 and 10, and decreased rapidly at pH 11 approximately 12 by increasing pH value. From the pH dependence of ks values, the pKa values (= 13.1 approximately 12.6) have been determined for these tocopherols. The ks values of ubiquinol-10 also remained constant between pH 4 and 9, and increased rapidly at pH 9. 5. Ubiquinol-10 is dibasic acid and can exist in three different molecular forms, depending on the pH value. By comparing the ks values with the mole fraction of each molecular form of ubiquinol-10, the reaction rate ks1 (= 1.21 x 105 M-1s-1) for the undissociated form, ks2 (= 1.04 x 106 M-1s-1) for monoanion and ks3 (= 0 M -1s -1) for dianion, and the pKa1 and pKa2 values (= 11.4 and 12.7) were determined. The ks2 value is 8.6 times as large as the ks1 value. Similar analyses were performed for ubiquinol-0 and hydroquinone. It was found that the relative ratio of ks values (100:21:20:2.9) of alpha-, beta-, gamma-, delta-tocopherols in micellar dispersion has good correlation with the relative biopotency ratios for rat fetal resorption, rat hemeolysis, and chicken muscle dystrophy. The relative antioxidant activities of alpha-tocopherol and ubiquinol-10 have been discussed based on the ks values obtained and their concentrations in serum and several tissues (heart, muscle, liver, kidney, and brain).[1]

References

Structure-Activity Relationship of the Free-Radical-Scavenging Reaction by Vitamin E (alpha-, beta-, gamma-, delta-Tocopherols) and Ubiquinol-10: pH Dependence of the Reaction Rates. Mukai, K., Tokunaga, A., Itoh, S., Kanesaki, Y., Ohara, K., Nagaoka, S., Abe, K. The journal of physical chemistry. B, Condensed matter, materials, surfaces, interfaces & biophysical (2007) [Pubmed]

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