Comparison between the free-radical-scavenging activities with
vitamin E and ubiquinol in biological systems based on their reaction rates: A
research account
Issue title: The Fifth Conference of the International CoQ10
Association, Kobe 2007 – 50th anniversary of CoQ10 discovery
Abstract: Detailed kinetic studies have been performed for the reaction of
aroxyl (ArO•) radical with vitamin E (α-, β-, γ-,
δ-tocopherol, TocH), ubiquinol-10, and related
antioxidants in micellar solution, using a stopped-flow spectrophotometer. The
second-order reaction rates (k_{s}) obtained increased in
the order of hydroquinone < tocol<δ-TocH<ubiquinol-0<γ-TocH∼ β-TocH<ubiquinol-10<α-TocH
at pH 4 ∼ 8. The antioxidants which have lower oxidation
potentials (E_{p}) showed higher reactivities. The reaction
rates obtained in micellar solution were pH dependent because of the
dissociation of OH groups in the antioxidants. For instance, by comparing the
k_{s} values with the mole fraction of each molecular form
of ubiquinol-10, the reaction rate k_{s1}
(1.21 × 10^{5}
M^{-1}s^{-1}) for undissociated form,
k_{s2} (1.04 ×
10^{6} M^{-1}s^{-1})
for monoanion and k_{s3} (0 M^{-1}s^{-1}) for dianion, and the
pK_{a1} and pK_{a2} values (11.4 and
12.7) were determined. It was found that the relative ratio of
k_{s} values (100:21:20:2.9) of α-,
β-, γ-, δ-tocopherols in micellar dispersion has good correlation with the
relative biological activities for rat fetal resorption, rat haemolysis, and
chicken muscle dystrophy. The relative antioxidant activities of
α-tocopherol and ubiquinol-10 have been discussed on
the basis of the products of the k_{s} values and their
concentrations in serum and several tissues (heart, muscle, liver, kidney, and
brain).
