Kinetic Study of Aroxyl Radical Scavenging and α-Tocopheroxyl Regeneration Rates of Pyrroloquinolinequinol (PQQH2, a Reduced Form of Pyrroloquinolinequinone) in Dimethyl Sulfoxide Solution: Finding of Synergistic Effect on the Reaction Rate due to the Coexistence of α-Tocopherol and PQQH2

Measurements of aroxyl radical (ArO center dot)-scavenging rate constants (k(s)(AOH)) of antioxidants (AOHs: pyrroloquinolinequinol (PQQH(2)), alpha-tocopherol (alpha-TocH), ubiquinol-10 (UQ(10)H(2)), epicatechin, epigallocatechin, epigallocatechin gallate, and caffeic acid) were performed in dimethyl sulfoxide (DMSO) solution, using stopped-flow spectrophotometry. The k(s)(AOH) values were measured not only for each AOH but also for the mixtures of two AOHs ((i) alpha-TocH and PQQH(2) and (ii) alpha-TocH and UQ(10)H(2)). A notable synergistic effect that the k(s)(AOH) values increase 1.72, 2.42, and 2.50 times for alpha-TocH, PQQH(2), and UQ(10)H(2), respectively, was observed for the solutions including two kinds of AOHs. Measurements of the regeneration rates of alpha-tocopheroxyl radical (alpha-Toc(center dot)) to alpha-TocH by PQQH(2) and UQ(10)H(2) were performed in DMSO, using double-mixing stopped-flow spectrophotometry. Second-order rate constants (k(r)) obtained for PQQH(2) and UQ(10)H(2) were 1.08 X 10(5) and 3.57 x 10(4) M-1 s(-1), respectively, indicating that the k(r), value of PQQH(2) is 3.0 times larger than that of UQ(10)H(2). It has been clarified that PQQH(2) and UQ(10)H(2) having two HO groups within a molecule may rapidly regenerate two molecules of alpha-Toc(center dot) to alpha-TocH. The result indicates that the prooxidant effect of alpha-Toc(center dot) is suppressed by the coexistence of PQQH(2) or UQ(10)H(2.)