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

摘要

Measurements of aroxyl radical (ArO)-scavenging rate constants (K_(AOH) antioxidants (AOHs: pyrroloquinolinequinol (PQQH2), α-tocophcrol (α-TocH), ubiquinol-10 (UQ_(10)H2), epicatechin, epigallocatcchin, epigalloca-techin 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) a-TocH and PQQH2 and (ii) α-TocH and UO_(10)H2). A notable synergistic effect that the k_s~(AOH) values increase 1.72, 2.42, and 2.50 times for a-TocH, PQQHj, and UO_(10)H2, respectively, was observed for the solutions including two kinds of AOHs. Measurements of the regeneration rates of tt-tocopheroxyl radical (α-Toc) to α-TocH by PQQH2 and UQ_(10)H2 were performed in DMSO, using double-mixing stopped-flow spectrophotometry. Second-order rate constants (k_x) obtained for PQQH2 and UQ_(10)H2 were 1.08 X 10~5 and 3.57 X 10~4 M~(-1) s~(-1), respectively, indicating that the k, value of PQQH2 is 3.0 times larger than that of UQ_(10)H2. It has been clarified that PQQH2 and UQ_(10)H2 having two HO groups within a molecule may rapidly regenerate two molecules of α-Toc to a-TocH. The result indicates that the prooxidant effect of a-Toc" is suppressed by the coexistence of PQQH2 or UQ_(10)H2.