Kinetic Study of the Antioxidant Activity of Pyrroloquinolinequinol (PQQH₂, a Reduced Form of Pyrroloquinolinequinone) in Micellar Solution

Kinetic study of the aroxyl radical-scavenging action of pyrroloquinolinequinol [PQQH(2), a reduced form of pyrroloquinolinequinone (PQQ)] and water-soluble antioxidants (vitamin C, cysteine, glutathione, and uric acid) has been performed. The second-order rate constants (k(s)) for the reaction of aroxyl radical with PQQH(2) and water-soluble antioxidants were measured in Triton X-100 micellar solution (5.0 wt %) (pH 7.4), using stopped-flow and UV-visible spectrophotometers. The k(s) values decreased in the order PQQH(2) > vitamin C >> cysteine > uric acid > glutathione. The aroxyl radical-scavenging activity of PQQH(2) was 7.4 times higher than that of vitamin C, which is well-known as the most active water-soluble antioxidant. Furthermore, PQQNa(2) (disodium salt of PQQ) was easily reduced to PQQH(2) by reaction of PQQNa(2) with glutathione and cysteine in buffer solution (pH 7.4) under nitrogen atmosphere. The result suggests that PQQ exists as a reduced form throughout the cell and plays a role as antioxidant.