Oxidation of Quercetin by Carotenoid Radical Generated in Illuminated Spinach Chloroplasts: The Effect of Ascorbate on Quercetin Oxidation

Carotenoid photobleaching in the presence of carbonylcyanidem-chlorophenylhydrazone (CCCP) was suppressed by quercetin,but not by ascorbate. When quercetin suppressed carotenoid photobleaching,quercetin was oxidized. The oxidation of quercetin was inhibitedby ascorbate with half-inhibition at about 10 µM. Ascorbatewas oxidized by CCCP-poisoned chloroplasts upon illumination.The rate of ascorbate oxidation in the presence of both ascorbateand quercetin was lower than that in the presence of ascorbatealone. Based on the present results, the physiological significanceof quercetin as an antioxidant and the redox reaction betweenascorbate and oxidized quercetin are discussed. (Received March 9, 1984; Accepted July 12, 1984)