Redox Reactions between Kaempferol and Illuminated Chloroplasts.

Bleaching of kaempferol by illuminated chloroplasts was observed at 380 nanometers. The photobleaching was stimulated by methyl viologen and suppressed by superoxide dismutase indicating the participation of O(2) (-) in the reaction. An electron transfer inhibitor on the oxidizing side of photosystem II, carbonylcyanide m-chlorophenylhydrazone (CCCP), stimulated the photobleaching and 3-(3,4-dichlorophenyl)-1,1-dimethylurea partially suppressed it. The stimulation by CCCP suggests that kaempferol is also bleached on the oxidizing side of photosystem II. The spectrum of kaempferol bleaching in the presence of methyl viologen was the same as that in the presence of CCCP having a maximum in absorbance decrease at around 380 nanometers. When kaempferol was oxidized by KMnO(2) or KO(2), the oxidized minus reduced difference spectra had also a negative peak at about 380 nanometers. The results suggest that kaempferol was oxidized by illuminated chloroplasts.The rate of kaempferol photooxidation increased as its concentration was increased from 1 to 100 micromolar. The rate of quercetin photooxidation also increased as its concentration was increased from 1 to 100 micromolar. Concentration of quercetin glycosides higher than 10 micromolar was required to detect their photobleaching by illuminated chloroplasts. From these results, it is postulated that flavonols function as antioxidants in chloroplasts.