Redox Reactions between Kaempferol and Illuminated Chloroplasts

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Abstract
Bleaching of kaempferol by illuminated spinach chloroplasts was observed at 380 nm. The photobleaching was stimulated by methyl viologen and suppressed by superoxide dismutase indicating the participation of O2- in the reaction. An electron transfer inhibitor on the oxidizing side of photosystem [PS] 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 PS 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 .apprx. 380 nm. When kaemperfol was oxidized by KMnO2 or KO2, the oxidized minus reduced, difference spectra had also a negative peak at .apprx. 380 nm. Apparently, kaempferol was oxidized by illuminated chloroplasts. The rate of kaempferol photooxidation increased as its concentration was increased from 1 to 100 .mu.M. The rate of quercetin photooxidation also increased as its concentration was increased from 1 to 100 .mu.M. Concentration of quercetin glycosides > 10 .mu.M was required to detect their photobleaching by illuminated chloroplasts. Evidently, flavonols function as antioxidants in chloroplasts.