Radical scavenger activity of different 3',4'-dihydroxyflavonols and 1,5-dicaffeoylquinic acid studied by inhibition of chemiluminescence.

To gain more insights into structure-activity relationships, four 3',4'-dihydroxyflavonols differing in the substitution of the A and C rings and 1,5-dicaffeolyquinic acid were evaluated for their ability to inhibit chemiluminescence of human neutrophils stimulated with opsonized zymosan or FMLP as well as in an enzymatic system with H2O2 and horseradish peroxidase. It could be shown that an additional o-dihydroxy structure in the A-ring, or a 6-methoxy group, respectively, has no significant influence, thus confirming the o-dihydroxy group of the B-ring as the most important structural feature for the radical scavenger activity. It can be supposed that the main effect of the tested flavonols is based on their inhibition of myeloperoxidase, besides inhibition of enzymes involved in activating the NADPH-oxidase, and a direct reaction with oxygen radicals. Inhibition of chemiluminescence by 1,5-dicaffeoylquinic acid was in the same order as those observed with the flavonols.