Hydroethidine as a probe for measuring superoxide formation rates during air oxidation of myricetin and quercetin

In the presence of the flavonols myricetin and quercetin, oxidation of hydroethidine (HE) by superoxide yielded ethidium (E+) instead of 2-hydroxyethidium (2-OH-E+). As a known pro-oxidant, myricetin alone was also found to be able to catalyze air oxidation of HE yielding exclusively E+. The reaction is inhibited by added superoxide dismutase, suggesting that superoxide is involved in the rate limiting step of the oxidation.