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The metabolism of the flavonol quercetin in human leukaemia (HL-60) cells was investigated. The fluorescence that is elicited by quercetin upon binding to a target protein was quickly attenuated in vital cells, while apoptotic cells showed persistent fluorescence. The dynamics of induction and loss of fluorescence in the cells were quantified by flow cytometry. Several potential metabolites of quercetin, apart from isorhamnetin, had weak or no fluorogenic properties with test proteins. HPLC analysis showed that quercetin was metabolised to several substances, among them glycosylated metabolites. The loss of fluorescence in vital cells offers the unique opportunity to directly observe the metabolic conversion of quercetin in human cells. [ABSTRACT FROM AUTHOR]