Flavonoids are commonly found in fruit and vegetables and have been shown to reach concentrations of several micromolars in human blood plasma. Flavonoids are also believed to have cancer chemoprotective properties. One hypothesis is that flavonoids are able to initiate apoptosis, especially in cancer cells, via a Ca(2+)-dependent mitochondrial pathway. This pathway can be activated through an exaggerated elevation of cytosolic [Ca(2+)], and sarcoplasmic/endoplasmic reticulum Ca(2+)-ATPases (SERCA) play an essential role in ameliorating such changes. In this study, we demonstrate that flavonoids (especially flavones) can inhibit the activity of Ca(2+)-ATPases isoforms SERCA1A and SERCA2B in the micromolar concentration range. Of the 25 flavonoids tested, 3,6-dihydroxyflavone (IC(50), 4.6 microM) and 3,3',4',5,7-pentahydroxyflavone (quercetin) (IC(50), 8.9 microM) were the most potent inhibitors. We show that polyhydroxylation of the flavones are important for inhibition, with hydroxylation at position 3 (for SERCA1A) and position 6 (for SERCA2B) being particularly relevant.