Estrogen sulfotransferase (EST) is the sole sulfotransferase expressed in normal human breast epithelial cells and has an important function in determining free estrogen hormone levels in these cells. In the present study we examined the inhibitory effect of the dietary polyphenols quercetin and resveratrol on EST activity, i.e. 17beta-estradiol (E2) sulfation. Both the compounds potently inhibited recombinant human EST in a competitive fashion with K(i) values of about 1 microM. In fact, both polyphenols could serve as substrates for EST. In order to extend the studies to more physiologically relevant conditions, we examined whether inhibition of EST also occurred in the intact cultured human mammary epithelial (HME) cells. The mean baseline EST activity (E2 sulfate formation) in the HME cells was 4.4 pmol/h per mg protein. The IC(50) for resveratrol was very similar to that for recombinant EST, i.e. about 1 microM. Surprisingly, quercetin was 10 times more potent in the HME cells with an IC(50) of about 0.1 microM, a concentration that should be possible to achieve from the normal dietary content of this flavonoid.