Tamoxifen has been reported to inhibit acidification of cytoplasmic organel
les in mammalian cells. Here, the mechanism of this inhibition is investiga
ted using in vitro assays on isolated organelles and liposomes. Tamoxifen i
nhibited ATP-dependent acidification in organelles from a variety of source
s, including isolated microsomes from mammalian cells, vacuoles from Saccha
romyces cerevisiae, and inverted membrane vesicles from Escherichia coil. T
amoxifen increased the ATPase activity of the vacuolar proton ATPase but de
creased the membrane potential (V-m) generated by this proton pump, suggest
ing that tamoxifen may act by increasing proton permeability. In liposomes,
tamoxifen increased the rate of pH dissipation. Studies comparing the effe
ct of tamoxifen on pH gradients using different salt conditions and with ot
her known ionophores suggest that tamoxifen affects transmembrane pH throug
h two independent mechanisms. First, as a lipophilic weak base, it partitio
ns into acidic vesicles, resulting in rapid neutralization. Second, it medi
ates coupled, electroneutral transport of proton or hydroxide with chloride
. An understanding of the biochemical mechanism(s) for the effects of tamox
ifen that are independent of the estrogen receptor could contribute to pred
icting side effects of tamoxifen and in designing screens to select for est
rogen-receptor antagonists without these side effects.