The effect of the anticancer drugs tamoxifen and hydroxytamoxifen on the calcium pump of isolated sarcoplasmic reticulum vesicles

The interactions of tamoxifen (TAM) and its active metabolite 4-hydroxytamoxifen (OHTAM) with the sarcoplasmic reticulum (SR) Ca2+-pump were investigated. The turnover of the Ca2+-ATPase is strongly inhibited by both drugs at low concentrations that do not significantly perturb the lipid organization of SR membranes. Moreover, TAM decreases Ca2+ accumulation by SR Ca2+-ATPase and increases in parallel the ATP hydrolysis, decreasing the energetic efficiency of the Ca2+-pump (Ca2+/ATP coupling ratio) by about 70% at 30 mu M. This uncoupling of ATP hydrolysis from Ca2+ accumulation is a putative consequence of structural defects induced on membranes, since the ATP hydrolysis at low residual Ca2+ (Ca2+ not supplemented) is also stimulated. On the other hand, OHTAM decreases the Ca2+ uptake to a greater extent than TAM but, unlike TAM, it inhibits ATP hydrolysis. Thus, the Ca2+/ATP ratio is decreased by about 47% at 30 mu M OHTAM; this effect is not a consequence of membrane disruption, since the ATP-splitting activity decreases in parallel to Ca2+ accumulation and no significant effect is detected for ATP hydrolysis at low residual Ca2+. The inhibition of the Ca2+-pump by OHTAM is putatively related to a direct interaction with the regulatory sites of the enzyme or interactive perturbations at the lipid-protein interface. The effect may result from a decrease of efficiency in the energy transmission and transduction between the ATP use at the catalytic site and the channeling process involved in Ca2+ translocation. Therefore, the effects of the drugs on the Ca2+-pump are different and rule out an unitary mechanism of action on the basis of bilayer structure perturbations. Copyright (C) 1996 Elsevier Science Ltd.