Custódio J B, Almeida L M, Madeira V M
Laboratório de Bioquímica, Faculdade de Farmácia, Universidade de Coimbra, Couraça dos Apóstolos, no. 51 r/c, 3000, Coimbra, Portugal; Centro de Neurociências, Universidade de Coimbra, 3049 Coimbra Codex, Portugal.
Toxicol In Vitro. 1996 Oct;10(5):523-31. doi: 10.1016/s0887-2333(96)00040-9.
The interactions of tamoxifen (TAM) and its active metabolite 4-hydroxytamoxifen (OHTAM) with the sarcoplasmic reticulum (SR) Ca(2+)-pump were investigated. The turnover of the Ca(2+)-ATPase is strongly inhibited by both drugs at low concentrations that do not significantly perturb the lipid organization of SR membranes. Moreover, TAM decreases Ca(2+) accumulation by SR Ca(2+)-ATPase and increases in parallel the ATP hydrolysis, decreasing the energetic efficiency of the Ca(2+)-pump (Ca (2+)ATP coupling ratio) by about 70% at 30 muM. This uncoupling of ATP hydrolysis from Ca(2+) accumulation is a putative consequence of structural defects induced on membranes, since the ATP hydrolysis at low residual Ca(2+) (Ca(2+) not supplemented) is also stimulated. On the other hand, OHTAM decreases the Ca(2+) uptake to a greater extent than TAM but, unlike TAM, it inhibits ATP hydrolysis. Thus, the Ca (2+)ATP ratio is decreased by about 47% at 30 muM OHTAM; this effect is not a consequence of membrane disruption, since the ATP-splitting activity decreases in parallel to Ca(2+) accumulation and no significant effect is detected for ATP hydrolysis at low residual Ca(2+). The inhibition of the Ca(2+)-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 Ca(2+) translocation. Therefore, the effects of the drugs on the Ca(2+)-pump are different and rule out an unitary mechanism of action on the basis of bilayer structure perturbations.
研究了他莫昔芬（TAM）及其活性代谢物4-羟基他莫昔芬（OHTAM）与肌浆网（SR）钙泵的相互作用。在不显著扰乱SR膜脂质组织的低浓度下，两种药物均强烈抑制钙ATP酶的周转。此外，TAM可降低SR钙ATP酶的钙积累，并同时增加ATP水解，在30μM时使钙泵的能量效率（钙-ATP偶联比）降低约70%。ATP水解与钙积累的这种解偶联是膜上诱导的结构缺陷的一个假定结果，因为在低残留钙（未补充钙）时的ATP水解也受到刺激。另一方面，OHTAM比TAM更能降低钙摄取，但与TAM不同的是，它抑制ATP水解。因此，在30μM OHTAM时，钙-ATP比降低约47%；这种效应不是膜破坏的结果，因为ATP水解活性与钙积累平行降低，并且在低残留钙时未检测到ATP水解有显著影响。OHTAM对钙泵的抑制作用可能与它与酶的调节位点直接相互作用或脂质-蛋白质界面的相互干扰有关。这种效应可能是由于催化位点的ATP利用与钙转运所涉及的通道过程之间的能量传递和转换效率降低所致。因此，药物对钙泵的作用不同，排除了基于双层结构扰动的单一作用机制。
