Fluorimetric studies of calmodulin interactions with antiestrogens

Recent cumulative data have shown that tamoxifen and its metabolites inhibit the activation of cAMP phosphodiesterase by calmodulin (CaM). In this study, the interaction of antiestrogens with CaM was investigated using a hydrophobic fluorescent probe, 2-p-toluidinylnaphthalene-6-sulfonate (TNS). Tamoxifen (TAM) enhanced the fluorescence of TNS bound to CaM and shifted the emission maximum to lower wavelengths. These effects were concentration-dependent. No change in the apparent affinity of TNS for CaM was noted in the presence of TAM. These results suggest that TAM bound to CaM at sites distinct from those of TNS and induced a change in TNS environment. Interaction of TAM metabolites with CaM depended on the degree of alteration of the dimethylaminoethoxy side-chain. Thus, N-desmethylation or N-di-desmethylation notably reduced the interaction of the drug with the macromolecule by 24 and 77% respectively. Side-chain deamination to the primary alcohol (metabolite Y) totally suppressed the interaction. The ability of these different metabolites to interact with CaM correlates with their efficiency to inhibit CaM-dependent cAMP phosphodiesterase and their growth inhibitory potency reported previously.