Calmodulin and alpha tocopherol as additional binding sites for doxorubicin.

The hydrophobic probes anthroylcholine (9AC), 8-anilino-1-napthalene sulfonate (ANSE), and 2-P-toluidinyl naphthalene 6-sulfonate (TNS) increased calcium-calmodulin (Ca2+-aM) fluorescence. This fluorescence was decreased by doxorubicin (DXR) in a dose-dependent fashion. The Ca2+ ion was an absolute requirement for the observed effects of DXR. DXR bound to the Ca2+-CaM complex (Kd = 4.2 X 10(-5) M, Bmax = 1.8) and to alpha tocopherol. The binding of untransformed (native) DXR to CaM was a reversible process. These data support a previous finding that DXR inhibits stimulation of calmodulin-deficient PDE (a CaM target enzyme) using either the Ca2+ CaM complex or alpha tocopherol by interacting with these agents, and suggest that other target enzymes for CaM may be similarly affected.