Doxorubicin and covalently crosslinked doxorubicin derivatives binding to purified cardiac thin-filament proteins in vitro

The binding of cardiac actin and tropomyosin to a cardiotoxic antineoplastic agent, doxorubicin, and its covalently crosslinked derivatives was investigated. The primary amino group of the daunosamine moiety of doxorubicin was blocked with fluorescein isothiocyanate. This doxorubicin derivative did not bind to Sepharose which was conjugated with cardiac actin. A doxorubicin dimer was made by covalently crosslinking one doxorubicin molecule to another identical doxorubicin molecule through the free amino group of each daunosamine moiety. This derivative demonstrated mobility different from parent doxorubicin on thin-layer chromatography, different elution pattern by column chromatography, and did not show binding affinity for actin. Exploring other purified thin-filament proteins, it was found that doxorubicin did bind to tropomyosin when gel filtration was performed on the protein drug mixture. The ability of tropomyosin to form paracrystal in vitro was not disturbed by a variety of concentrations of doxorubicin. These data support the concept that the doxorubicin solitary free amino group is the site which is responsible for this ligand to bind to actin and may relate to its cardiotoxic effects.