Interactions of daunomycin and melanotropin-daunomycin with DNA.

Abstract

The specific recognition of fluorescein–melanotropin and melanotropin–ferritin–fluorescein conjugates by melanotropin (MSH) receptors on the surface of murine melanoma cells suggested the possibility of specific site-directed chemotherapy of melanomas1–3. An MSH–daunomycin conjugate has been shown to be more toxic to mouse melanoma cells than free daunomycin, and, at the same concentration, it was not toxic to mouse 3T3 fibroblast cells. Unconjugated daunomycin was equally toxic to both cell lines4. We report here investigation of the interactions of purified DNA with free daunomycin and with MSH–daunomycin conjugate using spectrophotometry and also an agarose gel assay that detects the unwinding of DNA due to ligand binding. We found an association constant of 4.8 × 106 M−1 and an unwinding angle of 10±1.5° for daunomycin, but no detectable binding and an association constant of <4.8 × 103 M−1 for MSH–daunomycin conjugate. We conclude that the cytotoxicity of MSH–daunomycin is not due to its binding to DNA.