The impact of spermine competition on the efficacy of DNA-binding Fe(II), Co(II), and Cu(II) complexes of dimeric chromomycin A3

Chromomycin (Chro) forms a 2:1 drug/metal complex through the chelation with Fe(II), Co(II), or Cu(II) ion. The effects of spermine on the interaction of Fe(II), Co(II), and Cu(II) complexes of dimeric Chro with DNA were studied. Circular dichroism (CD) measurements revealed that spermine strongly competed for the Fe(II) and Cu(II) cations in dimeric Chro-DNA complexes, and disrupted the structures of these complexes. However, the DNA-CoII(Chro)2 complex showed extreme resistance to spermine-mediated competition for the Co(II) cation. According to surface plasmon resonance (SPR) experiments, a 6 mM concentration of spermine completely abolished the DNA-binding activity of FeII(Chro)2 and CuII(Chro)2 and interfered with the associative binding of CoII(Chro)2 complexes to DNA duplexes, but only slightly affected dissociation. In DNA integrity assays, lower concentrations of spermine (1 and 2 mM) promoted DNA strand cleavage by CuII(Chro)2, whereas various concentrations of spermine protected plasmid DNA from damage caused by either CoII(Chro)2 or FeII(Chro)2. Additionally, DNA condensation was observed in the reactions of DNA, spermine, and FeII(Chro)2. Despite the fact that CuII(Chro)2 and FeII(Chro)2 demonstrated lower DNA-binding activity than CoII(Chro)2 in the absence of spermine, while CuII(Chro)2 and FeII(Chro)2 exhibited greater cytoxicity against HepG2 cells than CoII(Chro)2, possibly due to competition of spermine for Fe(II) or Cu(II) in the dimeric Chro complex in the nucleus of the cancer cells. Our results should have significant relevance to future developments in metalloantibiotics for cancer therapy.