Irreversible binding of reductively activated streptonigrin to nucleic acids in the presence of metals

The binding of streptonigrin (SN) to nucleic acids was studied in the presence of reducing agents and metals. Incubation of chemically reduced SN with DNA in vitro resulted in irreversible binding and complexes containing 1 mol of SN per 250 nucleotides were obtained. The presence of Zn²⁺ increased this binding considerably to give complexes containing 1 mol of SN per 80 nucleotides. On the other hand, Mg²⁺ decreased this binding. More drug was bound to the denatured DNA than to the native DNA. Maximum binding was obtained when SN was reduced in the presence of DNA. Increased binding was also obtained when the fully reduced SN was incubated with DNA. Considerably more SN was bound to DNA when activated enzymatically than with NaBH₄. Studies with synthetic polynucleotides in the presence of Zn²⁺ suggested that while SN has a high affinity for guanine residues, cytosine and adenine residues also serve as excellent substrates.These studies indicate that the active intermediate that binds to nucleic acids is unstable and may be derived from the fully reduced drug. These in vitro studies further suggest that Zn²⁺ plays an important role in the binding of SN to DNA and may have implications for the biological actions of SN if similar reactions occurred in vivo.