Radioprotection of plasmid DNA by oligolysines

Abstract

Purpose: To define ionic conditions under which oligolysines condense DNA as assayed by radioprotection of a plasmid substrate. And to compare these conditions with those required by the well‐characterized ligands spermidine and hexammine cobalt (III). This will enable a reversible compaction model for plasmid DNA to be devised that models more closely mammalian chromatin than those based on polyamines.

Materials and methods: Aqueous solutions containing plasmid DNA, sodium perchlorate and one of the five ligands trilysine, tetralysine, pentalysine, spermidine, or hexammine cobalt (III) were subjected to gamma‐irradiation. The yields of the resulting single‐strand breaks were quantified by gel electrophoresis. The effects of tetralysine and pentalysine were also examined by light scattering.

Results: The combination of low concentrations of the ligand and high concentrations of sodium perchlorate produced a relatively high yield of single‐strand breaks. In contrast, the combination of high concentrations of the ligand and low concentrations of sodium perchlorate resulted in an approximately 25‐fold lower single‐strand break yield. The transition between these two break yields took place over very narrow concentration ranges of the ligand. A large change in light scattering occurred at the same concentration. The radioprotective ability of the ligands decreased in the order pentalysine>tetralysine>hexammine cobalt (III)>spermidine>trilysine.

Conclusions: The effect of the oligolysines is qualitatively very similar to the previously reported radioprotection produced under similar conditions by the polyamines spermidine and spermine. It is caused by condensation of the DNA into a highly compacted form. As peptides, oligolysines are structurally more closely related than other ligands to naturally occurring DNA condensing agents such as histone proteins. Therefore, they may form the basis of a model system suitable for studying DNA damage produced by the direct effect of ionizing radiation (ionization of the DNA itself).