Protection of DNA against direct radiation damage by complex formation with positively charged polypeptides

Abstract

Radioprotection of DNA from direct-type radiation damage by histones has been studied in model systems using complexes of positively charged polypeptides (PCPs) with DNA. PCPs bind to DNA via ionic interactions mimicking the mode of DNA-histone binding. Direct radiation damage to DNA in films of DNA-PCP complexes was quantified as unaltered base release, which correlates closely with DNA strand breaks. All types of PCPs tested protected DNA from radiation, with the maximum radioprotection being approximately 2.5-fold compared with non-complexed DNA. Conformational changes of the DNA induced by PCPs or repair of free radical damage on the DNA sugar moiety by PCPs are considered the most feasible mechanisms of radioprotection of DNA. The degree of radioprotection of DNA by polylysine (PL) increased dramatically on going from pure DNA to a molar ratio of PL monomer:DNA nucleotide approximately 1:2, while a further increase in the PL:DNA ratio did not offer more radioprotection. This concentration dependence is in agreement with the model of PCP binding to DNA that assumes preferential binding of positively charged side groups to DNA phosphates in the minor groove, so that the maximum occupancy of all minor-groove PCP binding sites is at a molar ratio of PCP:DNA = 1:2.

FIG. 1

Dose dependence of total base release from dry (Γ = 2.5) DNA, DNA-PL (PL:DNA = 10:1), DNA-PR (PR:DNA = 3.2:1), and DNA-PLT (PLT:DNA = 8.5:1) films X-irradiated under aerobic conditions in nine-well plates. The films are labeled in the graph.

FIG. 2

Dependence of radiation chemical yields of total base release from dry (Γ = 2.5) DNA and DNA-PL films X-irradiated under aerobic conditions in nine-well plates on the PL:DNA ratio. Radiation yields are normalized to the radiation yield of total base release from the DNA films.

FIG. 3

Dependence of radiation chemical yield of total base release (G_t) from DNA and DNA-PL films X-irradiated under aerobic conditions in nine-well plates upon film hydration. Squares, DNA films; open circles, PL-DNA films with PL:DNA = 3.5:1; closed circles, PL-DNA films with PL:DNA = 10:1. Inset shows the dependence of G_t on the estimated partial hydration of the DNA moiety, Γ′_DNA-PL, in the DNA-PL films with PL:DNA = 3.5:1.