RADIOPROTECTION OF DNA BY POLYAMINES

Putrescine, spermidine and spermine are natural polyamines bearing at neutral pH the net electrical charges +2, +3 and +4 respectively. We r eport here the radioprotective effect of these polyamines on the radio lysis of pBR322 plasmid DNA. We observe a very efficient protection ag ainst fast neutron-induced single and double-strand breakage in the pr esence of spermine and spermidine, and a significantly less efficient protection in the presence of putrescine. An ionic strength dependence is observed for spermidine and spermine, but not for putrescine. Circ ular dichroism measurements show spermidine- and spermine-induced stru ctural modifications of DNA, i.e. the formation of tightly packaged co ndensates in the concentration range corresponding to radioprotection. No structural change is observed for concentrations of putrescine aff ording radioprotection. We explain the radioprotection by: (1) the sca venging of OH. radicals in the bulk, essentially observed in the case of putrescine; (2) a local scavenging at the sites of binding of polya mines; and (3) the reduced accessibility of the attack sites in the co ndensed stuctures induced by spermine or spermidine.