EFFECT OF POLYAMINE-INDUCED COMPACTION AND AGGREGATION OF DNA ON THE FORMATION OF RADIATION-INDUCED STRAND BREAKS - QUANTITATIVE MODELS FORCELLULAR RADIATION-DAMAGE

The yield of DNA single-strand breaks, G(SSB), upon gamma irradiation of SV40 DNA and SV40 minichromosomes in aqueous solution under aerobic conditions was determined at physiological ionic strength in the pres ence of various potential radioprotective agents. Putrescine (PUT), sp ermidine (SPD), glutathione, trans-4,5-dihydroxy-1,2-dithiane, 2-merca ptoethyl disulfide and cystamine, all at 0.1-10 mM, spermine (SPM, 0.1 -1 mM) and WR-33278 (WRSSWR, 0.1-2 mM) lowered G(SSB) of SV40 DNA. The se results were expected from the ability of these agents to scavenge OH radical in the bulk solution. However, SPD, above 10 mM, and SPM an d WRSSWR, each above 2 mM, produced dramatic radioprotection attribute d to polyamine-induced compaction and aggregation of the DNA (PICA eff ect). The DNA of SV40 minichromosomes was inherently less radiosensiti ve and was subject to a PICA effect at lower polyamine concentrations, i.e. similar to 5 mM SPD, similar to 0.6 mM SPM and similar to 0.5 mM WRSSWR. The PICA effect decreased G(SSB) for SV40 DNA and minichromos omes by one to two orders of magnitude, depending upon the scavenging capacity of the medium. The final yields were similar for SV40 DNA and minichromosomes and were comparable to the corresponding yield determ ined for cells. Results for the yield of double-strand breaks indicate d that the yield of double-strand breaks, G(DSB), for DNA and minichro mosomes is subject to a PICA effect by SPM and SPD comparable to that measured for G(SSB). Values of G(SSB) for SV40 DNA and minichromosomes subjected to the PICA effect were well approximated by calculations b ased upon a 30-nm cylinder assumed to model their condensed states. Th e results indicate that a major fraction of the formation of SSBs in c ondensed DNA and minichromosomes results from nonscavengeable radical intermediates. Minichromosomes subjected to the PICA effect of 2 mM SP M were protected against formation of radiation-induced SSBs 1.5-fold by 20 mM DTT but 5-fold by 10 mM DTT plus 10 mM WR-1065 relative to 2 mM SPM alone. Thus WR-1065 is capable of providing marked protection o f compacted and aggregated minichromosomes, a protection ascribed to t he chemical repair of DNA radicals by WR-1065. (C) 1997 by Radiation R esearch Society.