Newton G L, Aguilera J A, Ward J F, Fahey R C
Department of Chemistry and Biochemistry, University of California, San Diego, La Jolla 92093, USA.
Radiat Res. 1997 Sep;148(3):272-84.
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 presence of various potential radioprotective agents. Putrescine (PUT), spermidine (SPD), glutathione, trans-4,5-dihydroxy-1,2-dithiane, 2-mercaptoethyl 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. These results were expected from the ability of these agents to scavenge OH radical in the bulk solution. However, SPD, above 10 mM, and SPM and WRSSWR, each above 2 mM, produced dramatic radioprotection attributed to polyamine-induced compaction and aggregation of the DNA (PICA effect). The DNA of SV40 minichromosomes was inherently less radiosensitive and was subject to a PICA effect at lower polyamine concentrations, i.e. approximately 5 mM SPD, approximately 0.6 mM SPM and approximately 0.5 mM WRSSWR. The PICA effect decreased G(SSB) for SV40 DNA and minichromosomes 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 determined for cells. Results for the yield of double-strand breaks indicated that the yield of double-strand breaks, G(DSB), for DNA and minichromosomes 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 based upon a 30-nm cylinder assumed to model their condensed states. The results indicate that a major fraction of the formation of SSBs in condensed DNA and minichromosomes results from nonscavengeable radical intermediates. Minichromosomes subjected to the PICA effect of 2 mM SPM 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 of compacted and aggregated minichromosomes, a protection ascribed to the chemical repair of DNA radicals by WR-1065.
在生理离子强度以及存在各种潜在辐射防护剂的条件下，测定了在有氧环境的水溶液中，γ射线辐照SV40 DNA和SV40微型染色体时DNA单链断裂的产额G(SSB)。腐胺（PUT）、亚精胺（SPD）、谷胱甘肽、反式-4,5-二羟基-1,2-二硫烷、2-巯基乙基二硫化物和胱胺，浓度均为0.1 - 10 mM，精胺（SPM，0.1 - 1 mM）和WR - 33278（WRSSWR，0.1 - 2 mM）均降低了SV40 DNA的G(SSB)。从这些试剂清除本体溶液中羟基自由基的能力来看，这些结果是预期的。然而，高于10 mM的SPD以及高于2 mM的SPM和WRSSWR产生了显著的辐射防护作用，这归因于多胺诱导的DNA压缩和聚集（PICA效应）。SV40微型染色体的DNA本身对辐射的敏感性较低，并且在较低的多胺浓度下，即大约5 mM SPD、大约0.6 mM SPM和大约0.5 mM WRSSWR时会受到PICA效应的影响。PICA效应使SV40 DNA和微型染色体的G(SSB)降低了一到两个数量级，这取决于介质的清除能力。SV40 DNA和微型染色体的最终产额相似，并且与在细胞中测定的相应产额相当。双链断裂产额的结果表明，DNA和微型染色体的双链断裂产额G(DSB)受到SPM和SPD的PICA效应影响，与G(SSB)所测得的情况相当。受到PICA效应影响的SV40 DNA和微型染色体的G(SSB)值，通过基于假定模拟其凝聚状态的30纳米圆柱体的计算得到了很好的近似。结果表明，凝聚的DNA和微型染色体中SSB形成的主要部分是由不可清除的自由基中间体导致的。受到2 mM SPM的PICA效应影响的微型染色体，相对于单独的2 mM SPM，20 mM二硫苏糖醇（DTT）使其免受辐射诱导的SSB形成的保护提高了1.5倍，但10 mM DTT加10 mM WR - 1065使其提高了5倍。因此，WR - 1065能够为压缩和聚集的微型染色体提供显著的保护，这种保护归因于WR - 1065对DNA自由基的化学修复作用。
