Chromatin structure and radiation-induced DNA strand breaks in human cells: soluble scavengers and DNA-bound proteins offer a better protection against single- than double-strand breaks.

In the present investigation we have studied the formation of DNA double-strand breaks (dsbs) in different chromatin substrates and made a comparison with our previous results on radiation-induced single-strand breaks (ssb). Removal of soluble scavengers increased the number of ssbs by a factor of 2, but increased dsbs only 1.2 times. Decondensation of the chromatin increased ssbs and dsbs to the same extent, by a factor of 3-4. Removal of DNA-bound proteins gave an additional increase in ssbs and dsbs by a factor of 14 and 5 respectively. The increase in both ssbs and dsbs was caused by OH radicals. The differential effect on ssbs and dsbs can be explained by assuming that dsbs are formed mainly by multiple hits from clusters of ionizations close to the DNA. When the DNA-bound proteins are removed, more distant radicals can reach the DNA, which favours ssb formation. Under those conditions dsbs also induced by single OH radicals become important. DNA-bound proteins protect DNA only to a limited extent by acting as scavengers of OH radicals. The largest part of the protection depends on the fact that DNA in the chromatin is made up of large, compact aggregates, where the distance between separate aggregates exceeds the effective range of the OH radicals and the amount of water inside the aggregates is too small to give a maximal contribution of OH radicals compared with when DNA is more evenly distributed in a given volume.