Radioprotection of human cell nuclear DNA by polyamines: Radiosensitivity of chromatin is influenced by tightly bound spermine

The polyamines putrescine (PUT) and spermine (SPM) were examined for their ability to protect human cell DNA against the formation of radiation-induce d double-strand breaks (DSBs), As observed previously, under conditions whe re polyamines were shown to be almost completely absent, association with n uclear matrix protein into a nucleoid, and organization into chromatin stru cture, protected DNA from induction of DSBs by factors of 4.5 and 95, respe ctively. At concentrations below 1 mM, PUT or SPM provided equivalent level s of protection to deproteinized nuclear DNA, consistent with their capacit y to scavenge radiation-induced radicals. At constant ionic strength, 5 ali SPM protected deproteinized DNA and nucleoid DNA and DNA in nuclear chroma tin by factors of 100 and 26, respectively. At 5 mM SPM provided 15 times g reater protection of deproteinized DNA than did PUT. Under physiologically relevant conditions, 5 mM SPM protected DNA in the intact nucleus from the induction of DSBs by a factor of 2 relative to DNA in the absence of SPM. S tudies of SPM binding during cellular fractionation revealed that a signifi cant fraction of the cellular SPM is tightly bound in the nucleus but can b e removed by extended washing. Thus the association of SPM with nuclear chr omatin appears to be a significant contributor to the resistance of the cel l's DNA to the induction of DSBs. (C) 1999 by Radiation Research Society.