HYPOCHLOROUS ACID N-CHLORAMINES ARE NATURALLY PRODUCED DNA-REPAIR INHIBITORS

Human mononuclear leukocytes (HML) respond to oxidative DNA damage by activation of ADP ribosylation and initiation of DNA repair synthesis (i.e. unscheduled DNA synthesis, UDS), whereas neutrophils do not, Whe n neutrophils are added to HML cultures in ratios up to 4:1 ADP ribosy lation becomes inhibited to similar to 50-60%, The ability of neutroph ils to inhibit HML ADP ribosylation was shown to be dependent on H2O2, chloride ions and myeloperoxidase, which in turn are factors known to govern HOCl and N-chloramine production by phagocytic cells, HOCl and a model N-chloramine, chloramine T, were shown to give a dose-depende nt inhibition of DNA repair using four independent estimates, namely A DP ribosylation, UDS and the repair of DNA strand breaks estimated by nucleoid sedimentation and alkaline elution profiles, All the DNA repa ir measurements used on HML were inhibited similar to 70-80% by 100 mu M doses of HOCl or chloramine T, which was considered a biologically relevant dose because: (i) viable neutrophils equal in concentration t o those found in blood could easily produce 100 mu M levels in shortte rm culture; (ii) 100 mu M doses of these agents were not acutely cytot oxic judged by trypan blue stained cells after 30-60 min exposure and under the conditions used for assay, but yet they abolished 86-95% of the growth response of HML to phytohemagglutinin.