Hypochlorite-induced damage to nucleosides: Formation of chloramines and nitrogen-centered radicals

Stimulated monocytes and neutrophils generate hypochlorite (HOCl) via the r elease of the enzyme myeloperoxidase and hydrogen peroxide. HOCl is a key b actericidal agent, but can also damage host tissue. As there is a strong li nk between chronic inflammation and some cancers, we have investigated HOCl damage to DNA bases. We show that reaction of HOCl with the exocyclic -NH2 groups of cytidine, adenosine, and guanosine, and the ring NH groups of al l bases, yields chloramines (RNHCl/RR ' NCl). These are the major initial p roducts. Chloramine decay can be accelerated by UV light and metal ions, an d these reactions, together with thermal decomposition, give rise to nucleo side-derived nitrogen-centered radicals. Evidence is presented for the rapi d addition of pyrimidine-derived nitrogen-centered radicals to another pare nt molecule to give dimers. Experiments with nucleoside mixtures show that the propensity for radical formation is cytidine > adenosine = guanosine > uridine = thymidine. These data are inconsistent with the selectivity of HO Cl attack and the stability of the resulting chloramines, but can be ration alized if chlorine transfer between bases is rapid and yields the most stab le chloramine, with such transfer preceding radical formation. Thus, though thymidine is the major initial site of chloramine formation, rapid chlorin e atom transfer generates cytidine and adenosine chloramines. These reactio ns rationalize the preferential formation of chlorinated cytidine and adeno sine in DNA.