Oxidation of 8-Oxo-7,8-dihydro-2′-deoxyguanosine by Oxyl Radicals Produced by Photolysis of Azo Compounds

Oxidative damage to 8-oxo-7,8-dihydroguanine (8-oxoG) bases initiated by photolysis of the water-soluble radical generator 2,2′-azobis(2- amidinopropane) dihydrochloride (AAPH) has been investigated by laser kinetic spectroscopy. In neutral oxygenated aqueous solutions, 355 nm photolysis of AAPH initiates efficient one-electron oxidation of the 8-oxodG nucleosides directly monitored by the appearance of the 8-oxodG^•++/8-oxodG(-H) ^•+ radicals at 325 nm. The reaction kinetics consist of a mechanism that includes the transformation of the 2-amidinoprop-2-peroxyl radicals (ROO^•+) derived from photolysis of AAPH to more reactive 2-amidinoprop-2-oxyl radicals (RO^•+), which directly react with the 8-oxoG bases. The major pathways for the formation of end products of 8-oxoG oxidation include the combination of the 8-oxodG ^•++/8-oxodG(-H)^•+ radicals with superoxide (O₂^•+-) and ROO^•+ radicals in approximately 1:1 ratios, as demonstrated by experiments with Cu,Zn superoxide dismutase, to form dehydroguanidinohydantoin (Gh_ox) derivatives. This mechanism was confirmed by analysis of the end products produced by the oxidation of two substrates: (1) the 8-oxoG derivative 2′,3′, 5′-tri-O-acetyl-7,8-dihydroguanosine (tri-O-Ac-8-oxoG) and (2) the 5′-d(CCATC[8-oxoG]CTACC) sequence. The major products isolated by HPLC and identified by mass spectrometry methods were the tri-O-Ac-Gh_ox and 5′-d(CCATC[Gh_ox]CTACC products.