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(center dot+)/8-oxodG(-H)(center dot) radicals at 325 um. The reaction kinetics consist of a mechanism that includes the transformation of the 2-amidinoprop-2-peroxyl radicals (ROO center dot) derived from photolysis of AAPH to more reactive 2-amidinoprop-2-oxyl radicals (RO center dot), 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(center dot+)/8-oxodG(-H)(center dot). radicals with superoxide (O-2(center dot-)) and ROO center dot 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.