Exposure of calf thymus DNA to hypochlorous acid/hypochlorite leads to extensive DNA base modification. Large concentration-dependent increases in pyrimidine oxidation products [thymine glycol (cis/trans), 5-hydroxycytosine, 5-hydroxyuracil, 5-hydroxyhydantoin] but not purine oxidation products (8-hydroxyguanine, 2- and 8-hydroxyadenine, FAPy guanine, FAPy adenine) were observed at pH 7.4. In addition, large increases in 5-chlorouracil (probably formed from 5-chlorocytosine during sample preparation), a novel chlorinated base, were observed. Addition of HOCl to DNA already damaged by •OH generated by a mixture of ascorbate, copper(II) chloride, and hydrogen peroxide showed that hypochlorous acid led to a loss of 8-hydroxyguanine, 2- and 8-hydroxyadenine, FAPy guanine, FAPy adenine, and 5-hydroxycytosine in a concentration- and pH-dependent manner. Nevertheless, time course studies suggested that the formation of purine oxidation products in isolated DNA by hypochlorous acid was not a major oxidation pathway. If this pattern of damage, especially the production of 5-chlorocytosine, is unique to hypochlorous acid, it might act as a “fingerprint” of damage to DNA by HOCl.
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