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Photo‐oxidation of 6‐Thioguanine by UVA: The Formation of Addition Products with Low Molecular Weight Thiol Compounds

The thiopurine, 6‐thioguanine (6‐TG) is present in the DNA of patients treated with the immunosuppressant and anticancer drugs azathioprine or mercaptopurine. The skin of these patients is selectively sensitive to UVA radiation—which comprises >90% of the UV light in incident sunlight—and they suffer high rates of skin cancer. UVA irradiation of DNA 6‐TG produces DNA lesions that may contribute to the development of cancer. Antioxidants can protect 6‐TG against UVA but 6‐TG oxidation products may undergo further reactions. We characterize some of these reactions and show that addition products are formed between UVA‐irradiated 6‐TG and N‐acetylcysteine and other low molecular weight thiol compounds including β‐mercaptoethanol, cysteine and the cysteine‐containing tripeptide glutathione (GSH). GSH is also adducted to 6‐TG‐containing oligodeoxynucleotides in an oxygen‐ and UVA‐dependent nucleophilic displacement reaction that involves an intermediate oxidized 6‐TG, guanine sulfonate (GSO3). These photochemical reactions of 6‐TG, particularly the formation of a covalent oligodeoxynucleotide–GSH complex, suggest that crosslinking of proteins or low molecular weight thiol compounds to DNA may be a previously unrecognized hazard in sunlight‐exposed cells of thiopurine‐treated patients.