Effect of N-nitrosamines carcinogenic for oesophagus on O6-alkyl-guanine-DNA-methyl transferase in rat oesophagus and liver.

Several O6-alkylGua adducts have been shown to be removed from DNA during its repair by transfer of the alkyl group to a cysteine residue in a specific AAP, with the formation of S-alkylcysteine. As the reaction is stoichiometric and irreversible, the AAP content of the cell can be reduced or depleted. In vivo depletion by a high dose of nitrosamine can be used to test for the formation of a repairable alkylation adduct at the O6-position of guanine. In addition, if the carcinogenic potency of a nitroso compound for a particular organ is related to the persistence of the adduct in DNA, potency would depend not on the level of alkylation attained after treatment, but on whether this was sufficient to deplete the AAP content of the organ concerned and so to slow down repair, i.e. depletion of AAP is a more relevant estimate of potency than is the initial extent of DNA alkylation. Dose-response studies on target and non-target organs showed that depletion of AAP correlated with organotropy for those nitrosamines known to methylate DNA, i.e. with NDMA for liver, and with NMBzA for oesophagus. With NDEA, the results supported the suggestion that other adducts in addition to O6-alkylGua may be involved. NMPhA, an oesophageal specific carcinogen, did not deplete AAP in oesophagus, and induced AAP in liver. This result adds to the evidence that NMPhA does not alkylate DNA.