Repair of alkylated purines in the hepatic DNA of mitochondria and nuclei in the rat

Further evidence for the preferential interaction of carcinogens with mitochondrial DNA (mtDNA) has been obtained. In rats treated with high doses of N-nitrosodimethylamine (NDMA) or N-nitroso-N-butylurea (NBU), hepatic mtDNA contains 1.4 times more O6-methyl-2'-deoxyguanosine (O6-MedG) or 2.3 times more O6-butyl-2'-deoxyguanosine (O6-BudG) than does nuclear DNA (nDNA). The kinetics of removal of O6-MeG from mtDNA and nDNA are similar at both high (20 mg/kg) and low (2 mg/kg) doses of NDMA, and the removal of O6-MeG can be increased by pretreating the animals with 2-acetylaminofluorene (AAF), indicating that O6-MeG is repaired in the mitochondrion by a mechanism similar to that which functions in the nucleus. In contrast, O6-BudG is removed very slowly from mtDNA and rapidly from nDNA, an observation which is consistent with the absence of a nucleotide excision mechanism in the mitochondrion and the repair of O6-BudG, predominantly by an excision mechanism, in the nucleus of mammalian cells. A 23-kd methyltransferase (MT) protein, similar to the one found in the nucleus, has been isolated from hepatic mitochondria and is present in mitochondria from which the outer membrane has been removed. It is suggested that O6-MeG, but not O6-BuG can be repaired from mtDNA by a MT protein that is nuclear encoded and transported across the mitochondrial membrane.