Accumulation of adenine DNA glycosylase-sensitive sites in human mitochondrial DNA

The mitochondrial respiratory chain inevitably produces reactive oxygen spe cies as byproducts of aerobic ATP synthesis. Mitochondrial DNA (mtDNA), whi ch is located close to the respiratory chain, is reported to contain much m ore 8-oxoguanine (8-oxoG), an oxidatively modified guanine base, than nucle ar DNA. Despite such a high amount of 8-oxoG in mt DNA (1-2 8-oxoG/10(4) G) , mtDNA is barely cleaved by an 8-oxoG DNA glycosylase or MutM, which speci fically excises 8-oxoG from a C:8-oxoG pair. We find here that about half o f human mtDNA molecules are cleaved by another 8-oxoG-recognizing enzyme, a n adenine DNA glycosylase or MutY, which excises adenine from an A:8-oxoG p air. The cleavage sites are mapped to adenines. The calculated number of Mu tY-sensitive sites in mtDNA is approximately 1.4/10(4) G. This value roughl y corresponds with the electrochemically measured amount of 8-oxoG in mtDNA (2.2/10(4) G), raising the possibility that 8-oxoG mainly accumulates as a n A:8-oxoG pair.