Fragmented mitochondrial DNA is the predominant carrier of oxidized DNA bases

Previous analyses indicated a high level of oxidative base modification in mitochondrial DNA, the extent of which raised questions about the methodological validity and biological implications. In the present study DNA was isolated from rat liver mitochondria under carefully controlled conditions, and the extent of base oxidation, DNA fragmentation, and nuclear DNA contamination were analyzed. DNA isolated from intact mitochondria treated with DNase consisted of 16.3 kilobase pairs, mostly circular, mitochondrial DNA molecules and a mixture of nuclear and mitochondrial DNA fragments, as identified by agarose gel electrophoresis and hybridization. High-performance liquid chromatography in combination with electrochemical detection confirmed that the overall level of 8-hydroxy-2'-deoxyguanosine, a marker commonly used in the analysis of base oxidation, is higher in mitochondrial than in nuclear DNA. Importantly, 8-hydroxy-2'-deoxyguanosine is relatively scarce in the 16.3 kilobase pair mitochondrial DNA molecules (0.051 pmol/mu g) but is present in high levels in mitochondrial DNA fragments (0.741 pmol/mu g). The fragments constitute about 18% of total mitochondrial DNA. The antitumor agent bleomycin, which binds to DNA, forms an iron complex capable of transferring electrons from Fe2+ to molecular oxygen. Exposure of mitochondria to bleomycin and iron resulted in nicking but not in a significant increase in base oxidation of 16.3 kilobase pair mitochondrial DNA, whereas the amount and the oxidation level of fragmented mitochondrial DNA significantly increased. These findings are relevant for a better understanding of the role of mitochondria in aging and various diseases and are consistent with the notion that despite the overall high DNA oxidation level, mitochondria can faithfully proliferate.