DNA and Aging -with special reference to mitochondria

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Aging-dependent changes of mitochondrial DNA and nuclear DNA (mtDNA/nDNA) ratios were measured using several tissues of rats. For accurate determination, we used LINE (long highly-repetitive sequence) which is present in nuclear DNA of rats. A 2 kbp LINE fragment which was obtained by Sall and PstI digestion of purified DNA was used for biotinylated probe (probe N) synthesis. mtDNA fragment A (6.2 kbp) of EcoRI digestion was used as a mitochondria-specific probe. Using these two probes, mtDNA/nDNA ratio can be obtained by a single experiment. The ratio of each point was calculated as average of at least three independent measurements.In heart muscle, mtDNA/nDNA ratio of 30 month-old rats decreased to about 30% of young (2 month) samples as in the case of rat liver. Experimental results described above and pervious years were those from rats on 60%-limited diet. In case of those fed ad libitum, age-dependent decrease of mtDNA/nDNA ratio was the same as limited samples in liver, however, slower decrease of the ration in heart muscle was observed. In kidney, the ratio stayd at the same level in case of the limited diet, but some decrease was evident with those fed ad libitum. In case of rats which were fed ad libitum, decrease of mtDNA was observed in cerebrum, but not in cerebellum.