DNA and Aging -with special reference to mitochondria

用大鼠几种组织测定了线粒体DNA和核DNA（mtDNA/nDNA）比值的增龄依赖性变化。为了准确测定，我们使用了存在于大鼠核DNA中的LINE（长高度重复序列）。将通过萨尔和Pst I消化纯化DNA获得的2kbp LINE片段用于生物素化探针（探针N）的合成。用EcoR Ⅰ酶切的mtDNA片段A（6.2kbp）作为特异性探针。使用这两种探针，可以通过一次实验获得mtDNA/nDNA比值。在心肌中，30月龄大鼠的mtDNA/nDNA比值与大鼠肝脏中的情况一样，下降到年轻（2月龄）样品的30%左右。上述和前几年的实验结果是来自60%限制饮食的大鼠。在自由采食的情况下，随年龄增长，线粒体DNA/nDNA比值的下降是相同的有限的样品在肝脏中，但在心肌中观察到的比例下降较慢。在肾脏中，在限制饮食的情况下，该比率保持在相同的水平，但在自由进食的情况下，该比率有所下降。在自由采食的情况下，在大脑中观察到mtDNA的减少，但在小脑中没有观察到。

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.