Incorporation of 32P i into mitochondrial and nuclear DNA in regenerating liver

Abstract

32P i incorporation into the liver DNA of normal and hepatectomized rats was studied. Four h after injection of 32P 1 into normal rats the mitochondrial DNA had a specific activity 6 times as great as did nuclear DNA. Twelve h after hepatectomy a 4-h incorporation of 32P i showed that the specific activity of mitochondrial DNA was 18 times as great as that of nuclear DNA and accounted for 50% of the 32P i incorporated into the total liver DNA. At 24 h post hepatectomy the rate of nuclear synthesis of DNA reached a maximum and the specific activities of nuclear and mitochondrial DNA were then equal after a 4-h incorporation. When normal rats were injected with 32P i and sacrificed at times between 1 and 14 days after injection it was found that the specific activity of the mitochondrial DNA did not change appreciably during that period. In hepatectomized animals, both nuclear and mitochondrial DNA reached a maximum specific activity 2 days after injection and then both decayed at the same rate. The results indicate that mitochondrial DNA, like nuclear DNA, is metabolically stable and that DNA synthesis in normal ratliver mitochondria is a continuous process. The base ratios of mitochondrial and nuclear DNA were slightly but consistently different. A metabolically active contaminant associated with purified mitochondrial DNA after enzymatic digestion to mononucleotides was identified as AMP.