The Association of Deoxyribonucleic Acid with Liver Microsomes

Rats and mice were given 3H-thymidine, and 18 to 24 hours later nuclei, mitochondria, and microsomes were isolated from the livers by differential centrifugation. The nucleic acids were extracted from the fractions by means of the phenol-sodium dodecyl sulfate procedure. DNA in amounts of 1% or less of the total tissue content was obtained from both the mitochondria and the microsomes. The specific radioactivity of the DNA samples followed the order mitochondrial g microsomal g nuclear. The radioactivity of the mitochondria- and microsome-associated DNA had half-lives of 8 to 9 days in vivo. The microsome-associated DNA was recovered mainly in the membranous portions of the microsomes, was removed by deoxyribonuclease, and closely resembled nuclear DNA in melting behavior, banding in CsCl, sedimentation rate, appearance under the electron microscope, and hybridization with cytoplasmic RNA. The possibility was examined that microsome-associated DNA represented a mixture of contaminating nuclear and mitochondrial DNA. Assays for succinic dehydrogenase, a mitochondrial marker enzyme, indicated a contamination of microsomes by mitochondria of ≤2%, a value insufficient to account for the results on this basis. In other experiments, the loss of DNA from nuclei and its adsorption by microsome were described; the specific radioactivity of the nuclear DNA released, however, was considerably lower than the radioactivity of the microsome-associated DNA, so nuclear leakage of DNA also did not appear to explain the results with microsomes. Microsomes were shown to have RNA polymerase activity but no evidence could be obtained for the involvement of microsome-associated DNA in this activity. The data obtained lead to the conclusion that although DNA has been found to be associated with microsomes and may be an integral component of the latter, alternate explanations based upon contamination by nuclear or mitochondrial DNA, or both, could also account for the recovery of DNA in the microsomal fraction. Since the data do not decide between these possibilities, further experiments will be required to decide if the DNA associated with microsomes is a true microsomal DNA.