Nature of the DNA associated with the nuclear envelope of regenerating liver

Our previous study showed that shortly after administration of [ 3H]thymidine to partially hepatectomized rats, DNA of the highest specific activity was found in a membrane fraction derived from the inner nuclear envelope. When DNA of regenerating liver was pulse-labeled at the beginning of the S- (DNA synthesis) period, the radioactivity shifted from the inner nuclear envelope to the nuclear pellet as the pulse time was increased. When DNA was pulse-labeled at various times during the S-period and the inner nuclear envelopes isolated shortly afterward, the specific activities of all fractions remained elevated. These results were consistent with the interpretation that both initiation and replication of DNA took place while in association with the inner nuclear envelope. Most of the newly labeled DNA of the inner nuclear envelope partitioned to the water-phenol interface during phenol extraction. Treatment of the nuclear membrane fraction with pronase, phospholipase A and sodium dodecyl sulfate resulted in the release to the aqueous phase of a major part of the DNA previously bound to the interface. Since these agents also disrupt lipoprotein structure, it is suggested that DNA of the inner nuclear envelope is associated in a complex which involves lipoproteins. Analysis of radioactive membrane-bound DNA by hydroxyapatite column chromatography and by filtration on nitrocellulose membrane showed that the DNA was double-stranded with significant single-stranded regions. The properties of the nuclear membrane-bound DNA in phenol extraction, hydroxyapatite chromatography and nitrocellulose membrane filtration were identical to those reported by other investigators for newly replicated DNA in mammalian cells; therefore it is suggested that the DNA associated with the nuclear membrane fraction is an intermediate in the replication of DNA in the regenerating rat liver.