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The intracellular distribution of thymidine phosphorylating kinases has been studied ill rat liver at different times after partial hepatectomy. After conventional fractionation in 0.25 M sucrose, thymidine kinase is present in all fractions, the highest proportion being found, however, in the supernatant; phosphorylation of thymidine to TDP and TTP is detected only in the supernatant fraction from 24 h onwards after hepatectomy. Nuclei prepared in 2.2 M sucrose or in non-aqueous media have the same level of thymidine kinase activity and can never phosphorylate thymidine to TDP and TTP. In the nuclear fractions, the absolute activity of thymidine kinase increases from 16 to 46 h after hepatectomy but represents a decreasing proportion of the activity of the homogenate. At the same time, the proportion of kinase activity rises markedly in the supernatant fraction with a maximum of 83% in 40 h regenerating livers. The implications of these results are discussed. [ABSTRACT FROM AUTHOR]

The kinetics of incorporation of labelled precursors into the DNA of isolated rat liver mitochondria has been studied after X-irradiation in vitro. Incorporation ofH-TTP was slightly inhibited during the initial period, followed by an increase of about 25 % of the specific activity. Incorporations ofH-TdR andH-BUdR were also stimulated by irradiation of mitochondria, although the amount of incorporated precursor was lower than that ofH-TTP. This radiation-induced incorporation of precursors was abolished by the addition of ethidium bromide or EDTA. Degradation of mt-DNA, indicated by the decrease of specific radioactivity, was observed after irradiation of pre-labelled mitochondria; in presence of EDTA the degradation was not detected. These results were discussed from the viewpoint of the repair synthesis of mitochondrial DNA. [ABSTRACT FROM AUTHOR]

Incorporation of thymidine into nuclear and mitochondrial DNA has been measured in the livers of normal rats and of rats killed 17 h or 24 h after partial hepatectomy. When total-body X-irradiation (500 or 1,500 R) is given at increasing intervals (up to 30 h) before sacrifice, a progressive decrease followed by a plateau of low level of incorporation is observed in the nuclear DNA (N-DNA) whereas the synthesis of mitochondrial DNA (M-DNA) first decreases until a minimal value is reached after 4 h in normal liver and 12 h in regenerating liver, then recovers at an exponential rate, quite independently of the nuclear DNA synthesis. The results suggest that irradiation can interfere with the enzymatic processes of both M-DNA and N-DNA syntheses, but that the persistence of the inhibition of N-DNA synthesis must be ascribed to a temporary block in the cell cycle. The SH-protector AET injected to non-irradiated rats exerts a very small inhibitory effect on normal liver N-DNA and M-DNA synthesis. In regenerating liver, both syntheses are affected rather similarly suggesting a common mechanism of inhibition through impairment of some step in the synthetic process. [ABSTRACT FROM AUTHOR]