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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]