Comparative sensitivity of the nuclear and mitochondrial DNA syntheses to X-irradiation and to the administration of a sulfhydryl radioprotector (AET) in normal and regenerating rat liver

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.