Total-body irradiation (1000 R) of rats delays the time at which maximal activity of the TdR phosphorylating kinases is reached in regenerating liver, if the irradiation is given before the appearance of kinase activity. The inhibition decreases when the lapse of time between irradiation and hepatectomy increases, indicating some repair in the protein-synthesizing system. When given during the period of increasing activity, 1000 R is no longer able to inhibit the kinase activity, but 1500 R exerts an inhibitory effect. The enzyme activity, therefore, is less radiosensitive than the synthesis of new enzyme molecules. When irradiation is given 20 hours after partial hepatectomy, at a time when kinase activity would no longer be inhibited, the rate of DNA synthesis measured at 24 hours is found to be decreased to about the same extent as DNA polymerase activity. A radioprotective dose of AET injected to rats before partial hepatectomy also delays the onset of kinase activity. This is considered as a symptom of the metabolic shock induced by the sulfhydryl protector, which may be of some importance in the development of the protective power. A protective effect of AET on the radiation-induced inhibition of kinases is observed when the rats are irradiated before partial hepatectomy. In rats irradiated after hepatectomy, the protective effect of AET is masked by its greater toxicity to hepatectomized animals.

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