The incorporation of${}^{14}{\rm C}\text{-acetate}$ and${}^{14}{\rm C}\text{-mevalonic}$ acid (MVA) into cholesterol was determined in liver slices from x-irradiated fasted mice. Those two compounds represent cholesterol precursors located before and behind the rate-limiting step of cholesterol synthesis, β-hydroxy-β-methylglutaryl-coenzyme A (HMG-CoA) reductase. The results indicate that the biosynthesis of cholesterol is increased from both precursors after irradiation. A latent period was observed before the increased cholesterol synthesis was manifested and 72 hours after the irradiation the highest incorporation was obtained. The extent of stimulation depends upon the radiation dose, but this is relevant only 48 and 72 hours postirradiation. The incorporation of acetate is increased up to twentyfold, while MVA incorporation is elevated by only fivefold. At least two different sites of cholesterol synthesis appears to be affected by the x-rays. One of these is located between acetate and MVA and is the more sensitive to x-irradiation; the other radiosensitive step(s) is between MVA and cholesterol. The elevated acetate incorporation is probably caused by an increase in the activity of HMG-CoA reductase, since the stimulation is markedly diminished by puromycin. Actinomycin D, however, is unsuitable for inhibition experiments, because this inhibitor itself stimulates acetate and MVA incorporation. The content of free cholesterol remained almost unchanged in the liver and in the blood of the irradiated mice. Esterified cholesterol declined in the liver but not in the blood. There appears to be no clear correlation between the increased synthesis of cholesterol in the liver slices and the cholesterol levels in liver or blood. Many of our data confirm the work reported by others in rats.

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