The influence of dietary protein on the incorporation of 14C-glycine and 32P into the ribonucleic acid of rat liver

Abstract 1. 1. Rats were maintained on diets either containing adequate amounts of protein or free from protein and, after fasting overnight, were injected with 32 P and with 14 C-2-glycine. They were killed at 3, 6 and 9 h thereafter. A further group of rats which had received the protein-containing diet were fed protein at the time of injection with the isotopes. 2. 2. The uptake of glycine into liver protein was essentially similar in the two groups in the post-absorptive state, but rose considerably when protein was fed at the time of injection. 3. 3. Labelling of RNA with 32 P and with glycine was much reduced in rats fasted overnight after the protein-containing diet. On feeding protein to this group, uptake of both isotopes rose to the level found with the protein-free diet. 4. 4. It is suggested that the low level of labelling in the groups fasted after the protein-containing diet was due to breakdown of RNA as soon as the supply of amino acids from the gut ceased, the products of breakdown causing dilution of isotopically labelled precursors of RNA in the acid-soluble fraction of liver. The feeding of protein, by restraining RNA breakdown and thus terminating dilution of these precursors, restored the level of labelling in RNA. This hypothesis is supported by a study of the effect of diet (a) on changes in the RNA content of the liver, (b) on 14 C-uptake by the purines of the acid-soluble fraction of liver, (c) on the uptake of 32 P by the RNA of different liver cell fractions and (d) on allantoin excretion. 5. 5. It is concluded that the rate of synthesis of liver RNA is a function of the available energy, whereas the stability and therefore amount of RNA in the liver is determined by the supply of amino acids for protein synthesis.