Protein synthesis in guinea-pig liver. Incorporation of radioactive amino acids into proteins of the microsome fraction in vivo

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Abstract
Guinea pigs were killed at various time intervals after the intravenous injection of a mixture of C14-labeled amino acids (Chlorella-protein hydrolysate). The liver-microsome fraction was separated into a number of subtractions of distinct composition by successive extractions with solutions of varying ionic strength and pH. The proteins of the microsome subtractions were labeled at widely differing rates. The protein fraction with the greatest turnover rate was associated with RNA and showed a maximum activity about 30 minutes after administration of labeled amino acids. Another microsome sub-fraction also contained ribonucleoprotein, but showed a much lower turnover rate. The microsome subtractions were analyzed for protein, RNA, DNA and lipid-soluble P. The RNA of several subtractions was abnormally unstable in perchloric acid. Several subtractions were examined in the ultracentrifuge and by electrophoresis, and results were compared with those obtained by other methods of fractionation of the microsomes. The possible role of RNA in protein synthesis is discussed.