Factors in the Stimulation of Protein Synthesis by Subcellular Preparations from Rat Liver

Abstract

Abstract The control of protein synthesis has been investigated in subcellular systems from livers of rats with plasma protein overproduction following nephrotoxic serum. The following observations were made. The microsomal fraction from nephrotic liver exhibited an increased synthesis of acid-insoluble protein and of plasma albumin, as measured by amino acid incorporation per mg of ribonucleic acid present. The synthesis of plasma albumin showed a much greater increase than did the synthesis of other proteins. This is in contrast to the situation in regenerating liver, in which the increase in plasma albumin synthesis was of the same order of magnitude as the increase in acid-insoluble protein. Ribosomes or polysomes, prepared from nephrotic liver with deoxycholate, did not differ from normal liver in their ability to synthesize protein. This finding, together with the observation that mixtures of control and nephrotic microsomal fractions behaved like the nephrotic system, suggested that the difference between control and nephrotic microsomes was not the result of any inherent difference in ribosomes but might be due to the presence of additional amounts of messenger RNA. Polysomes prepared from nephrotic liver without the use of deoxycholate but still containing small amounts of attached membrane protein showed slight but significant increases in plasma albumin synthesis, averaging 33%. Cell sap from nephrotic liver was more active than that from controls in promoting protein and plasma albumin synthesis by microsomes. This difference was ascribed to the presence of increased amounts of messenger RNA in this fraction on the basis of indirect evidence.