THE ROLE OF MICROSOMES IN THE INCORPORATION OF AMINO ACIDS INTO PROTEINS

Abstract

These studies are an extension of previous findings of a relationship between ribose nucleic acid and protein synthesis (Brachet, Caspersson, Borsook, Keller, Siekevitz, Mirsky). In vivo experiments have been described which show that 70 per cent or more of the initial incorporation of leucine-C¹⁴ or valine-C¹⁴ in the liver occurs in the protein of the microsome fraction, which contains most of the cytoplasmic ribose nucleic acid. A straight line rate of incorporation of valine-C¹⁴ into microsome protein has been found during the first few minutes after intravenous injection of the labeled amino acid. The uptake of leucine or valine into microsome protein in these experiments is approximately 2 per cent per hour. Experiments on a cell-free system have also been reported, showing that the microsome fraction and the "soluble fraction" of the liver cell are both essential for incorporation. This incorporation occurs almost entirely into the protein of the microsome fraction.