THE SEQUENTIAL SYNTHESIS OF THE POLYPEPTIDE CHAIN OF SERUM ALBUMIN BY THE MICROSOME FRACTION OF RAT LIVER

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Abstract
Microsomal albumin was prepared from the isolated microsome fraction of regenerating rat liver after incubation with cell sap, an energy source, and [S35] methionine, [C14] isoleucine or [C14] leucine for different periods of time. The distribution of the isotopes in albumin was determined by separation of tryptic peptides from the protein. Radioactivity was measured in peptides either qualitatively by radioautography or quantitatively by labelling with both H3 and C14. A gradient of radioactivity existed at all times in albumin isolated after incubating microsomes. The shorter the incubation time the fewer were the peptides labelled in albumin, but the peptides with highest specific activity after short incubation times corresponded to those with highest specific activities after long incubation times. Leucine released from the C-terminus of albumin had a higher specific activity than the mean specific activity of the remaining leucine residues. The peptide with the highest specific activity in albumin was probably derived from the C-terminus of the protein. [C14] Glutamic acid was incorporated into the N-terminus of albumin after incubating the microsome fraction with this isotopically labelled amino acid, cell sap and a source of energy. The specific activity of the N-terminal glutamic acid under these conditions was less than the mean specific activity of the remaining glutamic acid and glutamine residues in albumin. The results were interpreted as reflecting a sequential synthesis of serum albumin in the isolated microsome fraction of rat liver. The direction of synthesis of albumin was from the N-terminus towards the C-terminus. The bulk of incorporation of radioactive amino acid into albumin in the isolated microsome fraction was due to completion of partially completed, pre-existing peptide and polypeptide chains. A limited synthesis of new chains of albumin did, however, occur.