The incorporation of [14C]leucine into serum albumin by the isolated microsome fraction from rat liver

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Abstract
A method is described for the extraction of serum albumin from microsomal extracts in high yield. The microsomal fraction from regenerating rat liver was incubated with C14 leucine and a source of high-energy phosphate. The particles were separated by centrifuging and serum albumin was released by ultrasonic vibrations. The albumin was purified finally by zone electrophoresis on cellulose acetate. A rat was intravenously injected with C14 leucine and after 30 minutes the animal was killed. Albumin was isolated from the microsome fraction of the liver. The radioactive albumin samples obtained under conditions in vitro and in vivo were denatured, oxidized and hydrolysed with trypsin. The peptides resulting were fractionated by ionophoresis on paper and the C14 leucine-containing peptides located by radioautography. Comparison of the radioautographs indicated about 20 bands of active peptides in each case. The peptides labelled were in general similar under the 2 conditions but the degree of labelling showed marked differences. The results are consistent with the synthesis of serum albumin by the isolated microsome fraction. They appear to exclude the incorporation of C14 leucine into serum albumin by attachment to the residues of the basic or acidic amino acids or by a limited exchange with leucine residues. The extent to which entirely new polypeptide chains are synthesized under the conditions in vitro is not at present known.