Studies on the synthesis of serum albumin by the isolated microsome fraction from rat liver

Abstract

The microsomal fraction from regenerating rat liver was incubated with radioactive amino acids, adenosine triphophate, phosphoenolpyruvate and cell sap. The microsomes were separated from the soluble proteins and were disintegrated with ultrasonics. The rat albumin released from the microsomes and in the soluble fraction of the incubation mixture was precipitated by the addition of an antiserum and its radioactivity was determined. The results indicated that there was an incorporation of amino acids into the microsomal albumin but not into the albumin in the medium. The rat albumin isolated from the microsomes and the medium after incubation as described above was purified by the trichloro-acetic acid-ethanol method and then subjected to electrophoresis on strips of cellulose acetate paper. The microsomal rat albumin retained its radioactivity throughout the course of these purification procedures. Similar results were obtained with microsomes from normal liver. Identical isolation procedures were applied to albumin isolated from liver microsomes 20 min. after injecting rats with radioactive amino acids. The radioactivity of the rat albumin was determined throughout the course of isolation. A comparison of the radioactivity of the insoluble microsomal protein and the microsomal albumin in the experiments in vitro and in vivo showed that the isolated microsomes suffered a relatively greater loss in their ability to synthesize albumin than insoluble protein. Results are consistent with the hypothesis that isolated rat-liver microsomes are able to synthesize albumin but that this albumin is not released in a soluble form.