The Correlation Between Morphological Structure and 



the Synthesis of Serum Albumin by the Microsome 



Fraction of the Rat Liver Cell 



P. N. Campbell 

 Courtauld Institute, Middlesex Hospital, London, England 



Our interest in serum albumin is based on the assumption that it is 

 typical of the soluble proteins synthesized by the liver. It happens to be a 

 rather convenient protein to study since it is homogeneous and not too 

 difficult to isolate in a pure form. I should like to describe some experiments 

 we have done in an attempt to determine the role of the various morpho- 

 logical structures that go to make up the microsome pellet in the synthesis 

 and secretion of serum albumin. 



Peters [3] has shown in his work with slices of chick liver that the 

 microsome fraction is the most active site in the cell for the synthesis of 

 albumin. We have for some time been working with the isolated microsome 

 fraction from rat liver. We now have good evidence that such a fraction is 

 able to synthesize serum albumin. Initially we demonstrated the incorpora- 

 tion of P^C]-amino acids into the serum albumin released from the micro- 

 some pellet by ultrasonics. The albumin was characterized by immuno- 

 logical, electrophoretic and solubility criteria [i]. More recently we have 

 been studying the pattern of incorporation of [^^C] -leucine under in vivo 

 and in vitro conditions. We hope by this means to determine whether there 

 is de novo synthesis under in vitro conditions. So far the results obtained 

 are consistent with this concept. 



Although the above experiments show that the microsome fraction is 

 an active site for albumin synthesis they do not preclude such synthesis by 

 other organelles such as the mitochondria. This fraction contains significant 

 amounts of serum albumin and may also be a site for synthesis. 



As explained by Dr. Porter in his paper the predominant structure seen 

 in electron micrographs of the microsome fraction of rat liver is the so- 

 called rough endoplasmic reticulum. As Dr. Porter has shown, this consists 

 of vesicles bearing ribonucleoprotein (RNP) particles on the outside. 



The experiments of Peters, and those on the isolated microsome 

 fraction, show that the albumin that is synthesized in the reticulum is at 

 first retained. In fact with the isolated microsome system the synthesized 

 albumin is not released into the soluble fraction of the incubation medium. 



