162 Zamecnik et al. 



Work in this laboratory has become confluent with this 

 cytochemical stream as a consequence of our interest in par- 

 ticulate cell constituents concerned in protein synthesis. In 

 agreement with the initial observation of Borsook and co- 

 workers (1950), we have found the microsome fraction of the 

 rat liver cell to be that centrifugally separable component of 

 the cytoplasm most rapidly labelled with ^^C-tagged amino 

 acids (Keller, Zamecnik and Loftfield, 1954). 



By means of sodium deoxycholate it has been possible to 

 separate the two above-mentioned principal components of 

 the "microsome" fraction, and to determine that the small, 

 dense granule (ribonucleoprotein particle) has a higher 

 specific radioactivity in its protein than does the vesicular or 

 fragmented membranous component (Littlefield et al., 1955). 

 The careful work of Palade and Siekewitz (1956) in tracing the 

 lineage of these microsomal constituents back to the small, 

 dense granules and membranes of the cell serves as a bridge 

 between cellular topography and cell fractionation studies. 

 Our conclusion about the high rate of labelling of the ribo- 

 nucleoprotein particle is based on experiments which were 

 carried out on whole rats. Their distinctive features were 



(1) the intravenous injection of ^*C-leucine or ^*C- valine and 



(2) the employment of labelling times of 2-10 minutes. These 

 conditions brought out maximal differences between the 

 specific activities of proteins located in the various cell 

 fractions, differences which became less distinguishable with 

 increased time periods. 



These studies also suggested that the ribonucleoprotein 

 particle in vivo was engaged in a rapid turnover process, in 

 which protein or large peptide fragments synthesized therein 

 were passed on to other parts of the cell. It has been calcu- 

 lated (Littlefield and Keller, unpublished) that the rate of 

 labelling of the cytoplasmic ribonucleoprotein particles of the 

 liver is sufficient to account for most of the protein synthesis 

 of the rat liver. This is not to imply that protein synthesis 

 may not occur also in the nucleus, but it does strengthen the 

 thought that the main pathway of synthesis of a protein 



