SITES WITHIN THE CELL 43 



after injection of labelled alanine, leucine or valine, the specific activity of 

 these amino acids in microsomal proteins was four times as high as in the 

 supernatant proteins and eight times as high as in mitochondrial proteins. 

 Quite similar results were obtained with plant tissues (Stephenson et al., 

 1956). Obviously, the microsomal fraction derives from cellular structures 

 which are more actively engaged in protein synthesis than the rest of the cell. 



Several workers later succeeded in subfractionating the microsomal 

 fraction into components which have different metabolic activities. By 

 treating microsome preparations with deoxycholate, Littlefield et al. 

 (1955) separated a nucleoprotein containing most of the RNA and a frac- 

 tion made of protein and lipid. AXXirey'et al. (1953), Daly et al. (1955) used 

 ribonuclease and salt extraction to obtain microsomal subfractions, Hultin 

 used extraction and reprecipitation with salt solutions. By a similar but 

 somewhat more elaborate procedure, Simkin and Work (1957) separated 

 several protein and nucleoprotein fractions. In every case, the most rapid 

 incorporation of amino acids is observed in a protein material which re- 

 mains closely associated with RNA. Moreover, during incorporation of 

 labelled amino acids, the specific activity of the ribonucleoproteins very 

 rapidly reaches a plateau whereas that of the other protein fractions keeps 

 increasing slowly (Fig. 19). 



If a tracer amount of radioactive amino acid is injected so as to present 

 the tissue with a highly radioactive precursor for a very short period only, 

 followed by dilution of the tracer, the specific activity of the ribonucleo- 

 protein particle rises abruptly during the first few minutes and then 

 decreases. To the contrary, the specific activity of the other microsomal 

 subfractions (e.g. the lipoproteins) and of the soluble fraction keeps increas- 

 ing for a rather long time. This is, qualitatively at least, what one would 

 expect if a microsomal ribonucleoprotein was an obliged intermediary 

 stage through which other protein fractions must pass (Littlefield et al., 

 1955; Littlefield and Keller, 1957). Very clear evidence for a microsomal 

 ribonucleoprotein intermediate in the synthesis of globin by rabbit 

 reticulocyte was also presented by Rabinowitz and Olson (1956, 1959) and 

 by Kruh et al. (1960). Similarly, serumalbumin appears first in liver 

 ribonucleoproteins (Peters, 1957, 1959; Takanami, 1960) and it is later 

 released as soluble protein. Antibodies are first detected in the particles of 

 lymph nodes (Kern et ah, 1959). 



Soluble proteins have often been found in a hidden form in the ribo- 

 nucleoprotein particles of the tissue which produces them. Earlier observa- 

 tions of this type were made by Jeener and Brachet (1944) for haemo- 

 globin in the 'small particles' of bone marrow cells and for the meianophore 

 expanding hormone in particles from hypophysis. More recently, Peters 

 (1957) and Elson (1958, 1959) found that newly made proteins can be set 

 free by destruction of the particles and Feldman et al. (1960) separated 



