INTERMEDIATE Rl-ACTIONS IN PROTEIN SYNTHESIS 



i6i 



almost all of the RNA of the niicrosoinc and have been termed rihoiuicleopro- 

 tein particles; the other microsomal material, soluble in deoxycholate, probably 

 represents the microsomal membranes with some associated soluble proteins 



(3I' 33)- 



Littletield et al. have shown that the ribonucleoprotein particles are the first 



part of the microsomes to incorporate amino acid into their protein (31 ). Follow- 

 ing injection of a saturating dose of C'*-leucine the liver was homogenized and 

 the microsome fraction was prepared and was separated further with sodium 

 deoxycholate. The deoxycholate-insoluble ribonucleoprotein particles were the 

 first to become labeled, as indicated in figure i. Shortly thereafter, the labeled 

 proteins were found in the remaining portion of the microsomes, and still later, 

 in the proteins of the supernatant fraction of the cell. 



The incorporation of amino acids into the ribonucleoprotein particles reached 

 a steady state and remained constant while that into the other protein fractions 

 proceeded at a linear rate. This suggests that the ribonucleoprotein particles are 

 the site of the synthesis of the peptide linkage of the proteins and subsequently 

 the proteins are released from these particles and appear in the deoxycholate- 

 soluble and supernatant cell fraction. 



Studies with cell-free homogenates have made it possible to define several 

 enzymatic steps and specific cofactor requirements. The pioneer tissue slice ex- 

 periments had indicated that some source of energy was required for formation 

 of peptide bonds (14, 42), and the initial homogenate described by Siekevitz 

 included the mitochondrial fraction (35). Later it was found that this fraction 

 could be replaced by adenosinetriphosphate (ATP) plus an ATP-generating sys- 



DEOXYCHOLATE 

 SOLUBLE 



DEOXYCHOLATE 

 INSOLUBLE 



SOLUBLE 

 PROTEIN OF CELL 



10 15 

 MINUTES 



20 



MICROSOMAL RNA 



NUCLEAR RNA 



-* *■ 



0.2 0.4 06 0.8 

 MG. RNA ADDED / FLASK 



Fig. I. Incorporation in I'ii'o of a large close of leucine-C* into tlic twu coiiiponcnts of the 

 microsomes and into the soluble proteins of the cell. [From (ji).| 



Fig. 2. Specificity of S-RNA as acceptor of the adenine nucleotide end group. [From (17).] 



