Amino Acid Incorporation by Liver Microsomes and 

 Ribonucleoprotein Particles 



Tore Hultin, Alexandra von der Decken, 

 Erik Arrhenius and Winfield S. Morgan* 



The Wenner-Gren Institute for Experimental Biology , 

 University of Stockholm, Sweden 



Experiments ranging over widely different groups of organisms have 

 shown that at least in the cytoplasm, but probably in all parts of the cell 

 [24, 25], the synthesis of proteins is intimately connected with a class of 

 submicroscopic particles (cf. [10]) remarkably rich in RNA.f By means 

 of isolated particles it has been possible to reconstruct, at the subcellular 

 level, essential parts of the protein-synthesizing mechanism, and model 

 experiments of various kinds have already yielded a rich supply of in- 

 formation about the enzymic pathways involved in the synthetic process. 

 However, in spite of the remarkable prosperity of the approach, the basic 

 function of the particles is still very vaguely understood, i.e. the reaction 

 by which individual, activated amino acids become linked together in a 

 predetermined order to polypeptide chains of specific configuration, 

 precursors of the final proteins. Neither has any definite answer been 

 given to the question of how the completed protein molecules are even- 

 tually released from the active sites on the particles. 



From the point of view of cell function a particularly important problem 

 pertaining to protein metabolism is how this fundamental process is 

 quantitatively and qualitatively regulated in growth and differentiation. 

 Unfortunately, our possibilities of reaching concrete answers to this 

 particular problem are even more limited at present, since no model 

 experiments have yet been devised which unambiguously reproduce such 

 regulatory mechanisms in vitro. 



In experiments with cell-free amino acid incorporation systems it is 

 often observed that the incorporation activity under optimal conditions 

 is approximately proportional to the amounts of RNP-particles present [5]. 



* Permanent address : Department of Pathology, Massachusetts General Hospital, 

 Boston, Mass., U.S.A. 



t Abbreviations : ATP, adenosine triphosphate ; GTP, guanosine triphosphate ; 

 PEP, phosphoenolpyruvate ; RN.A., ribonucleic acid ; RNP-particles, ribonucleo- 

 protein particles. 



