37. NUCLEIC ACID AND PROTEIN SYNTHESIS 363 



have been found associated with whole mammalian microsomes (cf. Pal- 

 ade 16a ). Only feeble activity remains when the membranous component is 

 removed, however. Cholic acid is activated by microsomes, 58 but the loca- 

 tion of the activity in subfractions of microsomes is not stated. 



It was observed early that rat liver microsomes catalyze a vigorous 

 pyrophosphate (PP)-ATP exchange reaction. 59 This was not influenced by 

 amino acids and hence could not be said to be due to amino acid activating 

 enzymes. Kenney furthermore reported that microsomes produce PP on 

 incubation with ATP, guanosine triphosphate (GTP), and uridine triphos- 

 phate (UTP). 60 The significance and exact anatomical location (i.e., ribo- 

 somes or membranes) of these reactions in microsomes remain obscure. 



Webster has found that a substantial portion of the total amino acid 

 activating activity in pea seedling extracts is associated with the ribonu- 

 cleoprotein particles and that it is difficult to remove by various physical 

 and chemical treatments. 61 Cohn, 62 and Rendi and Hultin 63 have found 

 such amino acid activating activity in detergent-prepared animal particles. 

 Peptidase activity has been found in E. coli particles. 64 In all these cases 

 the particles are thought to be relatively free of membrane components. 



Several workers have reported the presence of ribonuclease in ribo- 

 somes. 16 ' 44 ' 46, 65 This is particularly interesting since so much of these 

 particles is composed of RNA. The enzyme usually manifests itself as an 

 apparent instability of the particle RNA under certain conditions. Ribo- 

 somes derived from reticulocytes, 66 on the other hand, do not appear to 

 have intrinsic ribonuclease activity. The significance of the ribonuclease 

 activity is unknown, but it is, thus far, the only enzyme which appears 

 likely to be an integral part of the ribosomes themselves. 



c. Evidence for a Direct Role of Ribosomes in Protein Synthesis 



Figure 1 is a schematic summary of the series of reactions thought to be 

 involved in the conversion of free C 14 -amino acids to peptide linkage in 

 protein. The inclusion of such chemical detail in this scheme has been made 

 possible chiefly as the result of studies on mammalian systems dating back 

 a little more than ten years. Although most of the data upon which the 



58 W. H. Elliott, Biochem. J. 65, 315 (1957). 



59 M. B. Hoagland, E. B. Keller, and P. C. Zamecnik, J. Biol. Chem. 218, 345 (1956). 



60 F. T. Kenney, S. P. Colowick, and E. Barbehenn, Arch. Biochem. Biophys. 69, 617 

 (1957). 



61 G. C. Webster, J. Biol. Chem. 220, 535 (1957). 



62 P. Cohn, Biochim. et Biophys. Acta 33, 284 (1959). 



63 R. Rendi and T. Hultin, Exptl. Cell Research 19, 253 (1960). 



64 B. J. McCarthy and E. T. Botton, Abstr. Meeting Biophys. Soc. p. 10 (1959). 



65 D. Elson, Biochim. et Biophys. Acta 27, 216 (1958). 

 S6 V. M. Ingram, personal communication (1960). 



