DIFFERENTIATION AND PROTEIN SYNTHESIS 117 



with that required for certain forms of mechanical work, e.g. that required 

 by muscles or for transferring substances across membranes against a 

 concentration gradient, and in fact one finds that the number of mito- 

 chondria in muscle cells or in the proximal tubular cells of the kidney is 

 much greater than that in protein-forming cells. 



The observation that ATP is implicated in peptide-bond formation 

 suggests that an activated amino acid is probably the intermediate in the 

 synthesis. Haogland et al. (1957) have obtained evidence of enzymes 

 which could effect activation. Enzymes which catalyse carboxyl activation 

 of at least two amino acids (tryptophane and tyrosine) are now known, and 

 probably there is an enzyme for each amino acid, i.e. according to Crick 

 (1958) twenty in all. The product of the reaction between the amino acid, 

 the activating enzyme and ATP is an amino acid-adenosine mono- 

 phosphate anhydride and it is supposed that compounds of this type are 

 intermediates (see Fig. 51). 



The reasonable expectation that the long peptide chains would be pro- 

 ceeded by the formation of short peptide sequences seems to be negatived 

 by several experiments. Small peptides are rarely found and ingested pep- 

 tides cannot be directly utilized, but are first broken down to amino acids. 

 There is, however, some confusing evidence on this point which cannot be 

 considered finally settled. 



Many efforts have been spent tracing the fate of labelled amino acids 

 when these are injected into animals or fed to micro-organisms. The 

 newly-formed protein is customarily isolated as a trichloracetic acid- 

 soluble fraction from homogenized cells. If the liver, for example, is 

 examined very soon after the administration of a labelled acid, the isotope 

 is found predominately in the microsome fraction. Siekevitz (1952) found 

 that incorporation of radioactive amino acids also occurred in cell-free 

 homogenates and that the radioactive label was again predominantly in the 

 microsome fraction. Further, a microsome preparation is effective when 

 cell sap is added, or if even a partially-purified mixture or activating 

 enzymes providing an energy source (ATP or GTP) is also added. 



The work of Zamecnik et al. (1956) shows further that the amino acid is 

 activated by the specific enzymes and ATP before incorporation and it is 

 probable that the activated acids are first transferred to a soluble RNA 

 (S-RNA). The transfer of this amino acid from soluble RNA to the 

 RNA particles requires the presence of guanine triphosphate. The 

 microsomal particles contain protein and the labelled acids are linked by 

 true peptide bonds. These steps are summarized in Fig. 51, p. 118. 



Other experiments have shown that the material bound to microsomal 

 particles can be dissociated from it and appears in new protein. Rabinovitz 

 and Olson (1957) prepared reticulocytes containing radioactive leucine and 

 incubated the microsomes isolated from these with fresh sap. In the 



