CELLULAR METABOLISM I25 



fact, presented models of peptide bond synthesis by selecting suitable 

 amino acid derivatives and by producing the conditions necessary for 

 reversal of hydrolysis. Whether this mechanism actually takes place in 

 living cells is not known. The synthesis of proteins, like that of glycogen, 

 must be due to the cooperation of a large number of enzymes. In cyto- 

 chrome containing living cells it does not occur in the absence of oxygen, 

 and it is plausible to postulate that proteins are, like carbohydrates and 

 fats, synthesized after their breakdown to amino acids and that peptide 

 bond formation requires the utilization of energy produced during the 

 oxidation of carbohydrates and fats. Support for this assumption is 

 given by the experiments of Cohen and McGilvery (23) on the forma- 

 tion of /j-amino-hyppuric acid from /j-aminobenzoic acid and glycine 

 by rat tissue : 



NH, ^^>COOH + NH,CH,COOH^ NH, €~>CO-NHCH,COOH + H,0 



This may be taken as a model of peptide bond synthesis. This reaction 

 could not proceed anaerobically nor in the presence of oxidative inhibi- 

 tors (HCN, NaNs). Inhibition of this reaction by arsenite, iodoacetate 

 and malonate favors the view that the energy for the synthesis is pro- 

 vided by oxidations in the Krebs cycle where there are a number of 

 — SH enzymes. Whether this energy is furnished by the high-energy 

 phosphate bonds produced in these oxidations is not yet established. It 

 must be emphasized however that proteins possess a great specificity 

 and complexity. Besides enzymes forming peptide bonds there must be 

 others which add these bonds together to form the complicated and 

 varied structures of the different proteins. 



Casper son (19, 20, 21) demonstrated that ribonucleic acid is present 

 in abundance in growing cells and gave some evidence in favor of the 

 view that it is essential for protein synthesis. The mechanism of action 

 of nucleic acids is not yet known. They must act in the last steps when 

 the architecture of the molecule acquires its specific characteristics. 

 Spiegelman and Kamen (80) believe that they are "the specific energy 

 donators which make possible reactions leading to protein and enzyme 

 synthesis." They found that new protein formation in yeast cells was 

 accompanied by a marked transfer of phosphate from the nucleoprotein. 

 However the phosphate groups in nucleoproteins are phosphate esters 

 which on hydrolysis provide small amount of energy. Northrop (64a) 

 has dealt extensively on this subject of the last stages of protein syn- 

 thesis. 



