5o6 MOLECULAR MECHANISMS OF DIFFERENTIATION 5 



tiation lies in the possibility that the production of different proteins is, at least 

 in part, controlled by the selective activation of the different enzymes which cata- 

 lyze the utilization of the individual amino acids. The activation of these enzymes 

 again may depend on the total metabolic energy available, or upon the specific 

 metabolite which acts as the carrier of the energy supply. It is apparent that this 

 aspect of the problem of protein synthesis in embryonic cells links it to possible 

 changes in intermediary metabolism of embryonic tissues. 



A second approach to the problem of protein formation has emerged from 

 studies of the patterns of enzymatic reactions by which amino acids are linked to 

 give the specific sequence which is characteristic for each protein species. The 

 work in this field has been summarized recently in three lucid reviews which provide 

 a valuable basis for the following discussion (Borsook, 1956a, b; Fruton, 1955 and 

 Steinberg, Vaughan and Anfinsen, 1956). 



Following Fruton's line of thought, peptide chain formation is brought about 

 by a stepwise addition of individual amino acids to form peptides of gradually 

 increasing chain length. The energy for the initial steps in this process is derived 

 from ATP, the end product of metabolic oxidation, and is utilized for the prodvic- 

 tion of certain key substances such as glutamine. Without further energy being 

 required, the amide group of glutamine can be replaced by amino acids resulting 

 in the formation of a dipeptide. In subsequent transpeptidation, formation of 

 larger peptides is achieved. Intracellular proteinases such as cathepsin are supposed 

 to be the catalysts for these reactions. Although conclusive evidence for the biolog- 

 ical significance of the enzymatic reactions indicated in this scheme is still want- 

 ing it could give meaning to the appearance of glutamine and of high concen- 

 trations of peptides in embryonic tissues mentioned in the earlier part of this 

 section. To which extent the association of a high activity of proteinase and pep- 

 tidases in centers of active growth (Urbani, 1955), the appearance of the enzyme 

 glutamotransferase (Rudnick and Waelsch, 1955) in embryonic tissues, or 

 the rapid rate of amidation in sea urchin embryos (Hultin, 1953d) can be asso- 

 ciated with a scheme of protein synthesis similar to Fruton's remains to be ex- 

 plored. 



Other experiments designed to investigate the mechanism of protein synthesis 

 were based on the following considerations. If proteins are synthesized by the 

 simultaneous formation of peptide bonds between all of the amino acids necessary 

 for a particular protein molecule, addition of a labelled amino acid to the amino 

 acid pool of a cell should result in a uniform distribution of the label in the different 

 portions of this molecule. If, however, individual peptides are synthesized at 

 different rates and then put together to form a complete protein molecule the 

 proportions of labelled amino acids in the various portions of the protein molecule 

 would be expected to be different. Anfinsen and Steinberg (1951); Steinberg and 

 Anfinsen (1952); Peters (1953); Flavin and Anfinsen (1954) and Vaughan and 

 Anfinsen (1954) using in vitro systems of liver, chick oviduct and pancreas tissue, 

 isolated serum albumin, ovalbumin, ribonuclease and insulin from the tissue 

 preparations incubated in the presence of the labelled amino acid. The analysis 

 of these isolated proteins showed a highly unequal distribution of the labelled 

 amino acid in the various peptides obtained from these proteins. In contrast. 



