728 NITROGEN METABOLISM AND GROWTH 9 



postgastrular period of development far overshadows the preceding stages. It is 

 here, therefore, that we may expect to find more satisfying answers to a variety 

 of questions. These may be outhned as follows: 



1. Protein synthesis 



a. Chemical limitations 



b. Cytological mechanisms 



c. Relationship with yolk storage 



2. Grow^th Regulation 



a. Differential growth 



b. Cessation of growth 



c. Genetic control 



3. Energy Sources in Development 



a. General 



b. Evaluation of Needham's thesis 



c. The Cleidoic Vertebrate Egg 



I. Protein Synthesis 



a. Chemical litnitations. It is appropriate that we rely heavily on a mature and 

 conservative assessment of the present status of this question by a biochemist 

 keenly interested in the problem. Fruton (1955) summarizes work on two fronts: 

 chemical, largely by his own group, and biological. Working with cathepsins of 

 animal material (Cathepsins A, B and C from beef spleen) and plant material 

 (papain and ficin), Fruton has characterized some of the peculiarities of synthetic, 

 transpeptidation reactions catalyzed by these enzymes. Important in the replace- 

 ment reactions are the structure of both replacement reagent and the compound 

 containing the sensitive CO-NH bond, and the fact that the replacement reagent 

 (amine) must be in uncharged form. Because transamidation reactions occur 

 preferentially near pH 7, Fruton suggests that "catalysis of replacement reactions 

 may represent a major physiological role of these enzymes". In experiments with 

 Cathepsin C from beef spleen, he was able, under carefully controlled conditions 

 to recover a hexapeptide, octapeptide and decapeptide, the first of which appeared 

 to be homogeneous, and the two latter mixtures of polypeptides of varying length. 

 Fruton has assembled information describing energy requirements in peptide 

 bond syntheses and cites evidence for relating ATP in a variety of reactions : 

 bacterial synthesis of CO-NH bond (Lipman, 1954); glutathione (Snoke et at., 

 1953)5 glutamine (Elliott, 1953); incorporation of isotopic amino acids into 

 proteins of tissue preparations (Siekewitz, 1952). He suggests that energy yielding 

 processes of the cell may be coupled with synthesis of peptide bonds of proteins 

 through a relatively small number of amides or peptides such as glutamine and 

 glutathione. Such hypotheses were advanced independently by Hanes et al. (1952) 

 and Waelsch (1952). Rudnick, Mela and Waelsch (1954) correlate protein 

 synthesis with glutamotransferase in the chick embryo. 



