II UTILIZATION OF PROTEIN PRECURSORS 5O5 



than in amphibian embryos. Similar to the results obtained in amphibians, amino 

 acid analogs diminished the incorporation of methionine-^^S. 



Effects of amino acid analogs on sea urchin development have also been reported 

 in a preliminary note (Gustafson and Horstadius, 1955). The results of the reported 

 experiments show that the amino acid analogs utilized (ethionine, thienylalanine, 

 phenyllactic acid, allylglycine, methionine sulfoxide and d-leucine and d-nor- 

 leucine) have a "vegetalizing" effect. From the reported data it is uncertain 

 whether the observed effects are actually due to interference with amino acid 

 utilization in protein formation. This can only be demonstrated by a reversal 

 of the abnormalities by addition of the corresponding normal amino acids, and 

 bv measurements of the incorporation rates of labelled amino acids with and 

 without amino acid analogs. Actvially "non-metabolic" effects observed with 

 other groups of apparent anti-metabolites were suggested earlier by Horstadius 

 and Gustafson (1954). Assuming that the anomalies are actually due to an 

 impaired amino acid utilization, the appearance of a "vegetalizing" effect 

 might indicate that the formation of specialized proteins is suppressed. This 

 suggestion is based on the fact that other "vegetalizing" agents, like lithivim, 

 prevent the production of enzymatically and serologically specific proteins which 

 appear in the course of sea urchin development. 



(c) Pathways of precursor utilization 



Assays of the changing quantities of amino acids occurring in the different 

 tissues of the developing embryo, determinations of the incorporation rates of 

 tracer amino acids, or the observations of the utilization of large molecular protein 

 precursors into embryonic cells, become meaningful only when such studies are 

 directed toward an understanding of the reactions by which these precursors are 

 transformed into the proteins of the embryo. Up to the present time the two main 

 lines of research dealing with the pathways of amino acid utilization in differenti- 

 ated tissues have hardly been considered in work with embryonic cells. 



One of these approaches comprises investigations of the mechanism by which 

 amino acids are activated preliminary to the formation of peptide bonds, the 

 primary reaction in protein synthesis. This work is of great importance because 

 it points to the mechanism by which metabolic energy is utilized in protein 

 synthesis. 



Stimulated by earlier ideas of Lipmann (1954), the result of the work of several 

 investigators (Hoagland, 1955 and Hoagland, Keller and Zamecnik, 1956) led 

 to the recognition that peptide bond synthesis involved the activation of amino 

 acid by a reaction involving ATP leaving the amino acid bound to an adenosine- 

 monophosphate radical. Specific enzymes catalyze this reaction for each individual 

 amino acid tested so far. Presumably in this intermediary form the amino acid 

 becomes attached to the site where peptide bond formation is catalyzed. Obser- 

 vation of a transient adsorption of free amino acids may be related to this step in 

 amino acid utilization (Britten, Roberts, French, 1955). 



Nothing is known, as yet, about the activating mechanism of amino acids in 

 embryonic tissues during proliferation and differentiation. The importance of the 

 recognition of this mechanism for the problem of protein synthesis during differen- 



Literalure f). 535 



