502 MOLECULAR MECHANISMS OF DIFFERENTIATION 5 



Synthesis of proteins which are highly specific cell products (e.g collagen, lens 

 proteins) presupposes in many instances an alignment of amino acids in a rather 

 unique seqvience. Such proteins would have to be built up from either small 

 peptides or free amino acids. On the other hand, during early embr oogenesis, 

 some high molecular precursors could possibly be utilized for energetically 

 "inexpensive" protein formation in the rapidly proliferating cells. Also, in some 

 species no net synthesis of proteins occurs during long periods of development 

 (sea urchin, amphibian). In these instances conversion of yolk proteins into 

 different cytoplasmic protein must take place and the possibility of utilization of 

 higher protein precursors for the formation of some cell proteins would seem 

 advantageous for rapid protein formation. 



The investigation of the types of precursors which are utilized in protein 

 synthesis is greatly aided by the use of amino acid analogs. These compounds inter- 

 fere primarily with the utilization of amino acids in protein formation and are 

 supposed to be indifferent with respect to protein formation from larger precursors 

 and lead, therefore, to the distinction of these two types of protein precursors. The 

 usefulness of amino acid analogs has been demonstrated in the field of micro- 

 biology. Some of the experiments which are of particular relevance in the context 

 of this discussion have been carried out by Halvorson and Spiegelman (1952) and 

 have been reviewed by Spiegelman, Halvorson and Ben-Ishai (1955). These 

 authors found a considerable amino acid pool in yeast cells. Just as in the case 

 of the sea urchin embryo, the amino acid content of yeasts proved to be a sensitive 

 index of the state of protein forming activity of the cell. Upon the stimulation of 

 growth as well as during induced enzyme synthesis, the quantity of this amino acid 

 "pool" diminished. Neither overall protein formation (growth), nor formation 

 of specific proteins (induced enzymes), nor a depletion of the amino acid pool 

 would take place in the presence of amino acid analogs (Halvorson and 

 Spiegelman, 1952). From this analysis it can be concluded that growth and the 

 formation of induced enzymes are dependent upon a supply of free amino acids. 



Amino acid analogs were introduced into the field of morphogenesis by Lehmann, 

 Bernhard, Hadorn and Liischer (1945) and by Erlenmeyer and Lehmann (1949). 

 In testing the effects of aminoketones as analogs of leucine it was found that these 

 substances inhibit the regeneration of the tail of amphibians. While the mitotic 

 activity and the histogenetic phase of the regenerative process were found to be 

 undisturbed, the formation of the blastema itself was impaired presumably by an 

 inhibition of cell migration (Erlenmeyer and Lehmann, 1949; Lehmann and 

 Dettelbach, 1952; Lehmann and Bretscher, 1952; Lehmann, 1954a, b; Lehmann, 

 Weber, Aebi, Baumler and Erlenmeyer, 1954). Further analysis of this effect 

 suggested a possible correlation of this inhibition with an increased proteolytic 

 activity (Lehmann and Lovtrup, 1956). Such a correlation seems to be corrobo- 

 rated by our experiments on the chick embryo. 



Testing the effects of amino acid analogs on explanted chick embryos it was 

 found that different analogs of the same amino acid gave similar abnormalities; 

 but, analogs of different amino acids gave characteristically different effects 

 (Herrmann, 1953b; Rothfels, 1954; Herrmann, Konigsberg, and Curry, 1955). 

 With ethionine, the analog of methionine, no abnormal morphogenesis could be 



