II UTILIZATION OF PROTEIN PRECURSORS 5OI 



that the free glycine equilibrates rapidly (within one hour) with the external 

 medium while the proteins lose their label only at a very slow rate, if at all. The 

 protein fraction does not lose statistically significant amounts of the label during a 

 six-hour incubation period. This shows that there is no immediate extensive 

 protein breakdown of the proteins of the explant. With the establishment of 

 conditions for rapid and slow growth of the explants (Britt and Herrmann, 1959), 

 possibilities for further explorations of the relation of tracer turnover to protein 

 accumulation seem indicated. 



Measurements of the uptake of tracer amino acids into isolated proteins have 

 been carried out so far in chick embryos only at a more advanced stage of develop- 

 ment (Herrmann, Lerman and White, 1958). In embryos at 14 days of incubation 

 the proteins collagen and actomyosin could be isolated from the leg musculature 

 at varying times after injection of glycine- i-'^^C directly into the embryonic 

 circulation. The amounts of total and of tracer glycine were determined in the 

 blood, in the free amino acid pool of the cells, and in the two proteins. Unex- 

 pectedly, it was found that the incorporation curves for the two proteins seemed 

 to differ qualitatively and quantitatively although the net accumulation of the 

 two proteins was found to be of the same order of magnitude. In attempting to 

 explain this discrepancy, it was suggested that in the formation of collagen and 

 myosin, the tracer amino acid is first rapidly incorporated into the proteins at the 

 site of their initial synthesis (microsomes). After the protein molecules are formed 

 and deposited as part of the extra- or intracellular structures, their rate of equilibe- 

 ration with the amino acid pool decreases markedly and may become neglibly 

 small. In order to account for the differences in the incorporation curves for the 

 two proteins, it seemed possible to assume that in case of the collagen determi- 

 nation, the active precursor was lost in the course of our preparation ; while in 

 case of the actomyosin fraction, the active portion of the incorporation portion 

 of this protein fraction was included in ovir preparation. Continuation of such 

 analyses seemed to open possibilities for a further study of the different phases of 

 protein formation and of the relation of tracer incorporation and net protein 

 accumvdation in growing tissues. 



In the preceding paragraphs data have been summarized dealing with the 

 utilization of amino acids and of larger building blocks (peptides, proteins) in the 

 formation of the cellular proteins of the embryo. The main line of evidence for 

 the utilization of large molecular precursors was the selectivity with which such 

 precursors are taken up by homologous organs. The mere presence of high con- 

 centrations of peptides in cell extracts suggests, possibly, that they play a more 

 prominent role in protein formation in the embryo than in adult tissues. The 

 utilization of amino acids was demonstrated by qualitative and quantitative varia- 

 tions of the amino acid pool and by the incorporation of labelled amino acids 

 into the proteins of various embryos. A sharper, qualitative and quantitative, 

 definition of the protein precursors used at various stages of development is, 

 in fact, of considerable interest. Utilization of larger precursors will give the formed 

 proteins a certain structural relationship to such precursors and the energy require- 

 ments for the formation of proteins from peptides are probably considerably 

 lower than for the synthesis de novo from individual amino acids. 



Literature p. S39 



