II UTILIZATION OF PROTEIN PRECURSORS 499 



Kavanau (1953 and 1954). The results of these analyses were interpreted as in- 

 dicating periodic phases of yolk degradation followed by new synthesis of embry- 

 onic proteins. The accumulation of non-protein amino acids, following the de- 

 crease in yolk protein content, was assumed to indicate that the yolk proteins are 

 first degraded into amino acids and peptides which serve as the precursors for new 

 synthesis of embryonic proteins. 



The utilization of amino acid storage depots has been approached by widely 

 different lines of investigation. The complex structure of yolk granules and the 

 changes occurring during the degradation of this material have been recorded in the 

 electron microscope by Bellairs ( 1 958) . An extensive study of the changes in the yolk 

 proteins of the frog egg and stimulating suggestions concerning the regulation of 

 their breakdown by a phosphorylation mechanism have been published by Barth 

 and Barth (1954) and the relation of phosphatase activity to yolk platelet utiliza- 

 tion was also investigated by Flickinger (1956). Studies of the amino acid content 

 of chick embryos (Williams, DaCosta, Newman and Marshall, 1954) and of the 

 disappearance of the proteins from the yolk and the egg white (Rupe and Farmer, 

 1955) have been initiated. 



Other interesting approaches to the problem of amino acid utilization in embryos 

 are the explorations of genetic control of the free amino acid content in Drosophila 

 and Ephestia (Hadorn, 1955; Chen, 1956b; and Chen and Kiihn, 1956) and 

 experiments designed to elucidate in chick embryos the relationship of hormonal 

 control (cortisone) of the synthesis of specific proteins (collagen) to the blood level 

 of one of the specific amino acid components (hydroxyproline) of collagen (Ro- 

 berts, Karnofsky and Frankel, 1951). 



As an alternative method, amino acid utilization in protein formation can be 

 measured by adding labelled amino acids to the amino acid pool and following 

 the incorporation of the label into the proteins. The advantage of this method is 

 the relative directness with which it indicates simultaneously both the rates of 

 protein formation and amino acid utilization. 



Using labelled alanine and glycine Hultin (1952) found, working with sea 

 urchin embryos, that the incorporation of these amino acids into a protein fraction 

 insoluble in isotonic KCl increases rapidly after fertilization and reaches a 

 maximum rate at about 12 hours of development (end of the blastula stage). 

 During the first eight hours of development the KCl soluble fraction exhibits a 

 much slower rate of incorporation than that of the insoluble one. After this period 

 a sharp increase in the rate of incorporation occurs in the soluble fraction up 

 to about the 12th hour of development, resuming thereafter the original slow 

 rate of incorporation, at least, until the 30th hour of development. This dis- 

 crepancy in the incorporation rates of the two fractions is attributed to the rapid 

 development of cellular particulate matter i.e. microsomes, mitochondria (insol- 

 uble fraction) beginning at fertilization and continuing during the first stages of 

 ontogenesis. Later, a temporary period of more rapid formation of soluble enzymes 

 ensues. 



Utilization of small molecular precursors in the formation of proteins in am- 

 phibian embryos could be demonstrated indirectly in experiments with '"^CO, 

 which penetrates rapidly into the embryo and is incorporated first into amino 



Literature p. S39 



