IV BIOSYNTHESIS OF AMINO ACIDS 55 



between the above mentioned amino acids. This method of establishing biochem- 

 ical relationships is called the "Isotope Competition" method, and will be 

 referred to frequently below. While the interpretation of isotope competition 

 experiments may sometimes be rather complicated, the studies are a welcome 

 supplement to those based on an analysis of biochemical mutants, conventional 

 isotope studies, or enzyme studies. 



Some of the metabolic relationships among the amino acids are illustrated in Fig. i. 

 It will be observed that six groups may be distinguished: 



(a) Histidine. The carbon of histidine is probably derived from a pentose derivative. 



{b) Serine family. When yeast cells were incubated with labelled serine, the protein 

 serine, glycine, and cysteine, had the same specific activity. However, tracer glycine 

 experiments with Torulopsis utilis, showed that cysteine and serine had only 15% of the 

 specific activity of protein glycine. (Abelson and Vogel, 1955). In E. coli, exogenous 

 threonine contributed to the serine family and serine contributed to the pyruvate family 

 (Abelson, 1954). 



{c) Aromatic family. A relationship exists between the aromatic amino acids, phenylala- 

 nine, tyrosine, and tryptophane, and the vitamin, /)-aminobenzoic acid. Much of the 

 carbon of these amino acids is derived from the phosphoenolpyruvate and erythrose 

 phosphate. 



[d) Pyruvate family. In T. utilis, labelled alanine is incorporated into protein bound 

 alanine, valine, and leucine. Non-labelled pyruvate reduces the incorporation of glucose- '■'C 

 into the above amino acids (Abelson, 1954). 



{e) Aspartic fainily. In yeast cells, labelled aspartate is incorporated into a number of 

 protein bound amino acids. On the basis of relative specific activity, these amino acids 

 fall into two groups. Aspartate, threonine, isoleucine, and methionine have approximately 

 the same specific activity. Glutamic, arginine, proline, and lysine have less than half the 

 specific activity of the former amino acids. 



(/) Glutamic family. Glutamate-i-'''C is incorporated into the following protein bound 

 amino acids of yeast cells: glutamic, proline, arginine. a-Ketoglutarate reduced the 

 incorporation of labelled glucose into these amino acids and into amino acids of the 

 aspartic family. Glutamic semialdehyde depressed the incorporation of glucose- '■*C into 

 glutamate and arginine of yeast cell protein. 



J. Essential amino acids 



Although many microorganisms and plants synthesize all of the amino acids 

 which are commonly found in protein, mutant strains are known which are in- 

 capable of synthesizing particular amino acids. An exogenous source of these amino 

 acids must be available if growth is to occur. Likewise, animal organisms manifest 

 nutritional requirements for particular amino acids, due to their inability to syn- 

 thesize these amino acids from the materials ordinarily available at a speed com- 

 mensurate with the demands for normal growth. The amino acids, histidine, iso- 

 leucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophane, and 

 valine must be supplied in the diet to promote the growth of the young mouse. 

 Arginine, as well as the former nine amino acids, is essential for the young rat, 

 while in the chick, glycine supplements are required along with the ten amino 

 acids required by the rat. 



In adult animals, the nutritional requirements of the animal may be investi- 

 gated by determining which amino acids are necessary for the maintenance of 

 nitrogen equilibrium. In man, eight amino acids are essential for nitrogen equilib- 



Literature p. 124 



