230 Essays in Biochemistry- 



acid, once thought to come from NH 3 or amide nitrogen, is probably 

 derived from aspartic acid, and, by analogy, the same may prove to 

 be true of the amino group of cytidylic acid. 



The role of the nitrogen of aspartic and glutamic acids, and their 

 amides, has been difficult to interpret for a number of reasons, some 

 of which have become apparent during the investigation of arginine 

 synthesis. In contrast to their amides, or to NH 3 , these two amino 

 acids are relatively impermeable to liver cells and usually give the 

 misleading impression of not being precursors when they are investi- 

 gated in tissue slices or the intact animal. On the other hand, glutamic 

 and aspartic acids can be rapidly synthesized within the respiring cell 

 from NH 3 and their respective keto acids (supplied by the operation 

 of the citric acid cycle) through reactions 10 and 12. NH 3 thus masks 

 their participation in the donation of nitrogen. Difficulties in detecting 

 a precursor, or in determining the actual pathway which a nitrogen 

 atom has taken, also arise whenever N 15 -labeled NH 3 is compared to 

 glutamic and aspartic acids, or to their amide groups, because rapid 

 isotope equilibration among all the nitrogens takes place through the 

 same reversible reactions in conjunction with reversible amide forma- 

 tion from NH :? . Further experimental difficulties are introduced by 

 the dependence of the synthetic mechanisms on ATP. In respiring 

 systems, just as the citric acid cycle, by facilitating the formation of 

 glutamic and aspartic acids from NH 3 , tends to obscure the nitrogen 

 source, it can also obscure the energy source by forming ATP. 



Our present problems of nitrogen metabolism come to us from the 

 pioneer accomplishments of earlier decades, when the enormous syn- 

 thetic potentialities of intracellular metabolism were clearly recognized 

 by investigators who initiated the experimental approach to cellular 

 activity, using surviving tissue or isotopes and the intact animal. 

 Perhaps it will be said of this decade that we are exploring nitrogen 

 metabolism at a higher level of magnification, where the enzyme is both 

 subject and tool. 



References 



1. H. A. Krebs and K. Henseleit, Z. physiol. Chem., 210, 33 (1932). 



2. P. P. Cohen and M. Hayano, ./. Biol. Chem,., 166, 239, 251 (1946). 



3. P. P. Cohen and M. Hayano, J. Biol. Chem., 172, 405 (1948). 



4. S. Ratner, J. Biol. Chem., 170, 761 (1947). 



5. S. Ratner and A. Pappas, J. Biol. Chem., 179, 1183, 1199 (1949). 



6. S. Grisolia, S. B. Koritz, and P. P. Cohen, /. Biol. Chem., 191, 181 (1951). 



7. S. Grisolia and P. P. Cohen, ./. Biol. Chem., 191, 189 (1951). 



8. S. Grisolia and P. P. Cohen, ./. Biol. Chem., 19S, 561 (1952). 



