(7) 



in structure and in enzymatic detachment of fumaric acid suggest that 

 a prior step occurs, similar to reaction 3, in which adenylosuccinic acid 

 is formed by condensation of inosinic acid with aspartic acid. 



The mechanisms of group synthesis or transfer discussed in the 

 preceding sections all function in the formation of compounds which 

 possess the ureicle, guanidine, or amidine configuration (cf. scheme 1). 

 It is interesting to find that the chemical lability associated with these 

 groupings has a counterpart in their manifold metabolic activities and 

 in what might be called their metabolic lability. 



Ammonia and Aspartic Acid 



When nitrogen excretion was intensively examined as a problem of 

 evolutionary development and survival, urea formation came to be 

 seen as a means of converting NH 3 into a form which is relatively 

 innocuous and easily excreted, well suited to the physiological limita- 

 tions of many terrestrial forms of life. Our attention was drawn to 

 this point of view before knowledge of nitrogen metabolism had been 

 extended to its present scope and complexity. Most of the nitrogen 

 is, of course, derived from amino acids, and it had been supposed that 

 the amino acids were directly broken down with the liberation of NH 3 . 



