138 THE AMINO ACIDS 



utilization. Protein products may function in other 

 ways than in the repair of tissues or in supplying 

 energy. It is highly probable that the organism uses 

 them, in part, for more specific and more immediate 

 needs. The discovery of substances absolutely essen- 

 tial to life, highly specific, and elaborated in special 

 organs, suggests that some part, at least, of the pro- 

 tein products set free in the gut may be used directly 

 by these organs as precursors of such specific sub- 

 stances. In adrenaline, for instance, we have an aro- 

 matic substance absolutely essential for the mainten- 

 ance of life, and it is probable that the suprarenal 

 gland requires a constant supply of some one of the 

 aromatic groups of the protein molecule to serve as 

 an indispensable basis for the elaboration of adrena- 

 line. If this be so, it is certain that the gland itself 

 could not, in starving animals, supply sufficient of such 

 a precursor to outlast the observed survival periods. 

 Since adrenaline must be produced at all costs, the 

 required precursor must, in starvation, be obtained by 

 tissue breakdown outside the gland. We may be sure, 

 however, that adrenaline is far from being the only 

 substance elaborated to which such considerations 

 apply. Similarly, in an animal upon a diet sufficient 

 to supply energy, but lacking in some essential group, 

 the minimal waste in the general tissues of the body 

 will be determined by the special need of the organs for 

 the missing group. On this basis we have a hypoth- 

 esis to account for the special protein-sparing prop- 

 erties of gelatin. It shares with protein certain 



