CHAPTER V 

 THEORIES OF PROTEIN METABOLISM 



Under the term protein metabolism are included all 

 the processes in the animal organism concerned with 

 the fate of protein whether introduced as food or 

 serving as tissue substance. The metabolic changes 

 are divisible into two distinct phases known as anabo- 

 lism, or building up processes, and catabolism, or de- 

 structive processes. Although it is definitely recog- 

 nized that metabolic activity is manifested in two dia- 

 metrically opposed directions, the successive stages in 

 either process are but vaguely understood. Only the 

 starting point and the final end products in each in- 

 stance can be stated with certainty, although here and 

 there individual stages in the processes under discus- 

 sion point in one or another direction, and thus give 

 indication of the type of activity that must have gone 

 before. For the unravelling of the mystery the first 

 requisite is a clear conception of the problem. In the 

 words of Liebig : "If we take the letters of a sentence 

 which we wish to decipher, and place them in a line, 

 we advance not a step towards the discovery of their 

 meaning. To resolve an enigma, we must have a per- 

 fectly clear conception of the problem. There are 

 many ways to the highest pinnacle of a mountain ; but 



