PROTEIN METABOLISM 89 



and a nitrogen free residue, serving as a ready source 

 of energy. 



On the other hand, after death of cells, the tissue 

 or organized protein, although also broken down into 

 two parts, finds a destiny unlike the products of food 

 protein disintegration. Tissue protein splits into a 

 nitrogen-containing and a nitrogen-free portion. Under 

 normal conditions the nitrogen-free part is trans- 

 formed to glycogen or fat which may be utilized for 

 purposes of energy. The portion containing nitrogen 

 is not broken down at once into urea but it leads to 

 the formation of a variety of substances which play 

 important roles in metabolism but are finally excreted 

 as urea. In the decomposition of tissue protein Speck 

 assigned to oxygen deficiency an exceedingly important 

 part. 



RUBNER 



In Rubner's theory of protein metabolism it is main- 

 tained that a study of metabolism cannot be considered 

 separately from the study of heat production. Ac- 

 cording to Rubner, therefore, metabolism must be 

 studied in connection with the exchange of energy. 

 In all of the metabolic changes undergone by protein 

 in the body reference is made to the accompanying 

 production of heat. Rubner believes in a "store" pro- 

 tein which may be compared to Voit's "circulating" 

 protein, and in a "wear and tear quota" necessary for 

 the repair of tissue waste. He assumes that most of 

 the protein after absorption is rapidly split into two 



