816 NUTRITION AND HEAT REGULATION. 



synthesis of its non-nitrogenous storage fuel. Fclin's estimations, 

 quoted above, showing that the urea of the urine rises and falls 

 promptly with the amounts of protein in the food, indicate that a 

 large part of the protein food undergoes this series of changes. 

 Since some of the nitrogenous food material must be actually 

 synthesized into living protoplasm, acting as a " tissue protein" in 

 Voit's sense, to make new tissue or to repair tissue waste, it follows 

 that a portion of the absorbed split products of digested protein 

 must undergo an entirely different process, but concerning the 

 intermediary steps of this process we have little or no knowledge. 

 From a quantitative standpoint it is probable that most of the 

 protein food in an average diet follows the history first described 

 and takes no part in the formation of tissue. 2. The urea of the 

 excretion may arise in the tissues at large from the breaking down 

 of the organized or living protein or of the protein material included 

 in the cell liquids. As has been stated in another v place most of 

 the living tissues contain intracellular enzymes capable of causing 

 hydrolysis of the proteins. When the excised tissues are kept warm 

 but protected from the action of the bacteria they undergo self 

 digestion or autolysis. As a result of this process the protein 

 molecule is split into its constituent parts, which in general are the 

 same as those formed during digestion. Assuming that these 

 intracellular enzymes act in this way during life it is evident that 

 the fate of the split products may be the same as that described 

 above. The ammonia compounds are converted to urea by the 

 liver and quite probably by other tissues; the monamino-bodies 

 lose their NH 2 group by a process of deamidization and the am- 

 monia compounds thus produced are in turn changed to urea, and 

 such compounds as arginin will probably under the influence of 

 arginase yield their nitrogen as urea. In the protein that undergoes 

 metabolism in the tissues, as well as in the protein of the food that 

 is metabolized before reaching the tissues at large, the urea is formed 

 by two general methods at least, by the intermediate production 

 of ammonia compounds and by the conversion of a guanidin residue. 

 For the sake of completeness a third method of producing urea in 

 the body may be added, namely, by being split off from uric acid. 

 This mode of origin is considered in the following paragraph. 



The Intermediary Metabolism of the Nucleoproteins. Nucleo- 

 proteins are taken in our food and likewise are found in the tissues 

 of the body. Their metabolism so far as the nuclein portion is 

 concerned gives rise to the formation of uric acid and the purin 

 bases. It will be remembered that in the urine we find some uric 

 acid and some xanthin and hypoxanthin. In the feces small 

 amounts may be detected of xanthin, hypoxanthin, adenin, and 

 guanin. These bodies all contain the purin group or radicle (see 



