METABOLISM 205 



rimeter, the products are carbon dioxide, water and nitrogen; 

 and the theoretical fuel value or calorific value of proteins 

 is in accordance with such a reaction. In the animal body, 

 however, the reaction does not take place in just this way. 

 The products formed here are carbon dioxide, water and complex 

 nitrogen compounds such as urea, uric acid and the purine 

 bases. The carbon dioxide and water are eliminated in the 

 respiration as in the case of carbohydrates and fat. 



Nitrogen Excretion in Urine. — The nitrogen-containing 

 compounds are all eliminated through the kindeys in the urine. 

 It is important to emphasize the fact that the urine is thus a 

 true excretion substance, carrying out from the body the end 

 products of metabolism, exactly analogous to the exhaled 

 breath and not like the feces, which are almost entirely a waste 

 product of undigested food material. The urine contains all 

 of the nitrogen of the katabolized protein and about go per cent 

 of this nitrogen is in the form of urea, only the other 10 per cent 

 being in the form of uric acid and the other nitrogen compounds 

 of urine. Thus a study of the urine nitrogen is the means we 

 have of studying the nitrogen portion of katabolized protein. 

 Also the total nitrogen of the urine is a measure of the total katab- 

 olism of protein and approximately every 16 grams of urine nitro- 

 gen represents 100 grams of protein that have been katabolized. 

 This explains the importance of nitrogen analysis of urine. 



Formation of Urea, etc. — Let us now examine in more detail 

 the way in which the protein nitrogen is converted into urea. 

 When protein is oxidized under the conditions present in the 

 muscle cells, or perhaps by a preceding hydrolysis of the protein 

 molecule, the nitrogen splits off in the form of ammonia. This 

 is readily understood when we remember that proteins are poly- 

 peptide compounds of amino-acids. Illustrating by a simple 

 amino-acid the hydrolysis occurring we have: 



H2N 

 H 



-CH2-COOH 



+ -> NH3 + CH2OH - COOH 



-OH 



