CHAPTER 7 



Nitrogen Excretion 



ATS AND CARBOHYDRATES are oxidizcd to water and carbon dioxide, 

 which are readily excreted; proteins and nucleic acids contain 

 nitrogen, which is excreted in various compounds according to the 

 physiology of the organism. Nitrogenous wastes may come from proteins (by 

 way of amino acids), from the purine and pyrimidine bases of nucleic acids, 

 and from miscellaneous nitrogenous compounds. *"'• ■'•'■ ■" 



CHEMICAL NATURE OF NITROGENOUS PRODUCTS 



Protein Degradation Products. At least six different types of compound 

 may be produced by the degradation of protein. 



Amino Acids. Proteins are hydrolyzed to polypeptides and dipeptides, and 

 these are broken down to their constituent amino acids. Amino acids may be 

 excreted without change in some animals. 



Ammonia. Amino acids undergo deamination, resulting in the liberation of 

 ammonia as follows: 



NH. O 



R_C-H + Vz O. ^ R-C + NH, 



^COOH COOH 



NH. OH 



R_c4^ + H.O ^ R-C-H + NHa 



^COOH ^COOH 



Ammonia is toxic and is low in concentration in the blood of many animals. 

 It is formed rapidly in drawn blood of vertebrates, but the amount present in 

 vivo in mammals is probably not more than 0.001-0.003 mg./lOO ml. ^'' The 

 amount present is similar (less than 0.1 mg./lOO ml.) in frogs, reptiles and 

 hshes. •'*- If the blood ammonia reaches 5 mg./ 100 ml. in rabbits death 

 results. ^ Some ammonia is excreted by the kidneys (about 0.5T gm. daily in 

 man). ''^ In mammals most of the urinary ammonia is formed from glutamine 

 by the kidney; the amount which comes from amino acids is much smaller 

 than it is in animals in which ammonia is an important excretory product. "^ 

 There are invertebrate animals which are more tolerant of ammonia. Blood 



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