NUTRITION 149 



a-amino acids contain the group CH(NH 2 )COOH, one 

 enzyme ought to be able to act upon them all with equal 

 ease. 



The relative value of proteins for maintaining nitrogenous 

 equilibrium in man are shown in the following table.* The 

 Biological value is probably related to the kinds of amino 

 acids in the protein. 



TABLE XXIX 

 Biological Value of Various Proteins. 



Relative value to 

 Source of protein meat proteins 



(Biological Value) 



Meat i4'74 



Milk 99-71 



Rice 88-32 



Potatoes J . . . . 78-89 



Beans 55-73 



Wheat Meal 39-56 



Maize 29-52 



The main nitrogenous end product, urea, is formed from 

 ammonium carbonate. If, however, an excessive amount of 

 acid is produced or administered the ammonia neutralises 

 the acid and is excreted as ammonium salts. This is one of 

 the ways in which neutrality is maintained in the body, and 

 the amount of ammonia in the urine is an indication of the 

 amount of acid to be neutralised. 



Another example of neutrality regulation is mentioned by 

 Dakin, that in the presence of acid the formation of lactic 

 acid from pyruvic aldehyde by glyoxalase is inhibited. As 

 mentioned on p. 112, both these processes can be explained 

 by Le Chatelier's Theorem. f 



The metabolism of carbohydrates and fats gives rise to 

 carbon dioxide and water. The intermediate stages are 

 discussed in Chapter VII. 



SULPHUR METABOLISM 



Sulphur metabolism presents certain features of interest. 

 Sulphur is excreted mainly as sulphates. These are produced 

 either from sulphates in the food or by oxidation of sulphur- 

 containing substances such as the amino acid cystine. The 

 taurine of the bile acid, taurocholic acid, is formed from 

 cystein by oxidation of the sulphur and removal of the 

 carboxyl group. 



* Abbreviated from K. Thomas, Arch. f. Physiol., 1909, p. 219. 

 f H, E. Roaf, Journ. Physiol., 1921, vol. 54 proc., p. Ixiv. 



