METABOLISM 201 



broken down to amino acids in digestion. Since proteins contain 

 widely varying proportions of the different amino acids, and a 

 given protein often lacks one or more of these compounds, it is 

 not surprising that different proteins vary greatly in their 

 usefulness and adequacy in the diet. The amount of protein 

 required thus is dependent on the kind of protein. If the body 

 requires a definite amount of a particular amino acid, that amino 

 acid must be present in the food protein unless the body is 

 capable of constructing it in its own workshop from other ma- 

 terial such as ammonia and residues of other amino acids, of 

 carbohydrates or other substances. Most interesting results have 

 been obtained in this field. 



Gelatine lacks tyrosine and tryptophane. Although gelatine 

 is a protein, if an otherwise adequate diet is fed containing gela- 

 tine as the sole protein constituent, the animal will die as surely 

 as if he were receiving no protein. Addition of these missing 

 amino acids to the diet, either directly or by adding a protein 

 which contains them will remedy the difficulty, and make the 

 diet adequate. Osborne and Mendel, and McCollum have con- 

 tributed greatly to our knowledge in this field. Zein, a protein 

 from corn, contains no tryptophane or lysine. On an otherwise 

 adequate diet containing zein as its sole protein, a young animal 

 declines and dies. On adding tryptophane to such a diet, the 

 animal no longer loses weight. It maintains about the same body 

 weight. Apparently tryptophane is necessary for body main- 

 tenance. But on such a diet the animal does not grow. If lysine 

 be added along with the tryptophane, the animal now grows 

 almost at a normal rate. Lysine thus appears to be necessary for 

 body growth. From the above discussion it is evident that the 

 body cannot synthesize tyrosine, tryptophane, or lysine, at least 

 in amounts sufficient for its needs and this is also true of cystine. 

 Some of the amino acids apparently can be built up by the body. 

 Thus after feeding benzoic acid, glycocoll will be excreted in 

 hippuric acid in quantities too large to be accounted for by the 

 available glycocoll in the organism. Probably some other ainino 

 acids, among them proline, also can be built up by the body. 



