312 Biological Chemistry. 



iron salts. This is due to the presence of homogentisic 

 acid, the amount of which can be readily determined. It 

 has been found that the amount excreted can be increased 

 by the addition of phenylalanine or tyrosine to the diet. 

 These substances appear, therefore, to be the products to 

 which homogentisic acid owes its origin. The chemical 

 relationships of these three substances are indicated in the 

 following formulae : 



COOH 



OH 



Tyrosine. Ho^ntisic p heny]alanine< 



It will be noticed that homogentisic acid contains two 

 hydroxyl groups in the o- and m-positions to the 

 CH 2 COOH group. There is no difficulty in con- 

 ceiving the origin of this group from the groups 

 CH 2 -CH(NH 2 )-COOH of alanine or tyrosine, but in the 

 latter substance the hydroxyl group is in the ^-position. 

 Homogentisic acid can only be derived from the latter by 

 a change in the position of the hydroxyl group in the 

 benzene ring. There are reasons for supposing that tyro- 

 sine passes through the following stages before conversion 

 into homogentisic acid ; * 



