Chemical Changes in Animal Organism. 313 



It is not possible in this place to give the chemical reasons 

 for assuming that these stages are passed through in the 

 transition from tyrosine to homogentisic acid. Most of 

 the intermediate products have, however, been prepared, 

 and it has been found that these, when superadded to a 

 given diet of an alkaptonuric subject, cause an increased 

 excretion of homogentisic acid in the urine. On the other 

 hand, substances like o- or m-tyrosine (that is, substances 

 with the hydroxyl group in the o- and m-position to the 

 CH 2 -CH(NH 2 )-COOH group instead of the p-position) 

 do not lead to an increased homogentisic acid output. It 

 has been supposed by Garrod and others that homogentisic 

 acid is a normal intermediary product of phenylalanine 

 and tyrosine catabolism, which in most individuals is 

 further oxidized in the body ; in the case of alkaptonuric 

 subjects, on the other hand, the capacity of degrading 

 homogentisic acid any further is wanting. If this is the 

 case, it can be readily understood that the alkaptonuric 

 individual affords a valuable subject for studying the inter- 

 mediary metabolism of aromatic substances derived from 

 proteins by hydrolysis. 



Experiments have been carried out on diabetic 

 subjects with the object of ascertaining the metabolic 



