114 THE AMINO ACIDS 



free liver was carried through alanine was not present 

 in significant quantities. From experiments of this 

 nature it is evident that the metabolic processes con- 

 cerned in protein metabolism are intimately associated 

 with those of the intermediary metabolism of carbo- 

 hydrate, and further that at times at least protein may 

 serve for both sources of nitrogen and carbonaceous 

 material. Protein, therefore, should not be regarded 

 in the strict sense merely as a purveyor of the nitrogen 

 which is essential for life processes. It is much more 

 than that, as has been demonstrated. 



ANOMALIES OF AMINO ACID METABOLISM 

 Alkaptonuria 



In previous pages it has been pointed out that the 

 normal organism is capable of demolishing the benzene 

 ring as found in tyrosine and phenylalanine for under 

 normal conditions no evidence of these substances can 

 be found in the urine. For the successive steps as- 

 sumed to occur in this destruction see p. 105. How- 

 ever, there are certain individuals who apparently are 

 unable to break open the aromatic ring and in the urine 

 is found an intermediary decomposition product, homo- 

 gentisic acid. Urine containing homogentisic acid 

 exhibits a tendency to turn dark on exposure to the air 

 and may show a strong reducing action. This condi- 

 tion, known as alkaptonuria, is of rare occurrence and 

 may last through life without affecting the health of 

 the individual. It can scarcely be regarded as of 



