116 THE AMINO ACIDS 



tisic acid is a step in the degradations of these amino 

 acids has been doubted by Dakin who believes that 

 there is in these subjects an abnormal formation of 

 homogentisic acid as well as an inability to destroy it 

 once formed. In accord with this idea he has shown 

 how tyrosine and phenylalanine may be destroyed with- 

 out homogentisic acid as an intermediary product (see 

 p. 103). On the other hand, it has been demonstrated 

 by Abderhalden that normal individuals may eliminate 

 homogentisic acid when excessive quantities of tyrosine 

 are fed. Also the administration of tyrosine or phenyl- 

 alanine, or of foods rich in tyrosine, that is proteins, 

 causes a significant increase in the excretion of homo- 

 gentisic acid. If alkaptonurics live on a protein-free 

 diet for short periods of time the excretion of homo- 

 gentisic acid is markedly diminished, but does not dis- 

 appear entirely. Undoubtedly the aromatic amino 

 acids formed from tissue metabolism do not suffer 

 destruction to any greater extent than those introduced 

 into the blood from the food protein. Why an alkap- 

 tonuric individual fails to destroy the tyrosine radicle 

 is still a matter of conjecture. 



Cystinuria and Diaminuria 



Under ordinary circumstances cystine, the sulphur 

 bearing amino acid fails to appear in the excreta, prob- 

 ably undergoing extensive destruction. The sulphur 

 is oxidized to sulphuric acid and eliminated as a sul- 

 phate in the urine. In certain individuals, however, 

 cystine appears in the urine and owing to its relatively 



