350 



ANIMAL METABOLISM 



0=C' 



H 



.C 



^ 



c 



o=c. 



c 



H 



,^ 



c. 



-CH2 



XHCOOH 



H 



2-Carboxy-2,3-dihydro- 

 5,6-dih5'droxyindoIe 



Hallochrome 



- CO2 



rearrangement 



HOC 



^ 



H 



C. 



H 



C- 



CH 



(O) 



HOCi::^ ^C^ ^CH 



c 



H 



H 



5,6-Dihydroxyindol('. 



tyrosinase 

 polymerization 



Melanin 



0=C' 

 0=C, 



■c 



H 





CH 



II 

 .CH 



H 



Indole-5,6-quinone 



Abnormal metabolism of tyrosine and phenylalanine is exhibited by 

 some people. In tlie condition known as alcaptonuria, tyrosine is con- 

 verted into homogentisic acid. This substance is excreted in the urine 



CHoCOOH 



CHoCOCOOH 



HC 



^^ 



^COH 



HOC 



^ 



.CH 



HC 



HC 



^ 



C, 



■^CH 



0' 

 H 



=^ 



C^ 

 H 



.CH 



Homogentisic acid 



Phenylpyruvic acid 



and causes it to turn black on standing exposed to air. In another con- 

 dition, phenylketonuria, large amounts of -phenylpyruvic acid are formed 

 from phenylalanine and are excreted in the urine. This apparently 

 results from an inability to convert phenylalanine to tyrosine. Both of 

 these conditions represent hereditary abnormalities of amino acid 

 metabolism. 



Certain amino acids, particularly glycine, have also been demonstrated 

 to be among the building blocks used by the animal in the biosynthesis 

 of purines and porphyrins. The various carbon and nitrogen atoms of 

 the purine skeleton come from the following sources: carbons 4 and 5, 



