ANIMAL METABOLISM 



349 



anhydride, creatinine, through the intermediary formation of creatine 

 phosphate. 



(53a) / 



HN=C COOH ..L > HN=C, 



\ / 



N— CHj 



CH, 



Creatine 



+ H3PO4 



H 



N— PO3H2 



(53b) 



N— CH2 



CH3 



Creatine phosphate 



H 



N 



HN=C CO 



\ / 



N— CH2 



CH3 



Creatinine 



Tyrosine, itself produced from phenylalanine, is the starting point for the 

 biosynthesis of melanin, the dark-colored pigment of the hair and skin, 

 and for the hormones, adrenalin, nor adrenalin, and thyroxine (see Chap. 

 11). Melanin is produced by the enzymatic oxidation of tyrosine to an 

 unstable intermediate product which polymerizes (reacts with itself) to 

 form the high molecular weight pigment. According to Mason the most 

 probable course of the process is as follows: 



NH2 



CH2— C— COOH 

 I H 



HC^ XH 



(0) 



NH2 



CH2— C— COOH 

 I H 



HC^ XH 



HC 



^. 



.CH 



OH 



L-Tyrosine 



tyrosinase 



rearrangement 



HOC:^ XH 



X 



I 

 OH 



Dihydroxyphenyl 

 alanine, or "Dopa' 



