344 



ANIMAL METABOLISM 



was formed in the animal by the transfer of methyl groups from choline 

 (reaction 39, Fig. 13-6) . ' 



COOH 



I 

 H2NCH 



I 



I 



CH2 



I 



SCH3 



L-Methionine 



+ (CHs)! (39) I- (CH3) 



COOH 



•I 

 H2NCH 



CH2 



I 

 CH2 



SH 



L-Homocysteine 

 I 



(CHa), 



N+ 

 I 

 CHj 



CHj 



I 



OH 



Choline 



A 

 (46) 



NH2 

 I 



CH2 



I 



OH 



Ethanolamine 



+ (CH3) 



COOH 



I 

 CHo 



I 



NH2 



Glycine 



V 



(CH3) 



- COa 

 (45) 



- H,0 



COOH 



(40) 



H,NCH 



COOH 



CH, H.NCH 



I I 



CHo— S— CH, 



L-Cystatliionine 



(41) 



COOH 



I 

 CO 



I 



CH2 



I 



CH3 



+ H,0, - NH3 

 (Ug** or Zn+*) 



COOH 



I 

 H2N— CH 



CH, 



I 



SH 



L-Cysteine 



(43) 



HCOOH 



Formic acid 



(44) 



COOH 



H2NCH 



I 

 CH2 



I 

 OH 



L-Serine 



- 2(H)^ 

 (42) 

 + 2(H) 



COOH COOH 



I I 



H.NCH H2NCH 



" I I 



CH,— S— S— CH2 



a-Ketobutyric acid L-Uysteine L-Cystine 



Fig. 13-6. Metabolic interrelationships of glycine, serine, methionine, 

 and cystine, and some methylation reactions in animal tissues. 



Without choline in the diet, homocysteine was unable to replace methio- 

 nine for rat growth. It was concluded that the animal was unable to 

 synthesize methyl groups needed for certain methylation reactions, but 

 could transfer them, by the process of transmethylation, from other 

 methylated substances, such as choline. Such substances are called 

 methyl donors and are said to contain labile methyl groups. An adequate 



