ANIMAL METABOLISM 



343 



in the fact that, in conjunction with transamination, it enables the body- 

 to convert one amino acid into another. 



Transamination. Two enzymes have been found in animal tissues 

 which catalyze the transfer of an amino group from an amino acid to a 

 keto acid. Each of these transamination reactions requires glutamic 

 acid as the amino group donor or a-ketoglutaric acid as the acceptor, 

 and pyridoxal phosphate as a coenzyme: 



COOH 



COOH 



COOH 



COOH 



(oxalacetic 



(37) 



transaminase) 



(pyruvic 



(3S) 



transaminase) 



The presence of these highly active transaminases in nearly all animal 

 tissues suggests that transamination is a major metabolic reaction. Two 

 additional amino acids, aspartic acid and alanine, are thus obtained from 

 sugar metabolism intermediates. There are indications that other amino 

 acids can take part in transamination also, but the importance of the 

 reaction in these cases is doubtful as far as normal metabolism is 

 concerned. 



Transmethylation. Many of the organic substances present in the 

 tissues of higher animals contain methyl groups attached to nitrogen or 

 to sulfur (examples: creatine, choline, methionine). Other substances 

 (for example, pyridine), not normally present, when fed to animals are 

 converted into methylated derivatives and are excreted in that form. 

 A clearer understanding of this process of methylation was obtained by 

 du Vigneaud from a study of methionine in relation to rat growth. He 

 found that this essential amino acid could be replaced by choline plus 

 homocysteine and proved, by using deuterium as a tracer, that methionine 



