468 ANIMAL BIOCHEMISTRY 



discussed in connection with folic acid. Intestinal bacteria convert 

 tryptophan to skatole, a component of intestinal gases. Plants trans- 

 form this same amino acid into indoleacetic acid, which serves as 

 a growth regulator (page 281). Animals use tryptophan to make 

 serotonin, which is a powerful vasoconstrictor and by its effect on 

 blood vessels must help control blood pressure. Another pathway 

 not indicated here leads from tryptophan to nicotinic acid and the 

 important vitamin nicotinamide in those species synthesizing this 

 vitamin. 



The relationships between glycine, serine, threonine, and the sulfur 

 amino acids are summarized in Figure 20-3. Glycine, together with 

 other intermediates, is used by animal cells in making proteins, 

 nucleic acids, purines and their nucleotides, porphyrins, and bile 

 pigments. In addition, glycine and serine undergo transamination 

 and degradation according to the mechanism shown on page 466. 



This same reaction system also reveals the mechanism of the inter- 

 conversion of glycine and serine, where the (— CH2OH) of Figure 

 20-3 represents the tetrahydrofolic acid derivative shown above. The 

 methyl groups required at other steps in the figure are shown as 

 ( — CH3) and may come from this same transfer intermediate or 

 methionine. The details of the transfer mechanism are still in doubt. 

 However, where more than one methyl group is indicated the inter- 

 mediate stages are known but omitted here to simplify the diagram. 

 Ultimately all necessary methyl groups come from ingested methionine 

 and serine or from glycine by way of glyoxalate. 



Choline and ethanolamine are components of phospholipides and 

 after being formed above become available for syntheses. Conversely 

 the ingested supply can be metabolized when the synthetic needs are 

 exceeded. The routes of metabolism yielding energy involve a-keto- 

 butyrate from threonine and methionine and hydroxypyruvate or 

 glyoxalate from glycine, serine, cystine, and cysteine. Serine and the 

 other intermediates converted to it can be converted to either carbo- 

 hydrates or fats for storage. 



Other amino acids, including leucine, isoleucine, valine, lysine, 

 and histidine, are abundant in the diets of animals and much is 

 known of their metabolism. However, in the interest of space the 

 reactions concerned have been omitted. It must not be construed that 

 these amino acids are less important than the others which have been 

 discussed. Rather the latter were chosen solely as examples to illus- 

 trate the complexity of amino acid metabolism and to reveal the 

 close connection of all metabolites. 



