66 



ESSENTIALS OF CHEMICAL PHYSIOLOGY 



phoric acid, stearic acid, and an alkaloid called choline. We are, 

 however, protected from the poisonous action of choline by the 

 bacteria, which break it up into carbonic acid, methane, and 

 ammonia. 



LEUCINE AND TYROSINE 



These two substances have been frequently mentioned in the 

 preceding pages. As types of the decomposition products of proteids 

 they are important, though probably only small quantities are 

 normally formed during digestion. 



They belong to the group of amido-acids. On p. 17 we have 

 given a list of the fatty acids ; if we replace one of the hydrogen 



Fig. 22. — Leucine crystals. 



Fig. 23.— Tyrosine crystals. 



atoms in a fatty acid by amidogen (NHg), we obtain what is called 

 an amido-acid. Take acetic acid : its formula is CH3.COOH ; replace 

 one H by NHg, and we get CH2.NH2.COOH, which is amido-acetic 

 acid or glycocine. If we take caproic acid — a term a little higher 

 in the series — its formula is C5H,,.C00H; amido-caproic acid is 

 CftHiQ.NHa.COOH, which is also called leucine. There are, however, 

 several isomeric amido-caproic acids. It was thought until quite 

 recently that leucine was the amido-acid of normal caproic acid, but 

 it has been shown to be a-amido-iso-butylacetic acid. The difference 

 in the structure of these two compounds may be represented by the 

 following graphic formulae : 



