CM. XXXII.] LEUCINE AND TYROSINE 499 



up io the lactic acid fermentation : this may go further and result in 

 the formation of carbonic acid, hydrogen, and butyric acid (see p. 

 391). Cellulose is broken up into carbonic acid and methane. This 

 is the chief cause of the gases in the intestine, the amount of which 

 is increased by vegetable food. 



ii. On fats. In addition to acting like steapsin, they produce 

 lower acids (valeric, butyric, ,etc.). The formation of acid products 

 from fats and carbohydrates gives to the intestinal contents an acid 

 reaction. Eecent researches show that the contents of the intestine 

 become acid much higher up than was formerly supposed. Organic 

 acids do not, however, hinder pancreatic digestion. 



iii On proteids. Fatty acids and amino-acids, especially leucine 

 and tyrosine, are produced ; but these putrefactive organisms have a 

 special action in addition, producing substances having an evil odour, 

 like indole (C 8 H 7 N), skatole (C 9 H 9 N), and phenol (C H 6 0). There 

 are also gaseous products in some cases. 



If excessive, putrefactive processes are harmful ; if within normal 

 limits, they are useful, helping the pancreatic juice, and, further, 

 preventing the entrance into the body of poisonous products. It is 

 possible that, in digestion, poisonous alkaloids are formed. Certainly 

 this is so in one well-known case. Lecithin, a material contained in 

 small quantities in many foods, and in large quantities in egg-yolk 

 and brain, is broken up by the pancreatic juice into glycerin, 

 phosphoric acid, fatty 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 beon frequently mentioned 'in the 

 preceding pages. They are important as types of the simpler decom- 

 position products of proteids. 



They belong to the group of amino-acids. On p. 393 we have 

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

 atoms in a fatty acid by the amino-group (NH 2 ), we obtain what is 

 called an amino-acid. Take acetic acid : its formula is CH 3 .COOH ; 

 replace one H by NH 2 , and we get CH 2 .NH 2 .COOH, which is amino- 

 acetic acid, or glycine. If we take caproic acid a term a little higher 

 in the series its formula is C 5 H n .COOH; amino-caproic acid is 

 C 5 H 10 .NH 2 .COOH, which is also called leucine. 



According to the way in which the amino-group is linked, a large 

 number of isomeric amino-caproic acids, all with the same empirical 

 formula, are theoretically possible. Some of these have been actu- 

 ally prepared in the laboratory; and chemical research has shown 

 that the amino-caproic acid called leucine formed during diges- 



