494 DIGESTION 



THE CHEMISTRY OF BILE 



The Bile Salts 



In most animals the bile salts consist of the sodium salts of glycocholic 

 and taurocholic acids. Each of these acids is composed of a part called 

 cholic acid which is more or less related to cholesterol, and of glycine 

 (CH 2 NH 2 COOH ammo-acetic acid) or taurine (C 2 H 7 NS0 3 ), a derivative 

 of cysteine, which is a-amino-/?-thiopropionic acid (CH 2 HS.CHNH 2 . 

 COOH). The exact form of cholic acid varies in different animals, that 

 of the pig, for example, being different from that of man. Bile salts are 

 an exclusive product of liver metabolism ; i. e., they are not formed in 

 any other part of the animal body. They give a very sensitive color 

 reaction known as Pettenkof er 's, which however is not specific of bile acids, 

 since it is also given by oleic acid and by many aromatic substances and 

 alcohols. It must be remembered that the part of the bile salts that is 

 characteristic of the liver is the cholic acid, the taurine and glycine 

 being present in other tissues and organs. 



When cholic acid is given to animals mixed with the food, the amount 

 of taurocholic acid excreted with the bile is increased, indicating that 

 there must be a store of taurine available in the organism. This store 

 can not, however, be large, for if the feeding with cholic acid is repeated 

 several times, it will be found that the taurocholic acid diminishes and 

 glycocholic acid takes its place; and this increased excretion of glyco- 

 cholic acid goes on just as long as cholic acid is fed. The reserve of 

 taurine in the animal body appears therefore to be limited, although it is 

 used in preference to glycine when there is an excess of cholic acid to be 

 neutralized. On the other hand, the store of glycine seems to be inexhaust- 

 ible. That there is no reserve of cholic acid itself in the body is indicated by 

 the fact that no increase in taurocholic acid excretion by the bile results 

 when cystine, the mother substance of taurine, is given with the food. 

 If both taurine and cholic acid be fed, however, the excretion of tauro- 

 cholic acid increases. 



The relative amounts of taurocholic and glycocholic acids in the bile of 

 different animals differ considerably. Human bile contains relatively 

 a small amount of taurocholic acid; on the other hand, the bile of the dog 

 contains a large excess of it. 



Cholesterol 



In human bile the percentage of this important substance is not high 

 (1.6 parts per 1000), but it is of great clinical importance because of the 

 fact that it may separate out as a precipitate forming gallstones. The 



