70 II. DIGESTIONS AND ABSORPTION OF FATS 



prevents the water-soluble proteases and amylases from coming into con- 

 tact with their substrates. Since proteins and carbohydrates are utilizable 

 only after having been hydrolyzed into their simplest units, any factors 

 which retard this hydrolysis will at the same time decrease absorption from 

 the gastrointestinal tract. 



(1) The Nature, Distribution, and Properties of the Bile Acids 



The most effective components of the bile which facilitate the absorption 

 of fats and of other lipids are the bile acids. Since the bile is normally 

 alkaline, such acids are usually in the form of their salts; hence the term 

 "bile salts" is a more accurate designation. However, in discussions of the 

 chemical nature of these substances and their transformations in the ani- 

 mal, they are almost universally considered to be acids. An especially 

 complete summary of the chemistry of the bile acids is to be found in So- 

 botka 398 and Fieser and Fieser. 399 



a. Distribution of the Bile Acids. The bile salts having the widest dis- 

 tribution in the animal kingdom are sodium glycocholate and sodium 

 taurocholate. Cholic acid is a component of both of these compounds, 

 but the bile salts differ from each other in that one has glycine conjugated 

 with the cholic acid while, in the second case, taurine replaces glycine. 

 The nature of these conjugated cholic acid compounds becomes evident 

 when they are hydrolyzed. Thus, when glycocholic acid is split, the reac- 

 tion proceeds as in (4). 



C 23 H 39 3 CONHCH 2 COOH + H 2 — ► C 23 H 39 3 COOH + H 2 NCH 2 COOH 



Glycocholic acid Cholic acid Glycine 



Hydrolysis of Glycocholic Acid (4) 



The reaction occurs in a similar manner in the case of taurocholic acid 



(5). 



C 23 H 39 3 CONHCH 2 CH 2 S0 2 OH + H 2 > C 23 H 39 3 COOH + HbNCHoCH^OaOH 



Taurocholic acid Cholic acid Taurine 



Hydrolysis of Taurocholic Acid (5) 



Cholic acid (Gr. x ^ 7 ?', bile) is 3,7,12-trihydroxycholanic acid. It is 

 derived from cholesterol, and possesses the cyclopentanophenanthrene 

 ring. Instead of belonging to the C 2 7 series as is the case with cholesterol, 

 cholic acid contains only 24 carbons. This is because the aliphatic side 



898 H. Sobotka, The Chemistry of the Sterids, Williams and Wilkins, Baltimore, 1938. 

 399 L. F. Fieser and M. Fieser, Natural Products Related to Phenanlhrene, 3rd ed., 

 Rcinhold. New York, 1949. 



