436 ANIMAL BIOCHEMISTRY 



components unless they too are altered in the bladder. Lipidcs, 

 inckiding cholesterol, fats, and lecithins, are supplemented in the 

 gall bladder while salts are withdrawn, maintaining a rather constant 

 salt concentration. 



No special function is known for the mucin and inorganic salts of 

 bile. The bile pigments are important as excretory forms of nitrogen 

 and are discussed in connection with nitrogen metabolism on page 

 471. These pigments are excreted in feces and represent waste 

 products of liver activity. Roles are not known for the fats and lec- 

 ithins of bile although they should be subject to the same digestive 

 processes as are analogous ingested materials. Cholesterol (see page 

 93) is partly derived from ingested animal materials, at least in 

 herbivores, is partly formed from plant sterols, and is partly synthe- 

 sized in the liver from acetate by a complex reaction pathway. Again 

 for cholesterol, the function in bile is not known. Much of it is 

 excreted in the feces, and in cases of atherosclerosis (hardening of the 

 arteries) cholesterol is deposited in artery walls. As a result there is 

 much interest in all aspects of the physiology of cholesterol. 



Bile salts form the most important biliary component, making the 

 major contribution to intestinal digestion. This group of organic 

 salts effectively emulsifies dietary lipides of all kinds. As a result of 

 this powerful surface activity, ingested fats subdivide into minute 

 droplets whose high surface-area-to-volume ratio makes them much 

 more efficiently hydrolyzed by lipase. Furthermore, the fats coating 

 particles of carbohydrate and protein are removed and digestion of 

 these food components is facilitated. Thus bile makes an important 

 contribution to many aspects of intestinal digestion though it does 

 not directly catalyze hydrolyses. When the flow of bile is blocked for 

 any reason, digestion is impaired in general, and most of the fat is 

 excreted and lost. Disease states of this type, which are termed 

 jaundice, most often result from physical obstruction of the bile duct 

 or from liver failure due to poisoning or infection as in infectious 

 hepatitis. The absence or shortage of bile during jaundice does not 

 permit proper emulsification of fats, and much fat cannot be digested 

 and is lost by excretion. 



The compounds termed bile salts are structurally related to and 

 may be derived in part from cholesterol. They are amides of complex 

 acids with glycine or taurine, +H3NCH2CH0SO3-, which is formed 

 from cystine and cysteine. In human bile four different acids occur, 

 the most abundant being cholic acid (see page 94). At the pH 

 of bile and intestinal contents, the bile salts exist as anions associated 

 with inorganic cations. 



