ROLE OF BILE IN LIPID ABSORPTION 91 



isolate the corresponding triketo acid, namely dehydrocholic acid. On the 

 other hand, he was unable to demonstrate a similar oxidation with the liver 

 brei of cattle. It was also demonstrated that the triketo acid originates 

 after reductodehydrocholic acid is given to guinea pigs (8) ; under such 

 conditions the product is excreted in the urine. 



Another example of oxidation of the hydroxy- to the keto-bile acids is the 

 conversion of ursodesoxycholic acid (3,7-dihydroxycholanic acid) to 3,7- 

 diketocholanic acid. Isaka 495 demonstrated this oxidation in tissue brei 

 of both human and rat cancer. In the case of rat carcinoma (Flexner), 

 an intermediate product was also identified, namely 3-hydroxy-7-keto- 

 cholanic acid. It is concluded that the reduction process or the oxidation 

 process may occur in the case of the bile acids, depending upon the oxygen 

 supply in the particular tissue involved. 



(d) Change in Isomeric Form. Kyogoku 496 has shown that, when 3a- 

 hydroxy-7,12-diketocholanic acid was administered to toads, its epimeric 

 form, 3/3-hydroxy-7,12-diketocholaiiic acid, could be detected in the urine. 

 A similar phenomenon was observed by Tukamoto, 493 who found 3/3- 

 hydroxy-G-ketoallocholanic acid in the urine of rabbits after the parenteral 

 administration of 3a-hydroxy-6-ketocholanic acid. In this case not only 

 did an epimerization occur at the 3 position, but the steroid structure 

 assumed an alio configuration. 



(2) The Action of Bile in Lipid Absorption 



It has long been recognized that the presence of bile in the intestine is a 

 necessary concomitant of the absorption of fats and other lipids. Rach- 

 ford 497 first demonstrated that pancreatic lipase was activated by bile, and 

 this observation has since been confirmed many times. According to 

 Loevenhart and Souder, 498 the amount of bile required for the activation 

 of pancreatic lipase varies with the type of substrate, and is a function of 

 the bile salt concentration. In the case of triglycerides composed of long- 

 chain fatty acids, 2-4% of bile salts are required to produce the maximum 

 activation of the lipase; in simple esters the optimum concentration of 

 bile salts amounts to only 0. 1%. However, a fair degree of lipolytic activ- 

 ity is known to obtain even in the complete absence of the bile salts. 



Another action of bile which facilitates the digestion and absorption of 



496 H. Isaka, /. Biochem. (Japan), 82, 131-135 (1940). 



496 K. Kyogoku, Z. physiol. Chem., 246, 99-105 (1937); 250, 253-257 (1937). 



497 B. K. Rachford, ./. Physiol, 12, 72-94 (1891). 



498 A. S. Loevenhart and C. G. Souder, ./. Biol. Chem., 2, 415-425 (1906-1907). 



