20 II. DIGESTION AND ABSORPTION OF FATS 



metabolism. In the present section, specific esterases concerned with the 

 lipids will be discussed, irrespective of whether their primary action is in 

 the gastrointestinal tract or in other parts of the animal, or whether they 

 may be of vegetable or of animal origin. 



a. Cholesterol Esterase. Although it has long been recognized that 

 enzyme systems exist in the animal body for the hydrolysis of cholesterol 

 esters, and also for the synthesis of cholesterol esters from free cholesterol 

 and a fatty acid molecule, the sites of formation of such enzymes are not 

 well known. Moreover, we have no information as to whether or not the 

 hydrolytic and synthetic mechanisms are mediated by the same or by 

 different enzymes. The work of Nieft and Deuel 118 seems to indicate that 

 different enzyme systems are involved in the hydrolytic and synthetic 

 reactions. 



(a) Hydrolytic Action of Cholesterol Esterases. About 75% of the choles- 

 terol in the blood is in the form of the ester. In 1911, Abderhalden 119 

 demonstrated that blood cholesterol is increased when cholesterol is fed, 

 either in the form of the free alcohol or as the ester. This suggests that 

 both hydrolyzing and esterifying enzymes are present in the gut. Thann- 

 hauser et al. m were the first to report the presence of cholesterol esterase in 

 pancreatic and intestinal juices. Cholesterol esterase has been found in 

 blood corpuscles. 121 



Cholesterol esterase was demonstrated in the liver of the horse and ox 

 by Kondo 121 as early as 1910. Schultz 122 performed the first valid experi- 

 ments which proved that cholesterol esters can be split by liver extracts. 

 Although Mueller 123 failed to confirm these results, Klein 124 adduced strong 

 evidence of the existence of a cholesterol-ester-hydrolyzing enzyme in the 

 liver. The latter conclusion was borne out by the data of Nieft and 

 Deuel. 118 Sperry and Brand 125 later demonstrated that cholesterol ester- 

 ase could exert both a synthetic and a hydrolytic action in liver preparations 

 of the enzyme, depending upon the conditions. 



The presence or absence of a hydrolyzing cholesterol esterase in the blood 

 is still a controversial subject. Although Kondo 121 did report this active 



118 M. L. Nieft and H. J. Deuel, Jr., J. Biol. Chem., 177, 143-150 (1949). 



119 E. Abderhalden, Biochemisches Handlezikon, Vol. Ill, Springer, Berlin, 191 1 , p. 178. 



120 S. J. Thannhauser, Deut. Arch. klin. Med., 141, 290-292 (1923); and W. Fleisch- 

 mann, ibid., 292-296; and Ecarius, ibid., 297-300; and P. V. Miller, H. Srhaber, and 

 C. Moncorps, ibid., 300-311. 



121 K. Kondo, Biochem. Z., 26, 243-251 (1910); 27, 427-435 (1910). 



122 J. H. Schultz, Biochem. Z., 42, 255-261 (1912). 



123 J. H. Mueller, /. Biol. Chem., 25, 561-565 (1916). 

 12 < W. Klein, Z. physiol. Chem., 254, 1-17 (1938). 



126 W. M. Sperry and F. C. Brand, J. Biol. Chem., 137, 377-387 (1941). 



