22 II. DIGESTION AND ABSORPTION OF FATS 



The other component, the so-called "co-factor," is a heat-stable, non- 

 dialyzable substance soluble in saturated ammonium sulfate solution. 

 The optimum pH was found to be 6.7 to 7.1. Yamamoto et aZ. 129 reported 

 the optimum pH of the cholesterol ester-hydrolyzing enzyme in pancreatin 

 as 6.6. It is destroyed by a temperature of 60°C, alkali treatment to a 

 pH of 10, or by digestion with purified trypsin. 130 



(6) Synthetic Action of Cholesterol Esterases. It has been recognized for a 

 long time that the animal must possess a mechanism for bringing about 

 esterification of cholesterol. Not only will cholesterol absorbed as the free 

 alcohol be later found as the ester, but there is every reason to believe that 

 the cholesterol synthesized de novo in the tissues can undergo esterification. 

 The nature and site of action of the enzyme system responsible for such 

 a reaction have only recently been investigated. 



Sperry 128 was the first to demonstrate in a clear-cut fashion that blood 

 serum contains a cholesterol-esterifying enzyme. He stated that the en- 

 zyme has an optimum pH. of 8.0; its activity is destroyed when serum is 

 heated to 55 to 60°C. Bile salts were shown to decrease the synthetic 

 action of cholesterol esterase and, when present in sufficient amount, to 

 inhibit it completely. 55 Swell and Tread well 133 demonstrated that free 

 cholesterol of human and dog sera was esterified during incubation and that 

 the reaction was inhibited by bile salts. They suggest that the synthetic 

 reaction is catalyzed by an enzyme of very low activity, or is non-enzymatic. 

 The pancreas is believed to be the sole source of the serum cholesterol ester- 

 ase in the case of the dog. 134 



Another possible site of the synthetic-acting cholesterol esterase is the 

 intestinal wall. Mueller 135 demonstrated that the lymph of cholesterol-fed 

 dogs contained cholesterol and cholesterol ester in a constant ratio, irrespec- 

 tive of whether the sterol was fed as an ester or as the free alcohol. Fro- 

 licher and Siillmann 136 observed a marked absolute increase in the choles- 

 terol esters in rabbit chyle after feeding free cholesterol in triolein, although 

 the proportions of free and of esterified cholesterol showed wide variations. 

 The presence of a cholesterol-esterifying system in the intestinal wall, as 

 well as in the liver, has been demonstrated by Nieft and Deuel. 118 This 

 system requires the concomitant presence of phosphate and of a source of 

 fatty acid. The enzyme exhibits maximum activity at a pH of 6.5. 



The pancreas also mediates the esterification of cholesterol. Schramm 



133 L. Swell and C. R. Treadwell, /. Biol. Chem., 185, 349-355 (1950). 



134 L. Swell and N. C. Kramer, Proc. Soc. Exptl. Biol. Med., 82, 197-198 (1953). 



135 J. H. Mueller, J. Biol. Chem., 27, 463-480 (1916). 



136 E. Frolicher and H. Siillmann, Bwchem. Z., 274, 21-33 (1934). 



