II. DIGESTION AND ABSOKPTION OF FATS 



Table 1 



The Classification, Occurrence, Substrate, and End-Products of Some Enzymes 



Concerned with the Metabolism of Lipids 



Principal sites 

 of occurrence 



Reaction products 



Lipase 



Esterases 



(a-group) 

 Cholesterase 



Lecithinase A 



Phytase 



Cholinesterase 

 Choline acetyls 



Chlorophyllase 



Sulfatase 



Gastric juice (?); 



pancreatic juice; 



waxy plants 

 Liver; plants 



Blood; other tissues 



Kidney, brain, intes- 

 tinal mucosa 

 Plants, especially 



Animal tissues 

 Nervous tissue; 



muscle 

 Green plants 



Neutral fats 



Esters of monohy- 



droxyalcohols 

 Cholesterol esters 



Lecithin 



Phytic acid 



Acetylcholine 

 Acetic acid + cho- 

 line 

 Chlorophyll 



Animal and plant tis- Sulfuric acid esters 

 sues 



Diglycerides, mono- 

 glycerides, fatty 

 acids, glycerol 



Alcohol -f- acid 



Cholesterol + fatty 

 acid 



Diglycerides, cho- 

 line, phosphate 



Inositol -f- phosphate 



Choline -f- acetate 

 Acetylcholine 



Phytol + chlorophyl- 



lide-a 

 Alcohol + sulfate 



The lipases, esterases, and other enzymes concerned with the metabolism 

 of lipids are water-soluble substances. The action of such molecules on 

 fats or esters which results in their hydrolysis can occur only if the enzyme 

 molecule comes into proximity with that of the lipid or ester. One site at 

 which proximity can be produced between these components is the water : 

 lipid interface. Since it is known that emulsification of fat precedes the 

 action of the enzyme, the area between the water and oil phases is greatly 

 increased. The large surface produced by this emulsification might be 

 sufficiently great to allow lipolysis to proceed at the speed that is known to 

 occur in the gut, in spite of the fact that the substrate is in a different phase 

 than is the enzyme. Schulman 2 has shown that the digestion of esters by 

 pancreatin is related to the nature of the surface film. The reaction rate is 

 related to the length of the alcohol and acid radical chain. Apparently 

 the acid radical plays the more important role ; complete inhibition usually 

 obtains when the acid contains 5 or 6 carbons or possesses the C 6 H 5 -ring. 

 Activity is noted when the acid is increased to a chain length of 8 carbon 

 atoms. On the other hand, although the length of the alcohol chain does 

 not influence the results in compounds otherwise indigestible, it was found 

 that the ethyl esters are somewhat more easily digested by pancreatin. 

 These variations in hydrolysis are interpreted by Schulman 2 as being re- 

 lated to difference in orientation of the esters in the surface films. In 1951 , 



2 J. H. Schulman, Trans. Faraday Soc, 37, 134-139 (1941). 



