l8o ORGANIC AGRICULTURAL CHEMISTRY 



This reaction is brought about in the digestive tract by means 

 of fat-hydrolyzing enzymes known by the general name of 

 lipolytic enzymes or lipases. It is well to consider the striking 

 fact that all of the digestive reactions of either carbohydrates, 

 proteins or fats are reactions of hydrolysis. The actions are all 

 brought about by means of enzymes, and digestive enzymes are 

 all hydrolytic. This statement has one exception if the enzymes 

 gastric rennin and pancreatic rennin are classed as digestive. 

 These two enzymes are coagulating enzymes, and, as they may 

 be considered as simply preparing the proteins for the real 

 action of digestion, they may be classed in a different group 

 than the true digestive enzymes. 



The Stomach 



Gastric Lipase. — Digestion of fats like that of proteins 

 begins in the stomach. In the gastric juice, in addition to the 

 enzymes already mentioned which produce protein hydrolysis, 

 there is present a lipolytic or fat-hydrolyzing enzyme known 

 as gastric lipase. This enzyme acts best in neutral solutions, 

 so that under normal conditions of stomach acidity its action 

 is not great, and the larger part of fat digestion is deferred till 

 later. The gastric lipase also acts best on emulsified fat, and 

 fat not so emulsified is very little, if at all, digested in the 

 stomach. In milk the fat is in the form of an emulsion and 

 this is also true in the yolk of eggs. In both of these cases in 

 man a large part of the fat digestion (78 per cent in egg yolk) 

 occurs in the stomach. On this account infants, before the 

 pancreatic juice is developed, are able to digest milk and egg 

 yolk. 



The Small Intestine 



Pancreatic Lipase. — The chief fat digestion takes place in 

 the small intestine due to the action of a fat-hydrolyzing enzyme 

 present in the pancreatic juice. This enzyme is known as 

 pancreatic lipase or as steapsin. The enzyme acts in alkaline 



