INTESTINAL DIGESTION 1 99 



digestive juices, however, and if found owes its presence to the excretory 

 processes of certain bacteria. Sucrases may also be obtained from 

 several vegetable sources. For investigational purposes it is ordinarily 

 obtained from yeast (see page 14). It exhibits its greatest activity 

 in the presence of a slight acidity, but if the acidity be increased to any 

 extent the reaction is inhibited. 



' Lactase is an enzyme which inverts lactose with the consequent 

 formation of glucose and galactose. Its action is entirely analogous, 

 in type, to that of sucrase. It has apparently been proven that lactase 

 occurs in the intestinal mucosa of the young of all animals which suckle 

 their offspring. 1 It may also occur in the intestinal mucosa of certain 

 adult animals if such animals be maintained upon a ration containing 

 more or less lactose. Fischer and Armstrong have demonstrated the 

 reversible action 2 of lactase. 



Maltase possesses the power of splitting maltose, the end-product 

 of the digestion of starch, into glucose. It was first discovered in the 

 urine and shortly after this time its presence was noted in the small 

 intestine and the saliva. Corn is sometimes used as the medium for 

 the preparation of the enzyme for experimental purposes. It occurs 

 in corn in a very active state. It was in connection with maltase that 

 the principles of the "reversibility of enzyme action" were first 

 demonstrated. 



Enterokinase possesses the power of "activating" trypsinogen see 

 Chapters I and X). In other words, trypsinogen as formed by the 

 pancreas has no proteolytic power, but when this inactive trypsino- 

 gen reaches the intestine and comes into contact with enterokinase the 

 latter transforms it into active trypsin. Enterokinase is not always 

 present in the intestinal juice since it is secreted only after the pan- 

 creatic juice reaches the intestine. It resembles the enzymes in that 

 its activity is destroyed by heat, but differs materially from this class 

 of bodies in that a certain quantity is capable of activating only a 

 definite quantity of trypsinogen. It is, however, generally classified 

 as an enzyme. Enterokinase has been detected in the higher animals, 

 and a kinase possessing similar properties has been shown to be present 

 in bacteria, fungi, impure fibrin, lymph glands, and snake-venom. 



The intestinal juice and the epithelium of the intestinal wall con- 

 tain enzymes capable of hydrolyzing nucleic acids and as these acids 

 are not acted upon by the gastric juice and probably not to any great 

 extent by pure pancreatic juice, the intestine apparently plays the chief 

 r61e in decomposition or digestion of these substances. At least two 



1 Mendel and Mitchell: American Journal of Physiology, 20, 81, 1907. 

 8 See p. 8. 



