224 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



enzymes is similar, the two processes are not identical, since in the first process 

 dextrose is the sugar formed, while in the second it is maltose. Moreover, 

 variations in temperature affect the two reactions differently. 



Conditions Influencing the Action of Ptyalin. Temperature. As in 

 the case of the other enzymes, ptyalin is very susceptible to changes of temper- 

 ature. At C. its activity is said to be suspended entirely. The intensity 

 of its action increases with increase of temperature from this point, and 

 reaches its maximum at about 40 C. If the temperature is raised much 

 beyond this point, the action of the ptyalin decreases, and at from 65 to 

 70 C. the enzyme is destroyed. In these latter points ptyalin differs from 

 diastase, the enzyme of malt. Diastase shows a maximum action at 50 C. 

 and is destroyed at 80 C. 



Effect of Reaction. The normal reaction of saliva is slightly alkaline. 

 Chittenden 1 has shown, however, that ptyalin acts as well, or even better, in 

 a perfectly neutral medium. A strong alkaline reaction retards or prevents 

 its action. The most marked influence is exerted by acids. Free hydrochloric 

 acid to the extent of only 0.003 per cent. (Chittenden) is sufficient to prac- 

 tically stop the amylolytic action of enzyme, and a slight increase in acidity not 

 only stops the action, but also destroys the enzyme. The latter fact is of 

 practical importance because it indicates that the action of ptyalin on starch 

 must be suspended after the food reaches the stomach. 



Condition of the Starch. It is a well-known fact that the conversion of starch 

 to sugar by enzymes takes place much more rapidly with cooked starch for 

 example, starch paste. In the latter condition sugar begins to appear in a 

 few minutes (one to four), provided a good enzyme solution is used. With 

 starch in a raw condition, on the contrary, it may be many minutes, or even 

 several hours, before sugar can be detected. The longer time required for 

 raw starch is partly explained by the well-known fact that the starch-grains 

 are surrounded by a layer of cellulose or cellulose-like material which resists 

 the action of ptyalin. When boiled, this layer breaks and the starch in the 

 interior becomes exposed. In addition, the starch itself is changed during the 

 boiling ; it takes up water, and in this hydrated condition is acted upon more 

 rapidly by the ptyalin. The practical value of cooking vegetable foods is 

 evident from these statements without further comment. 



Physiological Value of Saliva. Although human saliva contains ptyalin, 

 and this enzyme is known to possess very energetic amylolytic properties, yet 

 it is probable that it has an insignificant action in normal digestion. The time 

 that food remains in the mouth is altogether too short to suppose that the starch 

 is profoundly affected by the ptyalin. It would seem that whatever change 

 takes place must be confined to the initial stages. After the mixed saliva and 

 food are swallowed the acid reaction of the gastric juice soon stops completely 

 all further amylolytic action. The complete digestion of the carbohydrates 

 takes place after the food (chyme) has reached the small intestine, under the 

 influence of the amylopsin of the pancreatic secretion. For these reasons it is 



1 Studies from the Laboratory of Physiological Chemistry of Yale College, vol. i., 1884. 



