150 PHYSIOLOGICAL CHEMISTRY 



activity is greatly decreased, and the addition of sodium chloride 

 will restore it. Phosphates also have a favorable effect on some 

 of the amylases. The same amylase, in the presence of chlorides, 

 may -act best at a pH different from its optimum in the presence 

 of phosphates. Some salts have a remarkable inhibiting effect 

 on ptyalin action, particularly salts of the heavy metals. The 

 speed of ptyalin action also is retarded by the end products 

 produced. This effect cannot be due simply to the reversibility 

 of the process, however, as sugars other than maltose also re- 

 tard ptyalin action. The sugars are believed to combine with 

 the ptyalin, thereby reducing its activity. The speed of diges- 

 tion by ptyalin is proportional to the amount of the enzyme. 



Starch is attacked by ptyalin and broken down into simpler 

 substances. The physical condition of the starch is important, 

 for raw starch is digested only with difficulty, whereas cooked 

 starch is digested very rapidly and completely under favorable 

 conditions of temperature, acidity, etc. Various products are 

 produced in the breaking up of starch, first the dextrins, of 

 which there undoubtedly are -several, and finally maltose and 

 isomaltose. The progress of the decomposition may be followed 

 by testing small portions of the liquid with iodine. The blue 

 color characteristic of starch gives place to a purplish, then a 

 red color, and finally no color whatever is produced. The 

 products corresponding to these stages are as follows: 



Color with Iodine 



Starch > Amylodextrin > Erythrodextrin -> Achroo-dextrin 

 blue blue red (no color) 



-^ Maltose 

 (no color) 



Maltose undoubtedly is split off even in the first stages of the 

 process. It also is possible to follow the progress of the reaction 

 by observing the time required for the clearing of an opalescent 

 starch solution, or the change in the optical activity of the 

 starch solution, or by the use of Mett's tubes filled with starch. 



