CH. VIII.] PTYALIN. 187 



that converts starch into dextrins ; dextrinase, that converts 

 dextrins into maltose ; and maltase, that converts maltose 

 into glucose. 



In the case of the action of ptyalin on starch as con- 

 ducted in vitro, the final product consists of about 80 per 

 cent, of maltose, the remaining 20 per cent, being a com- 

 paratively simple dextrin called " stable " dextrin, owing 

 to its resistance to the further action of the enzyme. But 

 if this dextrin be isolated the action of ptyalin is to hy- 

 drolyse it very slowly and incompletely to equal molecular 

 parts of maltose and glucose. 



The optimum reaction for ptyalin is at PH = 6*7.* 

 The enzyme is rapidly destroyed should the reaction be 

 markedly acid to this, as it is during full digestion in the 

 stomach. But the presence of proteins, which function as 

 buffers, prevent the hydrochloric acid secreted in the early 

 stages of digestion from causing too high a concentration 

 of hydrogen-ions for the action of the ptyalin. This, 

 combined with the absence of active mechanical move- 

 ments in the cardiac end of the stomach, allows salivary 

 digestion to be carried on in the stomach for about 30 

 minutes after the ingestion of a mixed meal. 



Ptyalin is remarkable in that it is inactive in the 

 absence of electrolytes. As the author first demonstrated, t 

 the influence of electrolytes on amylolytic action is depen- 

 dent on the negative ion (kation). These have not all the 

 same activating power, the chloridion being the most 

 effective. A satisfactory explanation of this effect has 

 not yet been advanced. 



The optimum concentration of sodium chloride is 

 between 0*02 per cent, and 2 per cent.-, between which 

 limits very slight differences can be observed, but the 

 difference between the action of the enzyme in a salt-free 

 mixture and in one containing 0*02 per cent, of sodium 



* It has recently been stated that the optimum reaction for malt diastase 

 is at P H = 4-9. 



f Journ. of Physiology, 1903. 





