320 CHEMISTR Y OF THE DIGESTIVE PROCESSES. 



Rate of zymolysis l or enzymic action. The rate at which diges- 

 tion goes on in any digestive fluid varies chiefly with the following 

 conditions, namely (1) The temperature, (2) the reaction, (3) the con- 

 centration of the products of digestion, (4) the concentration of the 

 digestive enzyme, (5) the condition of the material to be digested. 



Temperature. The digestive enzymes are very sensitive to changes 

 in temperature ; they all act most energetically at or slightly above the 

 body temperature. The point of greatest activity is called the optimum 

 point ; as the temperature varies, either above or below this point, the 

 rapidity of action of the enzyme slackens ; and, as the interval apart from 

 the optimum point is increased, a point is finally reached at which the 

 action of the enzyme is no longer appreciable. Any temperature 

 markedly above that of optimum action slowly destroys the enzyme, 

 and this destructive action in all cases becomes very rapid at tempera- 

 tures varying (between 50 and 65 C.) with the particular ferment, the 

 reaction of the fluid in which it is so heated, and the degree of its dilu- 

 tion. 2 On the other hand, low temperatures, though they slow and 

 finally stop ferment action, do not destroy the ferment ; this recovers its 

 activity completely when the temperature is again raised, even though 

 the temperature has been kept at 5 C. for several hours. 3 



Reaction. The variation in chemical reaction of the fluid in which 

 they act has a similar effect on enzymes to that of variation in tempera- 

 ture. For each of the digestive enzymes there is a particular reaction, 

 and degree of that reaction, at which it acts with maximum power. A 

 departure from this degree of acidity or alkalinity causes a more or less 

 rapid diminution in the speed with which the enzyme acts, and a 

 sufficient amount of departure from the optimum reaction causes the 

 destruction of the enzyme. Some of the enzymes act in solutions of 

 either acid, neutral, or alkaline reaction, provided always that the 

 reaction does not stray too widely from that at which they act best ; 

 examples of such are ptyalin and trypsin. Others only act with one 

 specific reaction, and are rapidly destroyed if the reaction changes from 

 this. Examples of these are pepsin, only active in acid solution, and 

 rapidly destroyed by a trace of alkalinity ; and the fat-splitting ferment 

 of the pancreas, active only in alkaline or neutral solutions, and rapidly 

 destroyed by acid. 



Accumulation of dissolved products of action. Accumulation of the 

 products of the action of an enzyme in the solution acts unfavourably 

 upon its continued action, slowing and finally altogether checking it. 4 

 This action may be to some extent prevented by removing the products 

 formed by dialysis, or diluting them by the addition of water. In the 

 latter case, however, the ferment is also diluted, and in the former, since 

 the products of digestion in most cases have no very high diffusive 

 power, the removal is very slow and incomplete. 



Eemoval of the digestive products by dialysis has, in addition, the 

 disadvantage that the digestive solution is diluted by the osmosis, due to 



1 This term is that proposed by Sheridan Lea, Journ. PhysioL, Cambridge and London, 

 1890, vol. xi. p. 254. 



2 v. Wittich, Arch.f. d. ges. PhysioL, Bonn, 1869, Bd. ii. S. 193 ; 1870, Bd. iii. S. 339. 



3 Bidder u. Schmidt, "Die Verdauungssafte, etc." 



4 Briicke, Sitzungsb. d. k. Alcad. d. Wissensch., Wien, 1862, Bd. xliii. S. 601 ; 

 "Vorlesungen," Wien, 1885, Bd. i. S. 312; Cohnheim, Virchow's Archiv, 1863, Bd. 

 xxviii. S. 241 ; Ktihne, " Lehrbuch der physiol. Chern.," 1866, S. 39, 51, 52 ; Sheridan Lea, 

 Journ. Physiol., Cambridge and London, 1890, vol. xi. p. 226. 



