118 ENZYMES. 



to apply the name of enzyme, and to apply to the process for which 

 its members are responsible the term zymolysis. 



The distinction between the organized and unorganized fer- 

 ments is, after all, probably a superficial and not a fundamental 

 one. The fermentative or zymolytic action is in both cases due 

 to a substance which cells produce. In the one case, that of an 

 organized ferment, such as yeast, this product is thrown out by the 

 cells of the yeast-plant while they are in contact with the substance 

 acted upon dextrose, for example. In the case of an unorgan- 

 ized ferment or enzyme trypsin, for instance the cells of the 

 pancreas which produce it remain in the organ, while the product 

 is poured out with the other constituents of the secretion and 

 brings about its action on the proteids at a distance from the cells. 

 In both instances it is the product of cells which produces the 

 change. There will here be discussed only the unorganized fer- 

 ments or enzymes. 



Some of the enzymes on analysis have been found to be very 

 similar in their composition to the proteids, but the consensus of 

 opinion is that they are not proteids, notwithstanding this resem- 

 blance. The failure to determine their exact composition is due 

 to the fact that as yet no enzyme has been obtained pure and free 

 from proteids ; and also because the quantity is, in any event, 

 exceedingly small. Like most proteids they are not diffusible, 

 and cannot therefore be separated from them by dialysis. 



Enzymes are soluble in water, and are precipitated by an excess 

 of absolute alcohol or by saturation with ammonium sulphate. 

 They are changed 'by alcohol only when the contact has existed 

 for a considerable time ; this period is, however, shorter in the 

 case of pepsin than in that of the other enzymes. 



Minute quantities of enzymes under proper conditions will 

 bring about zymolytic changes in considerable quantities of the 

 substances upon which they act, apparently without suffering any 

 diminution. Thus, 1 part of renniii will coagulate 800,000 parts 

 of milk, and pepsin will dissolve in seven hours 500,000 times its 

 weight of fibrin. The conditions under which they act vary for 

 each enzyme ; but, as a rule, high temperatures destroy and low 

 temperatures inhibit, while for each there is a temperature at which 

 its action is the most pronounced ; this is called the " optimum '' 

 temperature. Thus for pepsin the optimum temperature is from 

 35 to 40 C., while below 1 C. its action ceases, as it does also 

 at 70 C., while boiling permanently destroys it. It has been 

 determined, however, that when perfectly dry the enzymes may be 

 heated to 160 C. without destroying their power. 



An interesting fact also connected with the enzymes is, that 

 when they have produced a considerable amount of their product 

 their action is diminished, and that if this new product accumu- 

 lates still more, the zymolytic action of the enzymes may be 



