352 ENZYMES 



general rule, enzymes will not pass through a parchment 

 membrane ; nevertheless Frankel and Hamburg found that on 

 subjecting a sample of diastase obtained from malt to dialysis, 

 they were able to effect a separation into two distinct en- 

 zymes ; one of these passed through the parchment and was 

 found to be a sugar-producing enzyme, while the other which 

 would not diffuse was able to liquefy starch. 



On the other hand, most enzymes will pass through a 

 porcelain filter, a fact which is made use of for separating the 

 active enzymes from living cells. Owing to adsorption on the 

 surface of the porcelain the filtration may, however, be accom- 

 panied by considerable loss of enzyme. 



Enzymes for the most part are soluble in water, or in 

 dilute salt solutions, or in glycerin. The lipases or fat-splitting 

 enzymes, however, whether of animal or of vegetable origin, 

 are characterized by their slight solubility in water. 



On the other hand, enzymes are precipitated from solution 

 by alcohol and by neutral salts such as ammonium sulphate. 



Enzymes exhibit in a marked degree the phenomenon of 

 adsorption,* and consequently are liable to be withdrawn out 

 of solutions by other substances, such as calcium phosphate or 

 uranyl phosphate, which may happen to be precipitated in 

 their presence. For the same reason they are extracted from 

 solution by shaking with charcoal, china clay, etc. The con- 

 ditions obtaining here are to a large extent dependent on the 

 electric charges of the substances concerned, a question which 

 has been considered in detail by Michaelis.f 



MODE OF ACTION OF ENZYMES. 



To explain the mode of action of inorganic catalysts, it is 

 frequently supposed that they form labile additive compounds 

 with one of the reacting substances which then react more 

 readily than the original substance would have done. 



Similarly, in the case of the enzymes, it is now generally 

 assumed that they enter into some form of loose combination 

 with the substrate ; in spite of this the enzyme is, in general, 

 not altered by the reaction but retains its original activity 



*SeeDauwe: "Hofm. Beitr.," 1905, 6, 426. 



fMichaelis: " Biochem. Zeit.," 1908, 10, 283; "Dynamik d. Oberflach- 

 enwirkung," Leipzig, 1909. 



