80 CONTEMPORARY SCIENCE 



would find its way into the surrounding layer of water. 

 Enzymes, like the proteins, are colloids. 



The fact that enzymes show colloidal properties, and 

 the fact that they are invariably associated with proteins, 

 made it seem probable that when ultimately isolated in 

 the pure condition, they would be found to be proteins. 

 Attempts to obtain pure enzymes have been many. The 

 general method of procedure in almost all cases consists 

 in first extracting with water 3 as already explained or 

 submitting the mass to much pressure (Buchner), if the 

 enzymes are "intracellular." 



Having obtained a solution, the next step is often that 

 of dialysis (Graham). Diffusible bodies, particularly in- 

 organic substances, are thereby separated. Three of the 

 classical investigators in this branch, Osborne, Peckel- 

 haring, and Fraenkel, have all employed this method. 



Now usually comes precipitation. Some substance 

 alcohol, acetone, or ammonium sulphate is added in 

 which the enzyme is insoluble. The precipitate so obtained 

 contains many impurities (proteins, certain carbohydrates, 

 etc.). To purify it, it is redissolved, re-dialyzed, and re- 

 precipitated many times. On occasion, a biological pro- 

 cedure, first suggested by Efifront, and put into practice 

 by Fraenkel, may be used. This consists in fermenting 

 the impure precipitate with yeast. The carbohydrate and 

 protein are thereby used up, but according to Fraenkel, the 

 enzyme is not touched. 



The laboriousness of such an operation may best be 

 gathered from a specific example. Let us take an experi- 

 ment from the work of Professor Sherman, of Columbia, 

 an active investigator. Here is his method for preparing 

 a starch-splitting enzyme from the pancreas: Mix thor- 

 oughly 20 grammes of pancreative powder a commercial 

 preparation with 200 cubic centimeters of 50 per cent, 

 alcohol at 15-20 C. [S. finds that much of the contained 



3 Often containing alcohol, toluene, or chloroform (as pre- 

 servatives). 



