THE NATURE OF ENZYMES 225 



polymerised to a high degree. Hence it would be more correct 

 to say that we do not know whether protein and enzymes have 

 ever been prepared in a state of purity, rather than, as is usually 

 done, that they never have been so prepared. 



One of the great difficulties in freeing enzymes from other 

 substances, such as proteins, is that they share the common 

 property of colloids, of being easily thrown mechanically out of 

 solution, by electrolytes or organic precipitants of proteins. 



The methods of attempted separation have differed in the case 

 of different enzymes, and cannot all be gone into in detail here. 



One general method is that of allowing the ferment to digest 

 out any substratum present naturally with it as much as possible 

 where it can be made to do this, as in the case of the proteo- 

 clastic enzymes, and then to precipitate it by means of some in- 

 different precipitate such as calcium phosphate in the case of 

 pepsin, or by the addition of collodium or cholesterin dissolved 

 in a mixture of alcohol and ether. Another method when the 

 ferment does not rapidly undergo alcohol coagulation is to remove 

 the accompanying protein by allowing the mixture to stand for 

 some weeks under alcohol, and then dissolve the ferment by means 

 of water, as in the case of fibrin ferment (thrombase). Another 

 method is to allow the strong solution to remain standing at the 

 freezing-point for some days, when the enzyme falls out in granular 

 form, as in the case of fibrin ferment precursor (pro thrombase) 

 in oxalated plasma, or of pepsin from pure gastric juice. 



The investigations have shown that enzymes are not in all cases 

 proteins. Thus the purer preparations of pepsin and invertase 

 do not give the protein colour tests. In elementary composition 

 the enzymes do, however, resemble the proteins more than any other 

 class of bodies. In addition to being salted out, or precipitated out 

 like colloids, the enzymes are further shown to be colloids by the 

 fact that they do not diffuse through parchment paper, or diffuse 

 with great slowness. 



THE SO-CALLED INORGANIC ENZYMES OR METAL SOLS. 



It has already been repeatedly stated that the enzymes are 

 a particular class of the bodies known as catalysts, which modify 

 the conditions of a reaction. The enzymes differ from most of 

 the inorganic catalysts, however, in that they are colloids, and 



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