ENZYMES. FERMENTATION 393 



The liquid, which oozed out, was filtered (i) through paper and (2) 

 through a clay candle to remove unbroken cells. 



The dried material is treated with water or with glycerin. The 

 solution is filtered and precipitated with alcohol. 



The autolytic extracts, or the liquids produced by mechanical dis- 

 integration, may be mixed with water or glycerol and precipitated with 

 alcohol. 



The alcohol precipitate is dissolved in water, reprecipitated with 

 alcohol, filtered off, washed with alcohol and ether and dried in vacua 

 over sulphuric acid. 



Too frequent solution and precipitation "by alcohol is avoided as 

 much enzyme is lost in the process. 



Aqueous or glycerin extracts of the dried material or the fresh 

 gland also contain the enzymes and may be used directly, as is usually 

 the case when enzymes are to be detected in tissues. 



Chemical Nature. 



The chemical constitution of enzymes is still quite unknown ; they 

 have been supposed to be proteins, nucleoproteins and carbohydrates 

 from the fact that the enzyme solution gave the reactions of these 

 classes of compounds. The purest preparations of invertase and amylase 

 that have been prepared have contained carbohydrate; the purest 

 preparation of pepsin has not contained n-ucleoprotein. 



Though the chemical nature of enzymes is unknown they belong 

 to the group of colloidal substances ; thus, they do not diffuse through 

 parchment paper and other membranes. 



Properties of Enzymes. 



(1) Enzymes can only be recognised by their activity. 



(2) Enzymes are specific in their action. An enzyme acts only 

 upon one compound, or a group of compounds, such as the fats and 

 proteins. The most striking instance of their specificity is observed 

 in the a- and /8- glucosides. The enzyme maltase acts only upon 

 #-glucosides : the enzyme emulsin only upon y8-glucosides. 



(3) Enzymes act by combination, or by adsorption, with the com- 

 pound upon which they act. From the combination of an enzyme 

 with the substance upon which it acts and its specific property, 

 arose the image of Emil Fischer, that the enzyme was to the sub- 

 stance as a key is to a lock. Only the proper key will open the 

 lock. In illustration of these properties Armstrong likened the speci- 

 ficity and combination to the fitting of a glove upon the hand. Only 

 the right-hand glove will fit the right hand There may be combina- 

 tion, but unless it is with every digit there is no enzyme action. 



(4) Enzymes act as catalysts, i.e. they increase the rate of a 

 reaction which is normally proceeding at so slow a rate that it cannot 

 be detected. 



