104 BACTERIOLOGICAL AND ENZYME CHEMISTRY 



change will be attempted later ; it is sufficient here to say that 

 the substances which give colour reactions with iodine are 

 known as dextrins, owing to their effect on polarised light ; the 

 sugar can be shown to be maltose. 



The nature of the substance present in the malt extract 

 which brings about the change has now to be considered. If 

 the extract is added to alcohol, a white precipitate is formed. 

 This precipitate can be shown to contain the active substance 

 or enzyme in question, which, as it is concerned in the breaking 

 down of starch, is known as ^ylase (or frequently ' diastase'). 

 Its preparation and investigation will illustrate very well 

 the properties and method of preparation of enzymes in 

 general.^ 



The following method may be used for the preparation 

 of the amylase of malt : 100 grams of ground malt (preferably 

 air dried) are digested with 250 c.c. of 20 per cent, alcohol 

 for four hours and then filtered. Strong alcohol is added to 

 the filtrate so long as a white flocculent precipitate is formed ; 

 this precipitate contains the amylase, it is allowed to stand 

 and the supernatant liquid poured off. The precipitate is 

 washed by decantation with a little strong alcohol, and after- 

 wards transferred to a smooth hardened filter and washed 

 repeatedly with small quantities of absolute alcohol. Portions 

 of the precipitate may now be examined in various ways. 

 By warming as much as will go on the end of a knife blade 

 with about 20 c.c. of starch solution the above described 

 changes in the starch solution will be found to take place. 

 The following reactions are characteristic of enzymes in 

 general : — 



1. A small portion of the precipitate is dissolved in the ■ 

 least possible quantity of water and a few drops of an alcohohc 

 solution of guiachum resin are added, together with a little 

 hydrogen peroxide. An intense greenish blue colour is obtained. 

 By taking different quantities of the aqueous solution it will 

 be found that the blue colour varies in proportion to the con- 



