188 CHEMISTRY OF PLANT LIFE 



rials, like aluminium hydroxide, or various hydrated clays. If 

 the dry preparations obtained in any of these ways are contam- 

 inated by carbohydrates, proteins, etc., these may be removed by 

 treatment with suitable digesting enzymes obtained from the 

 saliva, gastric, and pancreatic juices, and the digested impurities 

 washed out with 60 to 80 per cent alcohol, leaving the enzyme 

 preparation in a purified but still active form. 



In any study of the " strength," or possible catalytic effects, 

 of an enzyme preparation, it is necessary, first, to determine what 

 particular reaction it affects, by qualitative tests with various 

 substrate materials, such as starch, sugars, glucosides, proteins, 

 etc., and then to determine quantitatively its accelerating effect 

 upon the reaction in question. The latter may be done by 

 measuring either the time required to carry a unit quantity of the 

 substrate material through any determined stage of chemical 

 change, or the quantity of the substrate which is changed in a unit 

 period of time. It would not be profitable to go into a detailed 

 discussion here of the methods of making these quantitative 

 measurements of enzyme activity. Such discussions must neces- 

 sarily be left to special treatises on methods of study of enzyme 

 action. It may be said, however, that generally both the quali- 

 tative tests for, and the quantitative measurements of, the accel- 

 erating influence of enzymes depend upon the observation of some 

 change in the physical properties of the substrate material, such 

 as the optical activity, electrical conductivity, or viscosity, of its 

 solution. In some cases, it is convenient to make an actual quan- 

 titative determination of the amount of end-products produced 

 in a given time, as in the inversion of cane sugar, the hydrolysis of 

 maltose, etc., but such determinations necessarily involve the 

 removal of some of the reaction mixture for the purposes of the 

 determinations, and are not, therefore, suitable for the study of 

 the progressive development of the reaction which is being studied. 



Enzymes are found in all parts of the animal organism and 

 those which are active in the digestion of food, the metabolism of 

 digested material, the coagulation of blood, etc., have been exten- 

 sively studied. A discussion of these animal enzymes would be 

 out of place in such a text as this, however, and the following list 

 includes only enzymes which are known to occur in plant tissues. 

 These well-known enzymes will serve as examples of the several 

 general types which have thus far been isolated and studied. 



