636 PHYSIOLOGICAL CHEMISTRY. 



precipitated at first dissolves on shaking, giving a blue solution which 

 is not reduced on heating. 



4. Sherer's reaction (applicable only to very pure leucine). Evapo- 

 rate carefully to dryness on a platinum foil a small portion of leucine 

 with a few drops of nitric acid. The residue is almost transparent, 

 and turns yellow or brown on the addition of caustic alkali. If this 

 mixture be again carefully concentrated, an oil-like drop is obtained, 

 which runs over the foil in a spheroidal state. 



Hydrolysis. 



In the animal body a certain kind of decomposition, called hydro- 

 lysis or hydrolytic cleavage, is particularly prominent. By cleavage 

 is meant the breaking up of a complex molecule into simpler ones, for 

 example, the splitting up of dextrose into alcohol and carbon dioxide : 



C 6 H 12 O 6 == 2C 2 H 5 OH + 2CO 2 , 



When the splitting is accompanied by the decomposition of water 

 and the taking up of its constituents by the decomposition-products, 

 it is known as hydrolytic cleavage or hydrolysis. The special kind 

 of hydrolysis is sometimes indicated by adding the suffix " lysis " to 

 a root designating the nature of the substance decomposed, as pro- 

 teolysis for hydrolytic cleavage of proteins. A familiar example of 

 this kind of cleavage is the inversion of cane-sugar by boiling with 

 acidified water : 



C 12 H 22 O n + H 2 : : C 6 H 12 6 + C 6 H 12 6 , 



Hydrolytic cleavage, as has been shown, can be brought about out- 

 side the body by heat, with or without the aid of acids or alkalies, 

 and by the action of certain substances called enzymes. 



Enzymes (ferments). 



Enzymes, as mentioned in Chapter 41, are substances that decom- 

 pose others without themselves undergoing permanent change, i. e., 

 they are catalysts. The activity of all enzymes is impeded by accu- 

 mulation of the products of the fermentation. The activity of enzymes 

 is controlled by the temperature and character of the solution in 

 which they act. There is a certain temperature at which every 

 enzyme is most active the "optimum" temperature. A higher 

 temperature first impairs, and then destroys its activity. All enzymes 

 are destroyed by heating to 100 C. (212 F.) with water. Cooling 

 impairs their activity ; but even after freezing they regain their 

 power when carefully brought to the proper temperature. Some act 



