60 PHYSIOLOGICAL CHEMISTRY 



ing chemical reactions, both breaking down substances and 

 building them up. 



Enzymes have been the object of much study in recent years, 

 but as yet very little is known of their chemical constitution. 

 There is some evidence indicating that perhaps some of the 

 members of the group may be, or may closely resemble proteins, 

 whereas others appear to be carbohydrates. It is altogether 

 probable that they will be found to vary much in their chemical 

 nature, just as they vary in their chemical activities. 



Although little is known of the chemical constitution of the 

 enzymes, many important facts are known concerning their 

 properties and the conditions governing their activities. One 

 of the striking facts of enzyme action is that these substances do 

 not appear combined with the final products which they produce. 

 They are responsible for the breaking down or building up of 

 many classes of compounds, but apparently they only change 

 the speed of reactions which, if given sufficient time, would go 

 on of themselves. Substances which show this behavior are 

 spoken of as catalytic agents, and are said to act by catalysis. 

 Enzymes have been defined as substances produced by living 

 cells, which act by catalysis. 



A striking fact in connection with the action of enzymes, is 

 that under their influence, reactions go on in the body at a low 

 temperature which, if duplicated in the laboratory without the 

 aid of enzymes, require very high temperatures or the action of 

 strong chemical reagents. This is a general property of cata- 

 lytic agent, for we know that in the presence of spongy plat- 

 inum, hydrogen and oxygen will combine at a temperature far 

 below that required in the absence of a catalyst. The final 

 point of equilibrium in the reaction is not altered, the speed 

 of the reactions is merely altered, and reactions which are 

 capable of being catalyzed are considered to be going on even 

 in the absence of the catalyst, though perhaps at an infinitely 

 slow rate. Catalysts, including enzymes, do not appear com- 

 bined with the final products of the reaction. They are tem- 

 porarily united to the substance acted on, the substrate, but 



