THE PLANT OXIDASES 217 



adopted in deciding whether a given reaction is enzymic 

 or an ordinary chemical change is to boil the solution. If 

 the reaction is no longer brought about, it may be pro- 

 visionally concluded that it is enzymic, its cessation being 

 due to the destruction of a thermolabile substance. But 

 enzymes have two other very important characteristics: 

 firstly, that a small quantity of the enzyme brings about 

 a relatively enormous transformation of the substrate ; and, 

 secondly, that the rate of this change is proportional to the 

 amount of enzyme present (provided the substrate is in 

 large excess), though the total amount transformed is 

 independent of it if a sufficient time be allowed to elapse. 

 It may be added that the enzymes are colloidal, and the 

 reactions they bring about or catalyze are in some cases 

 reversible, the point of equilibrium being usually very near 

 that of complete change in one direction. Furthermore, 

 their action is as a rule specific, one enzyme only acting on 

 one substrate, or on one class of substrates, and may in 

 many instances be inhibited entirely, or reduced in velocity 

 by very minute quantities of paralyzors. 



EFFECT OF HEAT ON OXIDASES. 



Now, the oxidizing substances of plants are destroyed by 

 heat, though in some cases peroxidases resist total inactiva- 

 tion even when their aqueous solutions are boiled for short 

 periods. Bach and Chodat (1903) have shown that with 

 laccase peroxidase the time required for complete destruc- 

 tion by boiling depends upon the concentration of the 

 enzyme. Thus a solution which necessitated boiling for 

 eighteen minutes to effect inactivation was found to be 

 quite inert after three minutes when diluted with twenty 

 times its volume of water. Alcoholic solutions are, how- 

 ever, inactivated by raising them to the temperature of 

 boiling water. 



