OCCURRENCE OF HYDROLYTIC PLANT ENZYMES 467 



The enzj^mes of dry tissues such as seeds and spores can endure tempera- 

 tures of 100° to 120° C, or even higher, for considerable periods without 

 suffering deleterious effects. The same is also true of dried enzyme extracts. 



The exact temperature at which a given enzyme will be destroyed varies 

 greatly, depending upon the conditions prevailing in the medium in which it 

 is dispersed. The reaction of the medium has a marked effect upon the 

 heat sensitivity of the enzyme. The presence of either the substrate or the 

 end products of an enzyme in the medium in which it is dispersed greatly 

 retards or may even entirely prevent its destruction at a temperature which 

 would otherwise result in its demolition. The substrate or end products of 

 an enzyme exert a similar protective effect against other destructive agents. 

 An example of this effect, as shown by one of the end products of an enzy- 

 matic reaction, is illustrated in Table 44. 



TABLE 44 THE ACTION OF FRUCTOSE IN PROTECTING A SUCRASE EXTRACT FROM DESTRUCTION 



BY ACIDS, ALKALIES, ALCOHOL, AND HEAT (dATA OF HUDSON AND PAINE, I9I0) 



Temperatures near or below the freezing point usually result in the in- 

 activation of enzymes, but not commonly in their destruction. When the 

 temperature is raised to within a suitable range enzymes which have been 

 exposed to relatively low temperatures are generally found to be little if any 

 impaired in their properties. 



Occurrence and Distribution of Hydrolytic Plant Enzymes.— The 

 hydrolytic enzymes most commonly found in green plants are amylase, mal- 

 tase, proteinases, lipases, sucrase, hemicellulases, inulase, emulsin, and the 

 pectic enzymes. Enzymes are very unequally distributed in the various organs 

 and tissues of plants, but there is probably no living cell in which some 

 enzymes, or at least their progenitors, the zymogens, do not occur. Certain 

 organs of a plant apparently contain one set of enzymes, other organs an- 

 other set. The leaves of the garden beet, for example-, are reported to contain 



