HISTOLOGY AND CHEMISTRY OF FUNGI 5 7 



as a thermolabile catalyst in solution. In connection with this defini- 

 tion, it is important to know that a catalytic agent is one which alters 

 the rate of a reaction without itself entering into the final product 

 (Ostwald, 1902), or which does not appear to take any immediate part 

 in the reaction, remains unaltered at the end of the reaction and can 

 be recovered again from the reaction product unaltered in quantity 

 and quality. 



Enzymes differ from ordinary inorganic catalysts in their sensitive- 

 ness to heat and light. They are destroyed at 100° C, and most of 

 them cannot be heated safely above 60° C.^ The velocity of the 

 reaction increases with a rise of temperature up to an optimum and 

 as the temperature is increased above the optimum the enzyme is 

 permanently inactivated. Enzymes retain activity even after ex- 

 posure to action of liquid air. Light in its ordinary form in the pres- 

 ence of oxygen and ultraviolet light independent of oxygen are de- 

 structive to enzymes. Again, enzymes possess most of the important 

 properties of colloidal solutions, such as their non-diffusibility. They 

 are soluble in water, in dilute salt solutions, or in glycerin. They 

 exhibit the phenomenon of adsorption. 



An important discovery has recently been made which has thrown 

 considerable light on the activity of enzymes, and that has been the 

 stimulation exercised by certain substances which have been called 

 activators and the inhibition exercised by other substances, which have 

 been called paralyzers. The activators are in some cases simple chem- 

 ical substances, such as acids, alkalis and salts, or they are complex 

 bodies of unknown chemic character, but they have this in common that 

 they can be separated from the enzyme by dialysis, and are not de- 

 stroyed by heating. An enzyme may be rendered inactive by the 

 removal of its activator, but it can be restored to activity by mixing 

 again with this substance. In the case of some enzymes, the inactive 

 substance, as it is formed in a cell may be called a zymogen, or profer- 

 ment, but when associated with the activator the active enzyme is 

 developed. An activator is inorganic. A kinase is a more or less 

 complex organic body which activates a proferment. 



Substances which reduce, or destroy, the activity of enzymes are 

 called paralyzers, which may be formed as products of enzymatic 



'Haas, Paul, and Hill, T. G.: An Introduction to the Chemistry of Plant 

 Products. 1913: 340-341. 



