ENZYMES 55 



reflected in a lowered rate of growth or may be seen in other behavior of 

 the fungus. With further increases in temperature, the enzyme systems 

 become less and less operative. So long as the temperature does not 

 exceed the point which produces irreversible inactivation, lowering the 

 temperature will enable the fungus to resume growth or other activity. 

 The temperature of inactivation is not fixed unless the length of exposure 

 is also considered. 



The effect of temperature upon growth is shown in Figs. 5 and 39. 

 The portions of the curves in the optimum temperature range represent a 

 balance between inactivation and increased rate of reaction. Above 

 optimum temperature, the rate of growth falls off abruptly. In a general 

 way the rate of growth parallels that expected of enzymatic processes. 



Hydrogen-ion concentration. Long ago it was recognized that strong 

 acids and alkalies were destructive to enzymes. A second effect was also 

 recognized: some enzj^mes exhibited maximum activity only in the pres- 

 ence of weakly acidic or alkaline solutions (see Chap. 8 for a discussion 

 of pH). The effect of pH on the activity of urease is shown in Fig. 25. 

 It should be noted that the pH optimum is dependent upon the concen- 

 tration of urea. 



Haldane (1930) compiled the pH optima of 105 enzymes and found 

 that the range extended from pH 2 to 10. However, all but nine of these 

 enzymes had pH optima between 4 and 8. Most fungi grow between 

 these limits. The effect of the pH of the medium upon the pH of the cell 

 contents is unknown in most instances. Biinning (1936) has reported 

 that the internal pH of the cells of Aspergillus niger is influenced by the 

 pH of the medium. The activities of the exoenzymes are affected by the 

 pH of the medium. 



Chemical reagents. Some enzymes are inactive or nearly so until they 

 have been treated with certain reagents. A group of the plant proteinases 

 which includes papain and bromelin are activated by hydrogen sulfide and 

 hydrogen cyanide (inhibitors for many enzymes), glutathione, and other 

 thiol compounds. These various activators do not act by removing 

 heavy metals (inactivators for many enzymes) but by reducing the disul- 

 fide linkage, — S — S — , to thiol (sulf hydril) , — SH. Neutral salts 

 activate some enzymes (emulsin, pancreatic amylase). The mode of 

 activation by neutral salts is unknown. Many of the metallic ions 

 (Mg++, Ca++, Fe++, Cu++, Mn++) are required for enzyme activity, but 

 it seems better to consider them as essential parts of some enzymes rather 

 than activators. 



Inhibitors are substances which reduce or destroy enzyme activity. 

 Inhibition may be reversible or irreversible. A few enzyme inhibitors are 

 cyanides, monoiodoacetate, fluoride, and the hea\^ metals (lead, copper, 

 mercury, silver, etc.). An inhibitor is active against certain enzymes and 



