474 



DIGESTION 



3000 



for the activity of each enzyme, as well as upper and lower limits of hydro- 

 gen ion concentration beyond which the enzyme is inactive, or may actually 

 be destroyed. The optimum hydrogen ion concentration for the activity of 

 any enzyme may vary, however, depending upon the source of the enzyme, 

 temperature, length of time the enzyme has been kept under any particular set 

 of conditions, etc. Typical curves for the relation between pH and the 

 activity of the enzyme amylase are shown in Fig. lOi. Curves expressing 



this relationship for other enzymes 

 follow somewhat similar trends, but 

 the optimum and limiting pH values 

 vary considerably. Most such curves 

 indicate that relatively small shifts 

 in pH value from the optimum re- 

 sult in marked changes in the activ- 

 ity of the enzyme. 



In general most hydrolytic plant 

 enzymes exhibit their maximum ac- 

 tivity in an acid medium — usually 

 between pH 4.0 and 7.0. Most 

 of the oxidizing-reducing enzymes 

 (Chap. XXX), on the other hand, 

 appear to attain their maximum 

 activity in neutral or alkaline so- 

 lutions. 



3. Hydration. — The effect of increased hydration upon the enzyme ac- 

 tivity of plant tissues is most easily demonstrated during the germination of 

 seeds. The enzyme activity, measured in terms of any specific enzyme 

 known to be present, is usually low in dry but viable seeds. As imbibition 

 of water proceeds during germination the activity of the enzyme increases 

 more or less progressively, an effect which can be ascribed, at least indi- 

 rectly, to the increase in the hydration of the tissues of the seed (Pickler, 

 1919). Sometimes, however, this apparent increase in the activity of an 

 enzyme with increase in the hydration of the tissues during seed germination 

 can be ascribed to the conversion of zymogens into enzymes. Similar effects 

 upon enzyme activity are undoubtedly shown in other plant tissues in which 

 considerable changes in hydration occur. 



4. Concentration of the Enzyme. — Since, with a few possible exceptions, 

 the chemical nature of enzymes is unknown, the concentration of an enzyme 

 extract usually cannot be determined or stated in any absolute terms. The 

 effects of relative concentrations of an enzyme extract upon the rate of 



234 56 789 10 



HYDROGEN ION CONCENTRATION EXPRESSED AS pH 



Fig. ioi. Relation between hydrogen 

 ion concentration and diastatic activity. 

 {A) pancreatic amylase, (5) amylase of 

 malt, (C) amylase of Aspergillus oryzae. 

 Data of Sherman, ct al. (1919). 



