ENZYMES AND THEIR ACTION 193 



vegetable proteases are usually most active in neutral or only 

 faintly alkaline solutions, and their activity is nearly always 

 inhibited by even traces of free acids. 



Most laboratory studies of proteolytic enzymes are carried on 

 with preparations of the powerful members of this class of enzymes 

 which are found in the digestive tract of animals, namely, the pep- 

 sin of the gastric juice, which acts in the acid medium. in the stom- 

 ach, and the trypsin of the pancreatic ju'ce, which acts in the 

 alkaline medium of the intestinal tract. But even these powerful 

 proteases require several hours for the transformation of an amount 

 of soluble albumin into its amino-acid constituents which is 

 equivalent to the amount of starch which is hydrolyzed to maltose 

 by diastase in a very few minutes. 



Enzymes which govern oxidative changes, known respectively, 

 as catalases and oxidases, are almost universally present in plants. 

 Catalase decomposes peroxides, with the liberation of free oxygen. 

 It is, therefore, necessary to the final step in the process of photo- 

 synthesis, as elucidated by Usher and Priestley (see page 26), 

 and serves to prevent the destructive action of hydrogen peroxide 

 upon chlorophyll. The almost universal presence of oxidases 

 in plant tissues has been repeatedly demonstrated. They are 

 present in especially large amounts in tissues which are being 

 acted upon by parasitic fungi or are combating unfavorable 

 conditions of growth. The oxidases, in such cases, seem to 

 be the agents by which the plant is able to stimulate its metabolic 

 activities to overcome the unfavorable environment for its normal 

 development. 



In vegetables and fruits, the common browning, or blackening, 

 of the tissues when cut surfaces are exposed to the air has been 

 demonstrated to be due to the catalytic oxidation of the tannins 

 or of certain amino-acids, especially tyrosin, under the influence 

 of the oxidases which are present in the tissues. In fact, most 

 pigmentation phenomena are due to changes in the oxygen con- 

 tent of the chromogens of the cells of the plant, under the influence 

 of the oxidases which are present in the protoplasm of the cells in 

 question. Hence, the oxidases may be said to be the controlling 

 agencies for both the energy-absorbing activities and for respiration 

 in plants. 



