192 CHEMISTRY OF PLANT LIFE 



by plants until after it has been hydrolyzed into its constituent 

 hexoses. 



The optimum temperature for invertase is 50 to 54; it is 

 killed if heated, in the moist condition, to 70. Its activity is 

 increased by the presence of small amounts of free acids; but is 

 inhibited by free alkalies. 



Zymase is the active alcoholic fermentation enzyme of yeasts. 

 It accelerates the well-known reaction for the conversion of hexose 

 sugars into alcohol and carbon dixoide, namely, 



Because of its scientific interest and industrial importance in the 

 fermentation industries, its action has been extensively studied. 

 It acts only in the presence of soluble phosphates and of a coen- 

 zyme (see below) which is dialyzable and not destroyed, which is 

 probably an organic ester of phosphoric acid. The significance 

 of the molecular configuration of the hexose sugars in their sus- 

 ceptibility to action by zymase has already been discussed in detail 

 (see page 56). 



The optimum temperature for zymase action is 28 to 30. 

 The enzyme is killed by heating to 45 to 50 in solution, or to 

 85 if in dry preparation. 



Proteases of the erepsin type, i.e., those which break proteins 

 down to amino-acids instead of only to the proteose or peptone 

 stage, as is characteristic of the enzymes of the trypsin type, are 

 widely distributed in plants. Except in the case of the two which 

 occur in large amounts in certain special fruits (papain in papaws, 

 and bromelin in pineapples), they are very difficult to prepare in 

 pure form for study. In general, all proteolytic actions, even when 

 accelerated by active enzymes, proceed much more slowly than 

 do the hydrolyses of carbohydrates or fats. It seems that meta- 

 bolic changes of the complex protein molecules are much more 

 difficult to bring about and take place much more slowly than do 

 those of the energy-producing types of compounds. 



The presence of proteolytic enzymes in most vegetative cells, 

 and in seeds, may be demonstrated, however, by studying the action 

 of extracts of these tissues upon soluble proteins. The best- 

 known example of this type of enzymes is the protease of yeast; 

 but similar ones may be found in germinating seeds. These 



