CH. XXVIII.] 



ENZYMES 



439 



in the case of yeast, the alcohol produced, and in the case of 

 bacteria acting on proteins, the phenol, cresol, etc., produced, first 

 stop the growth of, and ultimately kill, these organisms. 



For a long time it was uncertain how micro-organisms were able 



boo' 



00 



7 



FIG. 316. Types of micro-organisms : a, micrococci arranged singly ; in twos, diplococci if all the 

 micrococci at a were grouped together they would be called staphylococci and in fours, sarcinse ; 

 b, micrococci in chains, streptococci ; c and d, bacilli of various kinds (one is represented with 

 a flagellum) ; e, various forms of spirilla ; /, spores, either free or in bacilli. 



to effect these chemical transformations. It is now, however, 

 definitely proved that they do so by producing agents of a chemical 

 nature, which are called enzymes. This was first demonstrated in 

 connection with the invertase of yeast cells, and with the enzyme 

 secreted by the micrococcus urese, which converts urea into ammonium 

 carbonate in putrefying urine. For a long time, however, efforts 

 to obtain from yeast cells an enzyme capable of bringing about the 

 alcoholic fermentation were unsuccessful. This is because the 

 enzyme does not leave the yeast cells, but acts intracellularly. 

 Buchner, by crushing the yeast cells, succeeded in obtaining from 

 them the long-sought enzyme (zymase) ; since then other enzymes 

 have been obtained from other microbes by similar means. 



Enzymes are also formed by the cells of the higher organisms, 

 both in animal and vegetable life. Familiar instances of these are 

 ptyalin, the starch-splitting enzyme of saliva, and pepsin, the 

 protein-splitting enzyme of gastric juice. The substance upon 

 which the enzyme acts is spoken of as the substrate. 



We may, therefore, place these essential facts concerning enzyme 

 action in the following tabular way : 



