MECHANISM OF METABOLISM. 141 



anything but a soluble compound of the yeast cell. Thus the alcoholase 

 was recognized. It was found later that yeast may be killed by alcohol, 

 ether or acetone without losing its fermenting power. 



This last method was applied later to lactic bacteria, and it was 

 proved that the lactic acid is also produced by an enzyme, lactacidase. 

 It is possible to kill the lactic bacteria cells so that they do not multiply 

 but still continue to form acid. It seems quite probable that other fer- 

 mentations of carbohydrates, like the butyric and the gassy fermentations, 

 are really due to enzymes. It is very difficult to give the experimental 

 proof, however. These enzymes are so sensitive that it requires much 

 experience to separate them from the cell, and it is also quite difficult to 

 obtain bacteria in quantities large enough for such experiments. 



The vinegar oxidase is an enzyme which remains in the cell of the 

 acetic bacterium, oxidizing alcohol to acetic acid. Its independence of 

 the living cell has been demonstrated by killing the cells with acetone. 



The PROTEOLYTIC ENDO-ENZYMES of yeasts, only, have been studied 

 extensively. That such enzymes exist is recognized by the observation 

 that certain microorganisms do not liquefy the gelatin until after they are 

 dead and the proteolytic enzymes diffuse out through the deteriorating 

 cell membranes. That yeast in the absence of sugar loses in weight, and 

 that leucin and other cleavage-products of protein are formed, was the 

 first indication of a proteolytic process in the yeast cells. By pressing 

 the juice out of the ground yeast cells, a liquid is obtained which liquefies 

 gelatin, digests casein, albumin and fibrin. The living yeast cell does 

 not attack these compounds, because they cannot diffuse into the cell 

 and the enzyme cannot diffuse out. The proteolytic endo-enzyme of yeast 

 is called endo-tryptase. Its object is apparently the regulation of the pro- 

 tein-content of the cell and perhaps it has some bearing on the formation 

 of cell plasma. The possible relation between enzymes and growth is 

 discussed in a following sub-chapter. 



If yeast is mixed with a weak antiseptic (chloroform, toluol) the pro- 

 teolytic process takes place quite rapidly. This process is called autolysis 

 (self-digestion). Similar autoly tic enzymes are found in other micro- 

 organisms. Autolysis is a well-known process in the higher animals. 

 To this is due the ripening of meat. 



Proteolytic er^do-enzymes must be expected in all microorganisms 

 which depend upon protein as food material only. These organisms 

 will produce certain enzymes which diffuse out of the cell and decompose 



