in] ENZYME ACTION 21 



afforded by the unicellular Fungus, Yeast (Saccharomyces), of which many 

 species and varieties are known. The feature of special interest in 

 connexion with the Yeast plant is its power of fermenting hexoses, with 

 the formation of alcohol and carbon dioxide, the process being carried 

 out by means of an enzyme, zymase. The reaction is generally 

 represented as follows, though there is little doubt that several stages 

 are involved : 



In addition to zymase, yeast contains the enzymes, invertase, protease, 

 peroxidase, catalase, reductase, glycogenase, carboxylase, a glucoside- 

 splitting enzyme, and some form of diastatic enzyme. The carboxylase 

 decomposes a large number of keto-acids, of which the most impor- 

 tant is pyruvic acid CH 3 CO COOH. The reaction involves the 

 formation of the corresponding aldehyde with the evolution of carbon 

 dioxide. Yeast also stores, as a reserve material, the polysaccharide, 

 glycogen, which occurs in animal tissues though it is rarely found in 

 plants : this is hydrolyzed by glycogenase into a monosaccharide. Finally, 

 yeast contains invertase, and most species, in addition, maltase, but from 

 a few species the latter enzyme is absent. Hence yeasts are able to 

 ferment the disaccharides, cane-sugar and maltose, since they can first 

 hydrolyze them to monosaccharides. 



As in the case of the enzymes of other tissues, those of yeast can be 

 made to carry out their functions after the death of the living protoplasm. 

 The method of demonstrating this is to "kill" the cells by means of 

 drying at 25-30 C., by treatment with a mixture of alcohol and ether, 

 or by treatment with acetone and ether. In this way the protoplasm is 

 destroyed, but the enzymes remain uninjured. Yeast treated thus has 

 been termed " zymin." 



From zymin some of the enzymes, e.g. invertase and the glucoside- 

 splitting enzyme, can be extracted with water: other enzymes, e.g. zymase 

 and maltase, are not so readily extracted. From the living cells the 

 enzymes are only obtained with difficulty, the extraction of yeast juice, 

 containing zymase and other enzymes, needing, by Buchner's method, 

 a pressure as great as 500 atmospheres. 



In connexion with alcoholic fermentation by zymase, the following 

 point is of special interest. For carrying out this process, two other 

 substances are necessary in addition to the enzyme, i.e. a co-enzyme of 

 unknown nature and a phosphate. The separation of zymase from the 

 co-enzyme has been accomplished by filtering expressed (Buchner) yeast 

 juice through a special form of gelatine filter under a pressure of 50 



