ENZYMES — KILBY 275 



square inch in a hydraulic press. An opalescent brownish-yellow 

 juice was obtained which was free from yeast cells, and this was used 

 for a number of animal experiments. It was not altogether satis- 

 factory, for it soon putrefied. The nature of the experiments excluded 

 the addition of the usual antiseptics, and so the Buchners decided 

 to add sugar as a preservative. To their surprise, fermentation began 

 immediately, the sugar being converted into alcohol in spite of the 

 complete absence of living yeast cells. 



Pasteur's thesis was thus disproved. But closer consideration shows 

 that both Pasteur and Liebig were partly right and partly wrong. 

 Fermentation could take place in the absence of living organisms, 

 and it was chemical in nature but not in the way Liebig had thought. 

 Certain ferments were produced by the living yeast and normally 

 functioned inside the cell to cause the breakdown of the sugar which 

 was serving as a food for the yeast. The nature of the yeast used 

 in making active juice is rather important. Some strains, such as 

 Munich "bottom" yeast (which rapidly settles to the bottom of fer- 

 mentation vats) gives active juices, whereas others, including some 

 Parisian and English "top" yeasts (which form a thick scum on the 

 surface), give quite inactive juices. It is possible that Pasteur may 

 have experimented with such a "top" yeast and been unable to prepare 

 any active extracts. A more fortunate selection might have enabled 

 him to anticipate the Buchners by more than 30 years. 



The production of this active juice meant that it was no longer 

 necessary to consider the possibility of some mysterious vital force 

 as being concerned in fermentation. The mystery should be explicable 

 in purely chemical terms. The juice could be easily made in quantity, 

 and an immense amount of work has since been done on its properties. 

 Its activity was originally ascribed to the presence of a ferment called 

 zymase, but it is now known that the break-down of sugar into alcohol 

 involves at least 15 separate and successive stages, and almost as many 

 ferments or enzymes. The unraveling of the intricacies of alcoholic 

 fermentation and the isolation of many of the intermediate compounds 

 and separate enzymes are among the great triumphs of biochemistry. 

 In 1925, Meyerhof published a method for preparing a juice from 

 muscle which would convert in a test tube glycogen (the animal analog 

 of starch) into lactic acid, a change that is known to occur in living 

 muscle and to provide the energy for contraction. This conversion 

 also has been found to take place in about 15 stages, which, except 

 for the initial and final ones, are identical with those in yeast. This 

 is a good example of the essential unity of biochemistry, where living 

 organisms of widely different character are found to involve the same 

 biochemical processes, with slight modifications to suit their particular 

 needs. 



