78 CONTEMPORARY SCIENCE 



quent experiments have but strengthened the conviction 

 that cells do produce these substances, and that the chemi- 

 cal changes are due not to the living organisms, but to the 

 lifeless substances (enzymes) to which these organisms 

 give rise. 



Minute in quantity, and tenaciously adhering to sub- 

 stances present, particularly protein, the isolation of an 

 enzyme in the pure state has become one of the most 

 difficult problems in physiological chemistry. Yet any 

 elementary student in the subject finds little difficulty in 

 performing simple experiments which convince him either 

 of its presence or of its absence. How are they done? 



The method consists essentially in making use of the 

 so-called "specificity" of enzymes. To use Fischer's 

 simile, just as one key fits one lock, so any one enzyme 

 will act on only a certain type of substance. Take, for 

 example, the enzyme found in saliva, ptyalin: it readily 

 acts on the carbohydrate, starch, but has absolutely no 

 action on protein. Again, take the pepsin of the stomach : 

 this enzyme breaks down proteins, but is without result 

 on carbohydrates. These instances may be multiplied in- 

 definitely. 



Some enzymes show their specificity to an even more 

 marked degree. In the yeast cell, for example, we find 

 one, sucrase, 1 which acts only on cane sugar (sucrose) ; 

 but on no other sugar or carbohydrate. A simple little 

 experiment demonstrates this beyond question. A yeast 

 cake is ground up very intimately with a little sand and 

 water, and the mass filtered. A small portion of the fil- 

 trate is added to a solution of cane sugar, the mixture 

 placed in an incubator kept at 38 C., and allowed to re- 

 main there for about 30 minutes or so. At the end of 

 that time the mixture, if heated with Fehling's solution, 2 



1 The ending "ase" denotes enzyme. 



2 This is the well-known alkaline copper solution used by all 

 medical men to test for sugar in the urine. The sugar in the 

 urine is not, as might be supposed, ordinary cane sugar, but 



