WHAT ARE ENZYMES? 79 



will yield a red-brick precipitate a result which could 

 not be obtained either with the cane sugar, or with the 

 enzyme solution alone. No other carbohydrate solution 

 or protein, or fat solution, for that matter can take 

 the place of the cane sugar; our enzyme will be without 

 effect. If we take our original yeast extract, and first 

 heat it to, say, the boiling point of water, then cool it, and 

 from here on repeat the experiment as before, no grape 

 sugar is obtained. If instead of heating the enzyme solu- 

 tion, we cool it, the action is considerably delayed. 



Some of the yeast extract may be poured into an ex- 

 cess of alcohol, the precipitate separated by filtration, and 

 redissolved in water. This solution will show all the 

 properties of the yeast extract. 



Evidently, then, the watery extract of yeast contains 

 something which has the power of breaking down cane 

 sugar. This something is exceedingly sensitive to heat, 

 rather less so to cold, and is precipitated together with 

 other substances (as could be shown) by alcohol. The 

 last three properties are characteristic not only of sucrase, 

 but of all enzymes to a greater or less degree. That a 

 minute quantity of enzyme can act upon an exceedingly 

 large quantity of substrate is also readily demonstrable. 

 The laws of catalysis hold firm. 



One other fact about enzymes is most important. Gra- 

 ham, as far back as 1861, found that certain substances 

 (cane sugar, salt, etc.) in solution, when placed in a dialy- 

 zer consisting of a parchment bag, which in its turn was 

 surrounded by water, would diffuse through the bag, 

 whereas others (proteins, gum, starch, etc.) would not. 

 The diffusible ones he named crystalloids, those non-dif- 

 fusible, colloids. If some of our original yeast extract 

 were placed in such a parchment bag, none of the enzyme 



grape sugar. Fehling's solution reacts with the latter, but not 

 with the former. 



The cane sugar is split or "hydrolyzed," by the sucrase, one of 

 the products being grape sugar. 



