224 SPECIFIC NATURE OF CATALYTIC ACTION 



posal. Thus, aspergillus cultivated on a nutrient medium con- 

 taining lactose or /3-methyl galactosid acquires the property of 

 hydrolysing these, while if grown upon a-methyl galactosid the 

 property is acquired of attacking this substance. 



The outcome of these investigations appears, then, to be that 

 by specific action must be understood entire conformity between 

 the particular enzyme and a corresponding molecular grouping 

 or structural arrangement in the molecule attacked, and not that 

 a single substance only is attacked by the same enzyme. 



It may perhaps, in conclusion, be pointed out that this serves 

 to explain what sometimes seems a most fantastic distribution of 

 certain enzymes in nature. Thus the stomach of the fish contains 

 a milk-curdling enzyme, similar to that of the mammalia, although 

 such an enzyme never comes in contact with milk, and never has in 

 the development of the fishes. The presence of such an enzyme 

 can scarcely be regarded as a provision of Nature for the coming 

 mammalia, and points to the fact that the milk-curdling enzyme 

 must have other functions than the coagulation of milk. Since 

 such ferments are also found in plants, it follows that they must, 

 like the sucroclastic enzymes, be adapted to some definite molecular 

 grouping upon which they act, and that milk coagulation can be 

 but one example of their activity. 



There is room for research in this region of biochemistry. 



THE CHEMICAL AND PHYSICAL NATURE OF ENZYMES. 



Little is known regarding the chemical nature of enzymes, 

 because all attempts to isolate them in a state of purity have 

 hitherto failed. In fact, there is nothing to give certainty that 

 at the end of any process the product in the case of such com- 

 plicated substances is pure, a remark which applies equally to 

 ordinary proteins. None of the criteria of purity in the case of 

 an ordinary crystalloid apply in the case of a colloid of complex 

 constitution, except constancy of percentage composition. It does 

 not crystallise out, 1 it has no definite melting-point, it does not 

 affect the freezing-point or boiling-point, 2 it cannot be synthesised 

 by reactions which can be followed in their course, and it is probably 



1 Unless when in combination with other bodies which confer crystalloid 

 properties. 



2 At least to such an extent that measurements can be made. 



