76 PHYSICOCHEMICAL BASIS OF PHYSIOLOGICAL PROCESSES 



solution is 0.002, this means that when volume, concentration of acid and 

 temperature are constant in a gram-molecular solution of sugar, 0.002 

 gram-molecule of sugar would be inverted the first minute and 0.002 

 gram each succeeding minute, provided we could keep the solution con- 

 stantly a gram-molecular one, that is, provided we could add sugar just 

 as quickly as it becomes inverted. 



At first sight it may appear of little practical importance to determine 

 K. In our present discussion concerning the nature of enzyme action, 

 it is however of great value for, whereas with inorganic catalysis K is 

 really of constant value, with enzyme action it is not so. Thus, when 

 cane sugar is inverted by sucrase an enzyme present in the intestine 

 and in yeast the constant gradually rises; for most other unimolecular 

 reactions mediated by enzymes it gradually falls ; for example, the action 

 of trypsin on proteins. 



Where there is a great excess of substance to be acted on, in compari- 

 son with the amount of enzyme present, it will be found that a more 

 constant value than K is obtained when we compute the absolute amount 

 of substance decomposed in a given time. In such a case, too, the 

 amount of change in a given time will be proportional to the amount of 

 enzyme present, indicating that some sort of combination between en- 

 zyme and substrate must be the first step in the fermentative process. 

 This fact has been noticed by us in connection with the hydrolysis of 

 glycogen in the liver. When there is an excess of glycogen present, the 

 amounts which disappear in equal intervals of time after death are the 

 same; when, on the contrary, there is not much glycogen, the amount 

 which disappears gradually declines, but, if K be computed by the above 

 equation, it is constant. 



To make these facts clear it may be well to pause for a moment to 

 consider an illustration. The conditions obtaining when there is a large 

 excess of substrate over enzyme may be compared to those governing 

 the removal of a pile of bricks from one place to another by a number of 

 men. The pile of bricks represents the substrate ; the men, the enzyme. 

 If each man works up to his capacity, it is plain that the number of 

 bricks transferred in a given time will 'not depend at all on the size of 

 the pile to be transferred. When, however, the pile of bricks gets small, 

 though the same number of men continue to Avork the number of bricks 

 transferred in a given time falls off, because the men interfere with one 

 another's activities in securing their loads from the pile. When a similar 

 stage is arrived at in enzyme processes, we have to use the velocity con- 

 stant to show how much work could be clone by the enzyme if the amount 

 of substrate were maintained of constant amount. 



In the large volume of recent work which has been done with the 



