CATALYSIS AND ENZYMES 313 



hydrolysis of a substrate into simpler molecules. As the equilibrium position is 

 approached, the opposing synthetic reaction becomes more and more marked by 

 mass action of the increasing products of hydrolysis, and, at the equilibrium 

 position, becomes equal in rate to the hydrolytic one. It is unnecessary to remind 

 the reader that what is spoken of here is the actual rate, that is, the total 

 amount of change in a given time, not the velocity constant, which is, of course, 

 independent of the active masses. 



Under the conditions in which most enzyme experiments are made in vitro, the 

 equilibrium position is so near that of complete hydrolysis, owing to the excess of 

 water present, that the synthetic reaction is too small to exercise any very 

 perceptible influence, so that. .other causes for the slowing of hydrolysis must be 

 sought for. When the conditions are such that the synthetic reaction is 

 considerable, as in the cases of lipase and emulsin quoted at the beginning of 

 this chapter, the reversibility of the reaction plays an important part. Fig. 80 

 (page 300) shows how the hydrolytic reaction in the case of lipase is favoured by 

 the presence of water. 



The second thing to be considered is that the rate of a catalysed reaction is 

 dependent on the concentration of the catalyst. If, then, anything happens 

 during the course of the reaction which diminishes the amount of enzyme present, 

 either actually, by destruction, or effectively, by paralysing its activity, the result 

 will be a progressive diminution in the rate of the reaction. We note that this 

 disappearance of enzyme may be irreversible, when there is actual destruction, or 

 reversible, when it is merely removed from the sphere of action, either by 

 temporary paralysis, caused by the products of the reaction, or by adsorption on 

 the surface of some substance present in the system, as is the case with many 

 of the so-called " anti-enzymes." 



It is well to mention here that the particular cases which are used in the following pages 

 for illustrative purposes are not to be taken by the reader as the only ones of the kind known. 

 Numerous others will be found in my monograph (1913, 2). 



We will take first the question of the actual destruction of enzyme. A solution 

 of any enzyme is found to lose its activity if kept. This is, in great part, due to 

 the complex colloidal state of these substances. The rate of this loss of activity 

 varies very much according to the individual case, and is accelerated by rise of 

 temperature. The time course of the process was investigated by Tammann (1892, 2) 

 in the case of emulsin. In the presence of substrate the rate of destruction is 

 much decreased (Bayliss and Starling, 1903; Vernon, 1904), although not entirely 

 prevented. This protective action of substrate has been ascribed to chemical 

 combination with the enzyme. Without denying that this may sometimes occur, 

 I have found that mere adsorption (Bayliss, 1911, 1) by charcoal is protective in 

 the case of trypsin, and there is no proof that this may not be the general 

 explanation. 



With very low concentration of enzyme, even in the presence of substrate, activity has 

 been found to disappear before completion of the reaction (Tammann, 1892, 2, and Bayliss, 1913, 1 , 

 p. 248); this fact has led to certain erroneous statements with regard to "false, equilibrium." 

 On the other hand, experiments made for the purpose (Bayliss, 1904) have shown that, in the 

 case of trypsin, there is no detectable loss during a few hours, although the velocity constant 

 has diminished considerably. The spontaneous destruction of enzyme is, therefore, not the 

 sole cause of the decrease in activity. 



Reversible Inactivation. It is found by experiment that the addition of certain 

 substances, amongst which are to be found the products of the reaction itself in 

 a large number of enzymic reactions, has a great effect on the activity of enzymes. 

 This action is brought about in several different ways : (1) Enzymes are colloids, 

 and therefore liable to aggregation or precipitation by a variety of agents. 

 Emulsin is precipitated by benzaldehyde, so that, even in the small quantities 

 produced in the course of the hydrolysis of amygdalin, it is probable that a certain 

 degree of aggregation and diminution of active surface is produced. We have seen 

 (page 301) that enzymes act by their surfaces, and further evidence will be given 

 presently. (2) Most enzymes are extraordinarily sensitive to changes of hydrogen 

 ion concentration. This is a common property of the colloidal state and points 



