OF VITAL PHENOMENA 87 



Many hypotheses have been made to account for this anomaly. 

 An increase in the substrate concentration may increase the vis- 

 cosity and therefore decrease the speed of diffusion. An in- 

 crease in the beginning concentration of the substrate means 

 an early accumulation of reaction end products which may be 

 detrimental to the reaction, either by starting a reaction in the 

 reverse direction or by combination with the enzyme. This 

 hypothesis is supported by the fact that the splitting of cane 

 sugar by invertase is retarded more by fructose than by glucose, 

 and mannose or glycerine have about the same inhibiting activity 

 as glucose. It has been supposed that the enzyme may become 

 saturated with the substrate, based on the fact that in some 

 cases the rate of reaction depends on the concentration of the 

 enzyme more than on the concentration of the substrate. 



In some cases the products of the reaction activate the enzyme 

 and hence the rate of reaction constantly increases. This type 

 of reaction is called autocatalytic, but it is to be remembered 

 that it does not catalyze itself in the sense of furnishing an 

 enzyme, but increases the rate by activating an enzyme already 

 present. Thus, alcoholic fermentation either with living or dead 

 yeast increases with the accumulation of the end products up 

 to a certain point. Hoyer (1907), in studying the saponification 

 of castor oil during the autolysis of ground castor beans, ob- 

 served that the reaction proceeded slowly until the H ion con- 

 centration reached a certain point, when it suddenly increased. 

 If the H ion concentration were increased at the start by the 

 addition of acids, the splitting of the oil was rapid from the 

 first. The higher fatty acids produced by the splitting of castor 

 oil were probably not sufficiently soluble to produce the required 

 PH alone, and were aided by C0 2 and lactic acid appearing in 

 the mixture. But we may imagine the splitting of the ester of 

 a lower fatty acid by the same enzyme, in which the fatty acid 

 produced would cause the rate of reaction to increase. This 

 might be called autocatalysis. According to M. Morse (1916) 

 the autolysis of ground animal tissues is autocatalytic, since the 

 rate of proteolysis is proportional to the PH which constantly 

 increases during the reaction in a characteristic curve. Bradley 

 and Taylor (1916) conclude that H ions act not on the enzyme 

 but on the substrate, increasing the number (and consequently 

 the total surface area) of the colloid particles. 



