AND VELOCITY OF REACTION 87 



Bredig have little practical weight, as, for example, where solid 

 substratum is used in determining activity in varying concentra- 

 tion of proteoclastic enzymes, or where the amount of substratum is 

 initially large compared to the amount of enzyme, and the measure- 

 ments are taken early in the course of the reaction before there 

 is any large alteration in percentage of the substratum. In such 

 cases none of the variations in the reaction detailed above occur 

 in the short stage of reaction utilised for the determinations, 

 and the degree of activation by the enzyme may be safely 

 taken as directly proportional to the amounts converted in equal 

 times. 



The law connecting velocity of action and concentration of 

 the enzyme varies with the nature of the enzyme. In many cases 

 where the question has been accurately examined in recent times, 

 the ordinary law which applies to inorganic catalysts applies also 

 to enzymes, viz. that the effect is in simple direct proportion to 

 the concentration in enzyme. There is this difference, however, 

 in the case of enzymes, that a maximum is soon reached beyond 

 which further addition of enzyme produces no noticeable effect 

 whatever, and it is hardly necessary to add that for concentra- 

 tions somewhat short of the maximum, the linear law does not 

 hold, as the linear portion of the curve gradually rounds off to the 

 asymptotic line which marks the maximum velocity with increasing 

 concentration. Having regard to the high molecular weight 

 which enzymes possess and correspondingly low molecular con- 

 centration, and also the low percentage amount present when 

 the maximum amount of increased effect with concentration is 

 obtained, we have here incontrovertible evidence of a difference 

 in mode of action of enzymes and inorganic catalysts ; such, for 

 example, as sucroclastic enzymes and the hydrogen ion of acids. 

 Here the sucroclastic enzyme is already in possession of its 

 maximum effect at a molecular concentration, at which the action 

 of the hydrogen ion is practically imperceptible, and the hydrogen 

 ion goes on increasing in effect, as the concentration is increased, 

 at a rate considerably greater than corresponds to the increased 

 ionic concentration, while the action of the enzyme remains at 

 a constant level. 



The enzymes which within the limits indicated above obey 

 the law of direct proportionality between concentration and 

 activity are : Invertase (O'Sullivan and Tompson, Henri, E. E- 



