198 EXPERIMENTAL OBSERVATIONS ON 



enzyme is simply inversely proportional to the two time- intervals 

 for production of the same percentage change. 



While it must be admitted that this method of varying time- 

 interval and constant percentage of conversion is the more scientific, 

 it must, however, be stated that it is in most cases of zymolytic 

 action most difficult or impossible to carry out experimentally. 

 For in most such cases we have no indicator to show when a certain 

 definite percentage of the total change has occurred, and a some- 

 what elaborate method of determination must be introduced 1 

 in order to discover the state of affairs in the solution, so that 

 the experimenter is reduced to making measurements at definite 

 time-intervals instead of at definite amounts of conversion. Nor 

 will it do to take the end-point of the reaction in most cases on 

 account of the slowness with which that point is reached, although 

 in certain cases where the end-point has special physical or 

 chemical characteristics this has been used as, for example, 

 the coagulation point for an enzyme such as rennin, or the dis- 

 appearance of the colour test with iodine in the case of starch and 

 diastase. 



It may be further added that in many cases the objections of 

 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 



1 This does not apply to the electrical conductivity method of Bay lisa, 

 who has utilised the method suggested by Bredig, and determined the times 

 at which equal changes in conductivity occur. 



