86 CONCENTRATION OF ENZYME 



of substratum has been large and not very widely varied before 

 the measurement has been taken, and especially those in which, 

 where possible, a solid substratum has been employed. 



As Bredig has pointed out, the result would be more certain, 

 and more definite conclusions could be drawn if, in such experi- 

 ments, instead of measuring the different amounts of substratum 

 converted in equal times by varying amounts of enzyme, deter- 

 minations were made of the varying intervals of time necessary 

 to convert the same percentage of substratum as the concentra- 

 tion of enzyme is changed. For in the latter case whatever the 

 law may be governing the course of the reaction, and as we 

 have seen above this may be somewhat complicated, since the 

 reaction in such case runs to the same stage, in deducing the 

 ratio of the increases in velocity, due to the two different con- 

 centrations of enzyme, this complicated factor eliminates out, 

 being the same in each case, and the ratio in the activation 

 by the two quantities of 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 measurement or determination must be made 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 disappear- 

 ance 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 



1 This does not apply to the electrical conductivity method of Bayliss, who 

 has utilised the method suggested by Bredig, and determined the times at which 

 equal changes in conductivity occur. 



