BRITTON CHANCE 



39 



This equation relates the velocity of disappearance (dx/dt) of hydrogen 

 peroxide to ki and k^ . It can be seen that either ki or k4 can be determined 

 for appropriate ratios of the substrate and hydrogen donor concentrations. 

 Conditions for the guaiacol assay of peroxidase activity which permit a 

 determination of these two reaction velocity constants and preliminary 

 values for the effect of temperature upon these two velocity constants are 

 reported (8). However, the range of concentration over which the de- 

 termination can be made is rather limited, as is the choice of hydrogen 

 donor molecules. This appears to be, however, one of the few cases in which 

 the principal velocity constants, ki and k4 , can separately be determined 

 from the overall reaction kinetics and this is because the enzyme-substrate 

 compounds in this reaction have already been studied directly. 



In catalase, the effect of temperature upon the velocity constant for the 



Fig. 1. Effect of temperature 

 upon the overall activity of 

 catalase in the decomposition 

 of hydrogen peroxide. Bacterial 

 catalase activity assayed by the 

 change of ultraviolet absorption 

 due to the disappearance of hy- 

 drogen peroxide (3). Data from 

 Biocheni. J. 



overall reaction is small (fig. 1). A study of the reaction mechanism shows 

 that the rate of disappearance (dx/dt) of substrate depends upon both 

 ki and ki and dx/dt increases linearly with the concentration of hydrogen 

 peroxide (9, 10) up to 0.5 m (11): 



dx/dt = — xe 



d 



(3) 



,l/k4 + 1/ki 



Fortunately, from observations of the steady state concentration of the 

 enzyme substrate compound a ratio of the velocity constants can be de- 

 termined : 



Pm 



e 



1 + ^ 



U) 



and their individual values calculated from the two equations above. 

 Thus, the effect of temperature on the overall reaction kinetics cannot be 



