ENZYME-SUBSTRATE COMPOUNDS 



a turnover number of 4 X 10^ sec.~^ at 25° C, the fastest 

 enzymatic process yet measured. The turnover number is 

 calculated from the quotient of the initial rate of peroxide 

 decomposition and the enzyme concentration (4.6 Af/sec./ 

 1.2 X 10-7 M = 4 X 107 sec.-^). This result differs dra- 

 matically from that of George, who used the manometric 

 method (35). 



Cone of HpO- 



(M) 



Fig. 2. Experimental study of catalase activity at high H2O2 con- 

 centrations by means of the quenching-flow apparatus (Ogura, unpub- 

 lished data and ref. 55). 



The ordinates represent the initial rate of H2O2, decomposition in 

 moles per liter per second in the presence of 1 .2 X 1 ~^ Af liver catalase. The 

 extrapolated maximal rate is 4.6 M/scc. at 30°G., pH 7.0. 



At very high peroxide concentrations the theoretical anal- 

 ysis above is inadequate to describe the experimental result and 

 it appears that an unidentified intermediate in catalase action 

 becomes rate-limiting in 5 Af peroxide. However, the life- 

 time of such an intermediate is very short; at the plateau of 

 Figure 2 the turnover time is only 0.693/4 X 10^ = 2 X 10-^ 

 sec. at 25° C* 



* In our previous discussion of the existence of ternary complexes in 

 catalase action we set the lifetune at a value less than 3 X 10~* sec. (11). 

 These data indicate that the lifetime may be 2 X 10~* sec, somewhat shorter 

 than our crude estimate. 



313 



