412 IX. HEMATIN ENZYMES, 11 



a minimal velocity of 8 X !'>" liter mole"' sec."'. Still higher values have 

 been estimated by Brdicka and co-workers (33^) on the basis of polaro- 

 graphic experiments. 



Between pK 6 and 8 the activity of catalase is not affected by pH, but 

 below pH 6 it decreases rapidly with decreasing pH, particularly in acetate 

 buffer (cf. Section '^.3.). 



The Michaelis constant was found to be about 0.03 M {72U2GJf7). Large 

 concentrations of hydrogen peroxide inhibit the action of the enzyme; 0.4 M 

 hydrogen peroxide, for instance, inhibits .50% (2f>Jf7). It is to be noted that 

 the oxidimetric estimation of catalase is carried out at a substrate concen- 

 tration (0.005 M) somewhat below the optimal concentration (0.07 M). The 

 greater k/Fep found by Zeile for pumpkin catalase {31 58), as compared with 

 liver catalase, may thus be due to the greater substrate affinity of the former. 



Inhibition by destruction of catalase. A form of inhibition of cata- 

 lase exists which differs essentially from inhibition by cyanide. 

 Lemberg and Legge {1705) have shown that the inhibition of the 

 catalatic action by ascorbic acid {2829) is due not to a decrease of k 

 but to an increase of k', i.e., to faster inactivation of catalase by 

 hydrogen peroxide. This also holds for inhibition by sulfhydryl com- 

 pounds, which inhibit at rather high concentrations — about 10~^ M 

 (Stern, 26Jf7). Waldschmidt-Leitz {2913) found the cysteine inhibi- 

 tion to be irreversible. It has been shown by Lemberg and Legge 

 that the destruction of catalase in the presence of ascorbic acid is 

 due to the action of hydrogen peroxide on ferrous heme iron and the 

 peroxidative autodestruction of catalase, with formation of bile pig- 

 ment hematin {cf. Chapter X).* Keilin and Hartree {11^90) have 

 observed that, in the presence of cysteine, glutathione, and azide, 

 the action of catalase is inhibited by carbon monoxide, and that this 

 inhibition is abolished by light. The inhibition by BAL (2,.3-dimer- 

 capto-1-propanol) is also increased by carbon monoxide {1699).\ 

 There is thus definite evidence for a change of valency of the catalase 

 iron under these conditions. 



Some "anticatalase" preparations of Battelli and Stern {191, 2237, 2662) 

 evidently contained reducing substances. In liver extracts catalase is acti- 

 vated by — S — S — compounds such as cystine and oxidized glutathione 

 and by othe^ oxidants (Balls and Hale, 126). Such effects have been described 



* This does not hold, however, for the effect of ascorhic acid on catala.se in the 

 presence of dilute hydrogen peroxide. Under these conditions, there is no evidence 

 that the catalase iron is reduced (Foulkes and Leml)erg, '.)21a). Carbon monoxide 

 does not increase the ascorbic acid inhibition; on the contrary, it decreases it by bind- 

 ing copper. ' 



t BAL does not remove the hematin iron, as postulated l)v Webb and van Heyningen 

 {3005a). 



