90 A SYMPOSIUM ON RESPIRATORY ENZYMES 



peroxide reaction contains the enzyme in the ferric state (61). 

 Carbon monoxide, which will inhibit many hemin catalyses where 

 ferrous iron is involved, has little or no specific effect on the catalase- 

 hydrogen peroxide reaction (61), other reports to the contrary 

 notwithstanding. Some years ago Haber and Willstaetter (19) pro- 

 posed a chain reaction schema for this catalysis which embodied, 

 as the initial step, a reduction of the enzyme iron to the ferrous 

 form. A little later the present writer modified this schema some- 

 what (55) with a view to avoiding the necessity of assuming a 

 ferri-ferro cycle. The initial step in that schema consisted in the 

 interaction of two adjacent porphyrin-bound ferri atoms with one 

 hydrogen peroxide atom to yield two monovalent OH-radicals which 

 could then propagate the chain. The iron was assumed to remain in the 

 trivalent form throughout. More recently Keilin and Hartree have 

 advanced a different hypothesis, incorporating the idea of the ferri- 

 ferro cycle (27): 



Step I 4 Fe"*^ + 2 H^O^ = 4 Fe"^ + 4 H" + 2 O, 



Step II 4 Fe^" + 4 H^ + O^ = 4 Fe^^^ + 2 H2O 



2 H2O2 = 2 H2O + O2 



The authors state that, in accordance with this schema, the 

 catalysis is greatly inhibited or even suspended in the absence of 

 free oxygen. It will be noted that the hydrogen peroxide is here 

 assigned the role of a specific reducer of the ferri form of catalase, 

 whereas molecular oxygen is considered as the oxidizing agent in 

 the regeneration of the ferri form. The concept of hydrogen peroxide 

 as a reducing agent in itself, although somewhat startling, is not 

 new and finds support in the earlier finding of Kuhn and Wasser- 

 mann (36) that ferric salts, in the presence of such complex formers 

 as a, a'-dipyridyl or o-phenanthroline, are quantitatively reduced by 

 hydrogen peroxide. But the schema is open to other objections, both 

 on theoretical and on experimental grounds. Thus Johnson and 

 van Schouwenburg (24), Weiss and Weil-Malherbe (80), Sumner and 

 Dounce (cf. 68), and the writer (61) failed to confirm the observation 

 of Keilin and Hartree that catalase is inactive under strictly an- 

 aerobic conditions. Furthermore, it seems somewhat strange that, 

 according to the schema of these workers, molecular oxygen should 

 be required for the reoxidation of the ferrous form of catalase in a 

 system containing hydrogen peroxide, which is generally considered 

 a more active oxidizing agent than oxygen, as Dr. M. Gorin has 

 pointed out, in a private communication to the writer. According 



