OXIDASES, PEROXIDASES, AND CATALASE 87 



vanced to explain the catalytic or semi-catalytic effects of iron and 

 other metal ions (37). As in the case of the hemins, we find a 

 widespread inclination to explain everything by a valency change 

 of the metal during catalysis: in the first stage the substrate reduces 

 the trivalent iron (or the bivalent copper) to bivalent iron (or mono- 

 valent copper); in the second stage the oxidizing agent, e.g., oxygen 

 or hydrogen peroxide, regenerates the ferric iron or cupric copper, 

 respectively. There are unquestionably instances where this simple 

 hypothesis will serve to explain all the facts, e.g., in the metal- 

 catalyzed oxidation of phenols by oxygen. But in many other in- 

 stances the hypothesis proves inadequate. Two of the main obstacles 

 are, first, the fact that bivalent iron salts and complexes are so 

 often superior in catalytic activity to the corresponding trivalent 

 compounds and, second, the phenomenon of the "primaerstoss" 

 (alpha-activity). By this we mean the frequent observation that 

 oxidation catalyses, in the presence of ferrous ions, exhibit an initial 

 phase of high velocity which is followed by a steady state of a 

 much lower reaction rate (beta-activity). The transition of ferrous 

 into ferric iron in the course of this process is accompanied by a 

 turnover of many more substrate molecules than would correspond 

 to the number of iron equivalents. 



Three different theories have been proposed to explain the facts. 

 All postulate the fonnation of labile and highly reactive intermedi- 

 ates which are formulated as peroxides by Manchot, as complex 

 compounds by Wieland, and as free radicals by Christiansen, Baeck- 

 stroem, and Haber. Manchot assumes that only the bivalent iron 

 is capable of forming peroxides of the type Fe^Og or, more recently, 

 of molperoxides of the type Fe'^^HoOg. Such peroxides could oxidize 

 two substrate equivalents and, subsequently, react with hydrogen 

 peroxide present in excess to yield inactive ferric iron, or they could 

 interact with excess hydrogen peroxide to form oxygen while the 

 ferrous iron is regenerated. Wieland, on the other hand, believes 

 that the ferrous iron forms a complex with the substrate or with 

 other substances present in the system with the possible inclusion 

 of the oxidizing agent. By the complex formation tlie substrate 

 hydrogen is "activated" and is thus made accessible to the attack 

 by the oxidant. The alpha-activity ("primaerstoss") is attributed to a 

 temporary protection of the active ferrous iron contained in the 

 complex against the transition into inactive ferric iron. In this way 

 one ferrous ion is enabled to oxidize a larger number of substrate 

 molecules, the oxidized molecules being released from the complex 



