88 A SYMPOSIUM ON RESPIRATORY ENZYMES 



and replaced by fresh substrate molecules. In the well-known chain 

 reaction schemas, as developed by a number of authors, the metal 

 is assumed to initiate the reaction by reacting with a substrate 

 molecule and producing a molecule of high energy content ("energy 

 chains" of Christiansen and of Baeckstroem) or monovalent radicals 

 of high reactivity ("radical chains" of Haber and Franck), where- 

 upon the chain is propagated by such intermediary radicals or 

 "hot" molecules without the further participation of the metal. 

 The chain length, i.e., the yield in product molecules per elementary 

 act initiating the process, is determined by the probability with 

 which two of the intermediary radicals or energy-rich molecules 

 will collide and inactivate each other, and by the presence or ab- 

 sence of specific inhibitors, so-called "chain-breakers." It is im- 

 portant to note that in the metal catalysis of sulfite oxidation, cupric 

 copper and ferrous iron but not ferric iron are able to initiate 

 reaction chains (14). In this connection, the work of Haber and 

 Weiss (17) on the decomposition of hydrogen peroxide by ferrous 

 salt and the nature of the alpha -activity is of particular interest. 

 As is well known, a small amount of ferrous salt, when brought 

 together with a large excess of hydrogen peroxide, is oxidized to 

 ferric salt with the simultaneous liberation of oxygen. The yield 

 depends upon the rate at which the two reactants mix; the ratio 

 AHgOa/AFe'^'^ may reach values as high as 15.6. The underlying chain 

 reaction is formulated by the authors as follows: 



FV" + H2O2 = Fe^^^OH + OH 

 OH + H2O2 = H2O + OH 

 OH + H2O2 = 02 + H2O + OH 

 Fe^^ + OH = Fe^^^OH 



The last equation depicts a chain-breaking reaction. If all the 

 Fe'^^ has been oxidized, the reaction stops.* It is perfectly true that 

 this process, like many reactions studied by Wieland and his stu- 

 dents, is not a true catalysis but an induced reaction. But it is like 

 the true catalyses in that small amounts of a promoter or inductor 

 bring about the reaction of a disproportionately large number of 

 substrate molecules. On the other hand, we know that all enzymes 

 are slowly but irreversibly "consumed" during the reactions which 

 they catalyze. Only a finite, although large, amount of protein or 

 hydrogen peroxide can be split by a given quantity of proteinase 



" For an analysis of the catalytic decomposition of hydrogen peroxide by 

 ferric salts in acid solution the reader is referred to the papers by Haber and 

 Weiss (18), and Kuhn and Wassermann (36). 



