OXIDASES, PEROXIDASES, AND CATALASE 95 



(10) Fe^^^ + HOr ^ Fe^- + HO2 



(11) Fe^^ + H2O2 = Fe^^^ + OH- + OH 



( 12 ) Fe+^ + OH = Fe*^^ + OH" 



The production of molecular oxygen is attributed to reactions 1 

 and 2a. 



It will be noted that the process, although involving free radicals, 

 is not a conventional chain reaction, since the enzyme iron plays 

 an active role in three stages: 10, 11, and 12. This is partly based on 

 the finding that in the model system platinum-hydrogen peroxide 

 an average chain length of only about five links may be assumed. 

 It is therefore considered probable that in the enzymatic catalysis, 

 under the usual experimental conditions, the chain reaction prac- 

 tically degenerates to a simple radical reaction. It will be recalled 

 that one of the criticisms directed against the chain reaction theory 

 of Haber and Willstaetter by Haldane was that one would expect 

 the rate to be proportional to the square root of the enzyme con- 

 centration rather than to the enzyme concentration itself, as is 

 actually the case. By postulating very short chains, as Weiss does, 

 the feature of the proportionality between reaction velocity, enzyme 

 concentration, and substrate concentration is retained without sacri- 

 ficing the essential concept of Haber, i.e., the postulate of inter- 

 mediate, monovalent radical formation. The further objection of 

 Haldane that the assumption of the same type of radicals (OH and 

 HO2) in various kinds of enzyme reactions, as was done by Haber 

 and Willstaetter, was in conflict with the well-known specificity 

 of the oxidizing enzymes, is also met if very short chains are as- 

 sumed. In this case the radicals are present only in so low a con- 

 centration that their oxidizing action on acceptors, e.g., iodide ion 

 or oxyphenols, remains below the threshold of sensitivity of the 

 analytical methods. All these considerations refer to the "normal" 

 course of the catalase reaction, i.e., to conditions where the enzyme 

 and the substrate concentration are within the range usually em- 

 ployed in kinetic studies and activity determinations. There can be 

 little doubt that the explosive type of hydrogen peroxide decompo- 

 sition, such as is produced in concentrated peroxide solutions when 

 a relatively large amount of enzyme is added, represents a chain 

 process with a long chain length. There may even be branched 

 chains, such as are assumed to occur during "knocking" in internal 

 combustion engines, when the relatively slow combustion along a 

 flame front of regular rate of progression changes over into detona- 

 tion (cf. Lewis and van Elbe, 40). 



