96 A SYMPOSIUM ON RESPIRATORY ENZYMES 



It is not feasible, within the space allotted to this discussion, to 

 cite all the experimental evidence and theoretical arguments in 

 favor of the intermediary formation of free radicals or of the chain 

 reaction character of the hydrogen peroxide catalysis or of oxidative 

 enzyme action in general. It must suffice, therefore, to refer to the 

 original publications where the existence of OH radicals during 

 hydrogen peroxide photolysis (73a) or the formation of the HO2 

 in reactions with molecular oxygen (78a) have been observed. The 

 experiments of Schwab et al. (53a) on the effect of typical chain- 

 breakers on the catalase reaction did not yield conclusive results. 

 But the experiments of Barron (5) on the effect of antioxidants 

 on the rate of oxidation of unsaturated fatty acids by hemochromo- 

 gens are very suggestive. The pioneer work of Michaelis on the 

 radical nature of the semiquinones, which arise through monovalent 

 reduction or oxidation of reversible dyestuffs makes it all but im- 

 perative to admit the intermediary existence of monovalent radicals 

 (monohydropyridine, monohydroalloxazine, monohydrothiamine, 

 etc.) during cellular respiration, unless reactions of high order are 

 postulated. 



Those who are interested in the manner in which the action of 

 other oxidative enzymes, e.g., peroxidase or dehydrogenases, may be 

 interpreted in terms of radical chain mechanisms are referred to the 

 papers by Haber and Willstaetter (19), Weiss (79), and the review 

 article by Moelwyn-Hughes (45). 



AUTOXIDIZABLE IrON COMPOUNDS 



We return to the starting point of the discussion: What makes 

 the iron atom and the oxygen molecule "click" during the primary 

 process? Why are some iron compounds autoxidizable and others 

 not? No satisfactory answer may at present be given to this ques- 

 tion, which certainly attracts the attention of many workers in the 

 field of biological oxidation. Only partial solutions have been of- 

 fered to this key problem. 



Ferrous iron is relatively stable in acid solution and is rapidly 

 oxidized to ferric iron by molecular oxygen in alkaline solution. 

 The state of the metal under these two conditions obviously differs 

 in one respect. In the acid medium the iron is present as ferrous ion, 

 whereas in alkaline solution it exists as non-ionized ferrous hydrox- 

 ide. Smythe (54), in an interesting paper, points out that the oxi- 

 dation of ferrous ion involves the separation of a negative charge 



