86 A SYMPOSIUM ON RESPIRATORY ENZYMES 



diflBcult to understand if the primary reaction of the enzyme with 

 oxygen were not a reversible equihbrium reaction. This is particu- 

 larly true for the photochemical dissociation of the iron-carbonyl 

 complex, where it is assumed that, under the influence of light, 

 carbon monoxide is reversibly exchanged with oxygen. All phe- 

 nomena may be satisfactorily explained on the basis of the assump- 

 tion that the primary step consists in the formation of a ferrous- 

 oxygen intermediate and that the further course of events is gov- 

 erned by the lifetime and reactivity of this complex. In the instance 

 of oxyhemoglobin the intermediate stage is fixed in a unique manner 

 under the influence of the globin component. In that of the oxidases, 

 the intermediate is very short-lived, with the possible exception of 

 the respiratory ferment in baker's yeast (75), where Warburg has 

 observed an absorption band which he tentatively attributes to an 

 oxygen addition compound of the enzyme. A unified theory of 

 oxidase, peroxidase, and catalase action could, then, be based on 

 postulating, with a fair degree of probability, the formation of 

 "moloxides" and "molperoxides," respectively, as the primary process 

 in the catalysis. The mechanism of the oxidation of hemoglobin to 

 methemoglobin by oxygen deviates from this schema because of 

 the high stability of oxyhemoglobin. The well-defined maximum 

 of reaction velocity at low oxygen pressures is in this instance 

 (10, 48) largely due to a decrease in the concentration of the free, 

 autoxidizable ferroform as the partial pressure of oxygen in the 

 system is increased. The rate of oxidation is proportional to the 

 concentration of reduced hemoglobin and to a function of the 

 oxygen pressure. In this case oxygen represents both a reactant and 

 an inhibitor, not only because of oxyhemoglobin formation but also, 

 perhaps, through breaking of chain reactions. 



On the Mechanics of Hemin Catalyses 



When considering reactions promoted by hemins, one almost in- 

 variably encounters the notion that they must all conform to the 

 pattern of the ferri-ferro cycle. There is no question that this is the 

 most handy explanation: everybody knows that the hemin iron may 

 exist in the reduced (Fe+^) and in the oxidized (Fe+++) state and that 

 this transformation may be accomplished in a reversible manner 

 even if no bases are linked to the heme. But like many generaliza- 

 tions, this concept as the only explanation is not only hazardous 

 but definitely too narrow. 



Let us go back for a moment to the theories that have been ad- 



