84 A SYMPOSIUM ON RESPIRATORY ENZYMES 



peroxidatic, and catalatic activity. But the enzyme peroxidase has 

 lost the power of splitting hydrogen peroxide without the presence 

 of an oxygen acceptor, and, conversely, the enzyme catalase is ap- 

 parently devoid of any peroxidatic activity. The faculty of reacting 

 with molecular oxygen, finally, is reserved to a small number of 

 autoxidizable hemin proteins, i.e., the respiratory fennents and the 

 Pasteur enzymes. The ultimate in specialization is reached with 

 hemoglobin, which alone of all iron derivatives and hemochromo- 

 gens will combine with but will not be oxidized by molecular oxy- 

 gen. The formally analogous reaction with carbon monoxide is 

 shared by most ferrous complexes, such as ferrous cysteine and 

 ferrohemochromogens of all types. As may be seen from Tables 4 

 and 5, at least two diflFerent types of hemins occur in hemin enzymes, 

 namely, the red protohemin and mixed-colored pheohemins. 



Enzyme-Substrate Intermediates 



It is diflBcult to understand why there should be so much con- 

 troversy over the existence of well-defined intermediates in the 

 course of enzymatic catalyses in general and hemin catalysis in 

 particular. Once it is admitted that enzymes, like all other catalysts, 

 exert their function not by mystical, long-range forces but by actu- 

 ally combining, at some stage of the process, with their substrates 

 and thus creating a new pathway which, although more complex, 

 yields a higher overall rate of reaction, it will depend solely on the 

 lifetime of these intermediates and on their spectroscopic or other 

 properties whether their existence can be detected by experimental 

 means. There are now on record several perfectly clear-cut cases 

 where such enzyme-substrate compounds have been demonstrated 

 by the spectroscope: the intermediate formed in the catalase-ethyl 

 hydrogen peroxide reaction (60), the complexes foiTiied between 

 peroxidase and various proportions of hydrogen peroxide (28), and 

 the less clear-cut observations on complexes between catalase, 

 hydrogen peroxide, and certain inhibitors, such as hydrazine and 

 hydroxy lamine (26). The first-named complex exhibits precisely the 

 behavior that the kinetic theories of Michaelis and Henri postulate 

 for an intermediate enzyme-substrate compound: it is unstable and 

 disappears at the rate at which the end products of the reaction are 

 formed. The peroxidase-hydrogen peroxide complexes are also un- 

 stable, but their decomposition cannot be due to a true peroxidatic 

 reaction, since it occurs also in the absence of oxygen acceptors. 

 However, the recent work by Karush (25) and Chance (12) on 



