42 A SYMPOSIUM ON RESPIRATORY ENZYMES 



THE PHYSICO-CHEMICAL MECHANISM OF 

 HYDROGEN TRANSPORT 



Kurt Stern, Yale University: 



Dr. Ball, on Thursday morning, spoke of a gap between the cyto- 

 chrome oxidase-cytochrome system and the substrate-dehydrogenase 

 systems. It appears that this gap may now be considered filled by 

 several flavoproteins, e.g., Hogness and Haas's cytochrome reductase, 

 Euler and Green's diaphorase (coenzyme factor), or Szent-Gyorgyi's 

 dicarboxyhc acid system, etc. 



It is incorrect to label the entire process of cell respiration one 

 of hydrogen transport, since the first stages, from oxygen through 

 the four iron atoms of the respiratory ferment and the three cyto- 

 chromes, are concerned exclusively with electron transfers. This 

 brings up a difficulty in the formulation of the elementary steps that 

 connect the iron systems with the dehydrogenase systems. The 

 oxido-reductive changes taking place in the former are one-electron 

 transfers; the dehydrogenation reactions, on the other hand, are 

 formulated as bivalent processes, involving the loss and uptake of 

 two hydrogen atoms per molecule. The most satisfactory way of re- 

 solving this dilemma without invoking the existence of highly 

 problematical trimolecular reactions is to assume, with Michaelis, 

 that the apparently bivalent dehydrogenations are actually two-step 

 processes, involving the transfer of one hydrogen atom or its equiva- 

 lent at a time, with the intermediate formation of semiquinoid radi- 

 cals. 



It seems to be no mere coincidence that the macromolecular prepa- 

 rations, called cytochrome oxidase by Keilin, contain a number of 

 components: the oxidase, cytochromes a and b, succinic dehydro- 

 genase, etc., which are all concerned with what Oppenheimer calls 

 the "terminal oxidation" of metabolites. It is reasonable to assume 

 that these particles represent functional units which contain these 

 catalysts in a spatial arrangement which facilitates the progress of 

 this important phase of cell respiration in a constant pattern and at 

 a constant and high rate. 



Dr. Potter: 



The dilemma of which Dr. Stern has spoken, that is, the mech- 

 anism for getting a bivalent dehydrogenation system to react with 

 a one-electron system, may possibly be resolved by the formation 

 of a complex made up of the proper components of the hydrogen 



