100 A SYMPOSIUM ON RESPIRATORY ENZYMES 



baker's yeast. It is thus seen that catalysts of the same type, al- 

 though differing in details, have been developed by nature to keep 

 in check both alcoholic and lactic acid fermentation in the presence 

 of oxygen. As Figure 2 indicates, oxygen is "mobilized" for its two 

 chief tasks in aerobic cell life by two autoxidizable hemin proteins 

 (Warburg and Pasteur enzymes) acting in analogous manner but 

 independently of each other. The effect of a variation of the oxygen 

 tension (and, for that matter, of carbon monoxide concentration in 

 inhibitor experiments) on the overall phenomena of cellular respira- 

 tion and aerobic fermentation will depend on the affinity which the 

 iron contained in the two types of enzymes has for these gases. 

 Just as is the case with the respiratory transport protein hemoglobin 

 in various vertebrate species, there will be variations in the gas- 

 dissociation curves for these enzymes from cell to cell. Depending 

 on whether a particular Pasteur catalyst has a higher or a lower 

 aflBnity for oxygen or carbon monoxide than the Warburg ferment 

 in the same cell, we will expect to find respiration more or less 

 readily afiFected by a lowering of oxygen tension or a given ratio 

 of carbon monoxide to oxygen than the Pasteur reaction in that cell. 

 If these gases do not vary in their effect on the two processes, as 

 Warren (unpublished observations) found to be true in the case of 

 bone marrow, it must be because the affinity of the enzymes for 

 the gases is equal. In general it would appear that in higher animal 

 tissues the Warburg ferment has a greater affinity for oxygen and a 

 lesser affinity for carbon monoxide than the Pasteur enzyme in the 

 same tissue (Laser effect, 39); in unicellular systems (certain bac- 

 teria, human myeolocytes) the reverse seems to be true (Kempner 

 effect, 29). It would appear that there are as many (slightly) different 

 Warburg and Pasteur enzymes as there are living forms. Probably 

 the difference resides in the protein rather than in the hemin group 

 of the molecules, except in the case of Azotobacter, which seems to 

 have a Warburg enzyme with a green hemin rather than a mixed- 

 colored or pheohemin in the prosthetic group (47). The reasons for 

 interposing a hypothetical ferrous iron catalyst between the Pasteur 

 enzyme and the fermentation system, as indicated in the figure, 

 cannot be explained here because of lack of space. 



In closing, some model experiments may be mentioned which are 

 being carried out at present in this laboratory. Some years ago, Lip- 

 mann (41) showed that a Pasteur effect in cell-free systems (yeast 

 and muscle fermentation extracts) may be produced by the addition 

 of suitable reversible dyestuffs of suflBciently high potential (see 



