Pasteur Effect 



FRITZ LIPMANN 



Massachusetts General Hospital 



WITH RESPECT TO their dependence on oxygen supply, organisms 

 may be classified into (1) strict aerobes, equipped only with 

 respiratory metabolic systems, (2) strict anaerobes, equipped only 

 with anaerobic fermentative metabolic systems, and (3) facultative 

 organisms, equipped with both respiratory and fermentative systems. 

 This commonly used classification should not be followed too rigidly, 

 however, for intermediate states between the main classes are com- 

 mon in nature, and adaptive interconversion has been widely ob- 

 served. 



The organisms in each of the first two groups rely exclusively on 

 one form of energy supply, respiratory or fermentative, respectively. 

 The third group, however, has developed the two mechanisms side 

 by side. It is with this latter group that we shall deal, and more 

 specifically with the interrelation between their respiratory and 

 fermentative mechanisms. 



Most doubly equipped organisms possess in the Pasteur effect 

 a regulatory device that enables them to use, as occasion demands, 

 either their aerobic or their anaerobic systems. By the operation 

 of this effect their fermentative apparatus is blocked in the presence 

 of sufficient oxygen, and energy is furnished almost exclusively by 

 the far more efficient and powerful respiratory apparatus. When 

 oxygen is lacking, however, the fermentation system is brought into 

 operation. 



The following example may serve to illustrate the energetic struc- 

 ture of a facultative anaerobic organism. A power plant uses as a 

 source of energy cheap water power; this may be compared to the 

 "cheap" respiratory energy. But because of seasonal variations of 

 flow the water power may not be entirely reliable and hence as a 

 safeguard against a deficiency in the supply of power a more ex- 

 pensively operating steam engine is built into the plant; this may 

 be compared to "expensive" fermentation. For obvious reasons the 

 plant will be equipped with a switch mechanism— its "Pasteur effect" 

 —which keeps the steam engine from functioning so long as the 

 water flow supplies suflBcient energy but throws it into operation 

 when water power is lacking. 



48 



