272 F. Lynen, et al. 



it is unlikely that the reversible isomerization of glucose- 

 6-phosphate to fructose-6-phosphate would be affected by 

 oxygen, the phosphofructokinase reaction must be the step 

 concerned. Engelhardt and Sakov (1943) considered the 

 aerobic inhibition of phosphofructokinase to be responsible 

 for the Pasteur effect. They assumed an oxidative inactivation 

 of the enzyme. This assumption, however, is not in agreement 

 with the finding that DNP and other uncoupling agents, even 

 in the presence of oxygen, suspend the enzyme inhibition. 

 Aisenberg and Potter (1957) also observed an aerobic decrease 

 in fructose diphosphate when they studied the Pasteur effect 

 in artificial systems consisting of rat liver mitochondria and 

 soluble fractions from rat brain and tumour tissue. They 

 proposed that a high-energy intermediate of oxidative phos- 

 phorylation linking the respiratory chain and ATP is respons- 

 ible for the inhibition of phosphofructokinase. While further 

 experimental evidence for this hypothesis is lacking, we prefer 

 a different explanation for the high glucose-6-phosphate level, 

 concomitant with the decrease in fructose diphosphate in our 

 experiments. It is based on our assumption, made before, 

 that only cytoplasmic ATP originating from fermentation can 

 be used for hexose phosphate phosphorylation. If this is 

 correct, the decrease in fructose diphosphate would be due to 

 the depletion of ATP in the cytoplasm and this would be 

 the direct consequence of the respiratory inhibition of 

 fermentation. 



In summary, the results of our experiments with yeast cells 

 indicate that the Pasteur effect is due to oxidative phosphoryl- 

 ation and enzyme localization in the different cell compart- 

 ments. The phosphorylation reactions going parallel with the 

 respiratory process exhaust the inorganic phosphate pool 

 available for the dehydrogenation of glyceraldehyde phos- 

 phate, thereby inhibiting the fermentative glucose degrada- 

 tion. The reduced glucose uptake of respiring cells compared 

 with that of fermenting cells is interpreted as an impairment 

 of the hexokinase reaction due to lack of ATP at the site of 

 glucose phosphorylation. The ATP formed during respiration 



