ANAEROBIC GLYCOLYSIS AND FERMENTATIONS 81 



not being inhibitable by iodoacetate. We must also consider the over-ail 

 reactions when different substrates are used: 



Glucose + 2 ADP + 2 P, -> 2 pyruvate + 2 ATP 



Glycogen + 3 ADP + 3 P, -> 2 pyruvate + 3 ATP 



Fructose- 1,6-diP + 4 ADP + 4 P, -> 2 pyruvate + 4 ATP 



Since the ATP, ADP, and P^ levels control glycolysis and determine which 

 reactions will be limiting, the effects of iodoacetate may be different with 

 these three substrates. For example, if the ATP level is low, inhibition with 

 glucose as the substrate might be greater than with fructose-l,6-diP be- 

 cause the former produces only half as much ATP as the latter. In other 

 words, iodoacetate would deplete the ATP faster with glucose as the sub- 

 strate and hence bring about a secondary inhibition of the hexokinase re- 

 action. Eacker (1954 a,b) has pointed out that in multienzyme systems the 

 results of inhibitions may be more complex than is usually supposed, and 

 that in the present case a partial inhibition of 3-PGDH may have a more 

 marked effect on glucose metabolism than on fructose- 1,6-diP metabolism. 

 In a system where no ATP has been added and where ATP may be hydro- 

 lyzing, the inhibition of ATP generation (although by no means necessarily 

 complete) may cause an almost total inhibition of glucose phosphorylation 

 and hence of glucose breakdown, whereas the inhibition of the metabolism 

 of fructose- 1,6-diP will be essentially the same as is exerted on 3-PGDH. 

 Perhaps if Beevers had added ATP to his extracts he would have observed 

 different results. The addition of ATP to cockroach muscle fibers glycolyz- 

 ing anaerobically relieves completely the inhibition by iodoacetate (Barron 

 and Tahmisian, 1948). 



Another factor to be taken into account is the rapidity with which 

 3-phosphoglyceraldehyde is formed and the level of concentration it reaches 

 during the inhibition, inasmuch as 3-phosphoglyceraldehyde protects the 

 enzyme 3-PGDH, the more effectively the higher the concentration. In 

 most cases there was no preincubation with iodoacetate, so the inhibitor 

 must act on 3-PGDH in the presence of the particular level of 3-phospho- 

 glyceraldehyde occurring with the substrate used. It is possible that under 

 certain conditions fructose- 1,6-diP may form 3-phosphoglyceraldehyde more 

 rapidly than glucose, and the 3-PGDH will be to some extent protected 

 against iodoacetate. That the steady-state level of 3-phosphoglyceraldehyde 

 may determine the degree of inhibition by iodoacetate is suggested by the 

 results of Holzer and Holzer (1953). The inhibition of anaerobic fermenta- 

 tion of glucose by yeast was determined after incubation of the yeast either 

 in air or in nitrogen. The inhibition was greater following anaerobic incuba- 

 tion, which was attributed to the lower concentration of 3-phosphoglyceral- 

 dehyde under anaerobic conditions. This mechanism may also explain the 



