ANAEROBIC GLYCOLYSIS AND FERMENTATIONS 69 



ficult since specificity will, in general, be lost. Of course, it is always ad- 

 visable and often necessary to obtain experimental evidence that the EM 

 pathway under the particular conditions is being blocked to a satisfactory 

 degree and selectivity, by methods to be discussed later. 



Comparative Susceptibility of the EM Pathway to lodoacetate in Different 

 Organisms and Tissues 



What has been said in the previous section refers to most mammalian 

 and bacterial EM pathways. This pathway has been demonstrated in all 

 types of organism, but is occasionally absent so that glucose metabolism 

 must be through other reactions. Furthermore, several instances of resist- 

 ance to inhibition by iodoacetate have been noted, as seen in Table 1-11, 

 most of these being in invertebrates, e.g., sea urchin and oyster eggs, and 

 cockroach and grasshopper muscles. Absence of glycolytic inhibition in fly 

 homogenates has also been reported (Chefurka, 1954); in this case, failure 

 to inhibit was possibly due to incubating with iodoacetate but without thiol 

 activators, so that 3-PGDH may have been in the disulfide state and re- 

 sistant. In Streptobaclerium casei, which quantitatively converts glucose to 

 lactate, a certain fraction of the lactate formation is resistant to iodoace- 

 tate (Field and Field, 1932). It is not known in these cases of resistance 

 whether the particular 3-PGDH is not as readily inhibited or whether other 

 pathways for lactate formation are present. We have noted that NADP- 

 dependent 3-PGDH is not inhibited as easily as the NAD-dependent en- 

 zyme, and perhaps resistant enzymes- occur in certain invertebrates. One 

 must also remember that the reduction of pyruvate (formed from any 

 source) to lactate might occur with NADH arising in any oxidation, or 

 with NADPH by a transfer to NAD. In any event, one must be prepared 

 to find iodoacetate-resistant EM pathways even under anaerobic conditions, 

 and for this reason the concentrations suggested in the previous section 

 will not always be adequate. If lactate or ethanol formation is measured, 

 one must remember that certain organisms can anerobically metabolize 

 pyruvate by other pathways, e.g., rumen microorganisms LC form acetate, 

 H2, and CO2 from pyruvate (Peel, 1960). 



Site of Inhibition in the EM Pathway 



The early location of the action of iodoacetate at 3-PGDH has been des- 

 cribed in the historical summary (page 6) and we shall now inquire into 

 the present state of this problem. In examining some of the evidence for 

 the inhibitory site we shall encounter results which will help us understand 

 the effects of iodoacetate in cells. 



(a) Comparison of sensitivities of EM pathway and its component enzymes. 

 It is difficult to compare the inhibitions of isolated 3-PGDH (Table 1-3) 



