INHIBITION OF ENZYMES 25 



75% in most cases. The arsenolysis and certainly the hydrolysis of acetyl-P 

 must not occur through reaction with the SH group attacked primarily by 

 iodoacetate. On the other hand, the hydrolysis of p-nitrophenylacetate is 

 quite readily inhibited (Park et al., 1961). This substrate is also not arsen- 

 olyzed, so the mechanism of the reaction must be different from that of 

 acetyl-P. 



Alcohol and Lactate Dehydrogenases 



The formation of ethanol or lactate in anaerobic glycolysis depends on 

 the alcohol and lactate dehydrogenases, respectively, through a coupled 

 oxidoreduction with 3-PGDH, and thus it is of some importance to deter- 

 mine as far as possible if the inhibition of glycolysis by iodoacetate is due 

 in part to effects on these enzymes. We have already seen that Dixon (1937) 

 found yeast alcohol dehydrogenase to be the most sensitive enzyme he tested 

 and believed it to be the site of inhibition of yeast fermentation, whereas 

 lactate dehydrogenase is scarcely inhibited so that the depression of muscle 

 glycolysis cannot be attributed to a block of this enzyme. There has been 

 some variability in the subsequent reports on the sensitivity of alcohol 

 dehydrogenase to iodoacetate (Table 1-4); however, it has generally been 

 confirmed that the yeast enzyme is fairly susceptible, although perhaps not 

 as much as originally reported by Dixon. A rather odd fact is that the most 

 recent work with the purest enzyme has indicated the least sensitivity. This 

 makes it very difficult to evaluate the role of alcohol dehydrogenase in 

 iodoacetate inhibition: the results of Dixon certainly suggest that in yeast 

 the inhibition of fermentation must to a great extent be attributed to an 

 effect on this enzyme as well as 3-PGDH, and the data of Mackworth (1948) 

 generally confirm this, but the results of Barron and Levine (1952) point 

 to only a minor role of alcohol dehydrogenase in the inhibition. Comparable 

 concentration-inhibition curves for each enzyme are lacking. The work of 

 Aldous (1952) is interesting because he incubated yeast with 1.5 mM iodo- 

 acetate for 30 min and then extracted the enzymes, finding 83% inhibition 

 of the alcohol dehydrogenase. The trouble here is that the pH was 4.5 to 

 facilitate penetration of the iodoacetate and glycolysis was blocked com- 

 pletely; thus one cannot compare the effects on 3-PGDH and alcohol de- 

 hydrogenase, nor can one use these results to estimate the intracellular 

 inhibition of alcohol dehydrogenase at other pH's. It may be noted that 

 pyruvate decarboxylase is inhibited 87%, lactate dehydrogenase 94%, and 

 even catalase 90%, so that under these conditions no specificity of any kind 

 was achieved. 



Lutwak-Mann (1938) emphasized that horse liver alcohol dehydrogenase 

 is much less sensitive to iodoacetate than the yeast enzyme. This is not due 

 to inactivation of the iodoacetate by the liver preparation, since incubation 

 of minced horse liver with iodoacetate (little inhibition produced) and then 

 testing this on the yeast enzyme led to marked inhibition. Inasmuch as so 



