INHIBITION OF ENZYMES 19 



probably at the level of triose-P oxidation. The confusion introduced by 

 Dixon (1937) in finding triose-P oxidation to be less sensitive to iodoacetate 

 than alcohol dehydrogenase — indicating that the site of action in the inhi- 

 bition of alcoholic fermentation is the alcohol dehydrogenase — was re- 

 solved partially by Green et al. (1937 b), who found that in the coupled ox- 

 idoreduction between triose-P and pyruvate the oxidation reaction is the 

 most sensitive, and completely by Adler et al. (1938), whose results differed 

 markedly from those of Dixon in that the oxidation of triose-P was inhib- 

 ited much more readily than alcohol dehydrogenase by iodoacetate. It was 

 now clear that in the various coupled reactions it is the 3-PGDH which is 

 involved in the inhibition by iodoacetate, and that the block of the forma- 

 tion of lactate or ethanol anaerobically is due to the suppression of the 

 production of NADH required for the reduction of pyruvate or acetaldehyde. 

 Although the inhibition of the oxidation of fructose-l,6-diP (in the pres- 

 ence of aldolase which is insensitive to iodoacetate) by iodoacetate had 

 been demonstrated in several preparations, a direct inhibition of 3-PGDH 

 was first reported by Cori et al. (1948) in a crystalline preparation of the 

 enzyme from rabbit muscle. It is interesting that Warburg and Christian 

 (1939) had crystalhzed yeast 3-PGDH but had not examined its susceptibi- 

 lity to iodoacetate. 



Inhibitions of 3-PGDH by iodoacetate are shown in Table 1-3. One notes 

 first that even within a single source there is great variability in the results, 

 due not only to the different incubation times but also to different tempera- 

 tures (which run from 0^ to 38^), pH's, and degrees of purity. One also 

 suspects, despite the difficulty in comparing the results of different workers, 

 that the enzymes from different sources vary in susceptibility to iodoacetate. 

 In general, the 3-PGDH from microorganisms seems to be less readily in- 

 hibited than from animal tissues, although the enzymes from most tissues 

 have never been tested directly. It also seems to be a general rule that NAD- 

 dependent 3-PGDH is more sensitive than NADP-dependent 3-PGDH, as 

 seen in the table from the results of Arnon on the enzymes from spinach 

 leaves. This has also been noted by Gibbs (1952) in preparations of 3-PGDH 

 from peas. We shall see that glycolysis is frequently less inhibited by iodo- 

 acetate in plants than in animal tissues, and it may be that the presence 

 of NADP-dependent 3-PGDH is at least partly responsible. There is much 

 indirect evidence that the 3-PGDH from various organisms and tissues is 

 readily inhibited by iodoacetate — Aspergillus niger, Streptomyces coelicolor, 

 Chlorella, sugar beet leaves, heart, brain, retina, etc., but only the enzyme 

 from rabbit muscle has been adequately studied in the pure state and quan- 

 titative results obtained. Thus most of our detailed knowledge of the inhi- 

 bition is based on muscle 3-PGDH and how much of this can be applied 

 to the enzymes from other tissues is not known. It is likely that incubation 

 of 3-PGDH from most animal tissues with iodoacetate at 0.1-0.5 mM for 



