INHIBITION OF ENZYMES 29 



gaard, 1956; Pfleiderer et al., 1958). Rabbit muscle lactate dehydrogenase 

 is more readily inhibited by iodoacetamide than iodoacetate, and yet rather 

 high concentrations and relatively long incubations (2.5 hr at 37°) are re- 

 quired to inhibit significantly (Dube et al., 1963). The enzyme is protected 

 by NAD and oxalate together, but not by either alone, and it was suggested 

 that a sluggish SH group occurs at the active site, although possibly other 

 groups are involved. Thus it is likely that the inhibition of muscle glycolysis 

 by reasonable concentrations of iodoacetate is not due at all to effects on 

 the lactate dehydrogenase. We shall see, furthermore, that the addition of 

 lactate to iodoacetate-treated tissues usually induces a good O2 uptake, in- 

 dicating that the dehydrogenase cannot be very significantly inhibited. 



Succinate Dehydrogenase 



Since the earlier work of Hopkins and Morgan (1938) showing the inhi- 

 bition of succinate dehydrogenase by iodoacetate and a variety of SH re- 

 agents, this enzyme has been one of the classic SH enzymes and has prob- 

 ably been studied more in this regard than any other. In this study the 

 methylene blue reduction technique was used and thus iodoacetate must 

 act on the dehydrogenase rather than the rest of the electron transport 

 sequence. The only other possible site in the sequence is just before the 

 cytochromes. The only data indicating an effect elsewhere than on the de- 

 hydrogenase is that of Millerd (1951), who found potato succinate oxidase 

 to be inhibited weakly by iodoacetate but succinate dehydrogenase not at 

 all; since similar results were found with malonate, one must assume the 

 operation of unknown factors. Some inhibitions are presented in Table 1-6. 



Succinate dehydrogenase must be classed as a moderately sensitive en- 

 zyme with respect to iodoacetate inhibition, and it is by no means as inhib- 

 itable as 3-PGDH. However, in some cases it appears that significant inhi- 

 bition would result from the use of iodoacetate at concentrations around 

 1 mM. It is not the most sensitive enzyme of the cycle (Yang, 1957) but at 

 1 mM it would be strongly inhibited, and these results are quite comparable 

 with those obtained on preparations from other mammalian tissues. One 

 characteristic of the enzyme is the sluggishness with which the groups react 

 with iodoacetate. Despite the fact that Hopkins and Morgan showed clearly 

 that the reaction is quite slow, very few investigators have preincubated 

 the enzyme before examination of the activity and fewer have mentioned 

 whether preincubation was carried out or not, as can be seen from the table. 

 Results with inhibitors such as iodoacetate are quite meaningless unless the 

 time relations are considered. In long-term experiments particularly, one 

 must be careful in using data from isolated enzyme preparations, since after 

 several hours the inhibition on succinate dehydrogenase may be much more 

 than is indicated. Of course, the sensitivity of the enzyme in the cell may 

 be different from that in the extracted state. Another characteristic is the 



