110 1. lODOACETATE AND lODOACETAMIDE 



of endogenous respiration from a variety of organisms and tissues are given 

 in Table 1-17. The degree of inhibition will depend on a number of factors: 

 (a) the substrate or substrates being oxidized, (b) the sensitivity of the par- 

 ticular 3-PGDH or EM pathway, (c) the inhibition of pathways other than 

 the EM pathway, (d) the permeability of the cells to iodoacetate, and the 

 external pH, and (e) the time of exposure to iodoacetate. The latter two 

 factors make it difficult to evaluate quantitatively the results in the table 

 and to compare the inhibitions in different organisms or tissues. In many 

 cases the inhibition progresses steadily with time so that a single value for 

 inhibition is meaningless, and furthermore it is quite possible that secondary 

 effects soon appear so that the inhibition is not solely due to the immediate 

 action of iodoacetate. It may be seen from the table that the iodoacetate 

 concentrations used have often been much higher than required to block 

 the EM pathway. Although the concentration within the cells is not known, 

 one must assume that the action is not specific on the EM pathway, since 

 concentrations of 1 mM or even lower have been shown frequently to in- 

 hibit pyruvate oxidation quite significantly. 



A few general deductions may be made from the results in Table 1-17. 

 Although endogenous respiration is usually inhibited by iodoacetate, it is 

 much less sensitive than anaerobic or aerobic glycolysis. If iodoacetate at 

 0.01-0.02 mM inhibits the EM pathway around 50% (page 63), it usually 

 requires over 100 times this to inhibit endogenous respiration comparably, 

 although in certain cells endogenous respiration is reasonably sensitive. The 

 principal reason for this is probably that the endogenous respiration com- 

 monly does not depend very strongly on carbohydrate. It may also be noted 

 that there is often an iodoacetate-resistant fraction of the endogenous respi- 

 ration (i.e., the inhibition levels off even though the iodoacetate concentra- 

 tion is raised above 10 mM). The nature of this resistant fraction is not 

 known, but it may consist mainly of simple oxidations of dicarboxylates 

 and amino acids. In fact, the increasing inhibition with time may occa- 

 sionally be due to the utilization and depletion of substrates oxidized 

 through iodoacetate-resistant pathways. 



Iodoacetate in common with other SH reagents stimulates the endoge- 

 nous respiration of certain cells at low concentrations. This has been ob- 

 served in Streptomyces griseus (HockenhuU et al., 1954 a), yeast (Lunds- 

 gaard, 1930 c), CJilorella pyrenoidosa (Kohn, 1935), the slime mold Physarum 

 polycepJialum (Allen and Price, 1950), sea urchin spermatozoa (Barron and 

 Goldinger, 1941; Barron et al., 1948), bull spermatozoa (Lardy and Phillips, 

 1943 a), toad retina (Hanawa et al., 1956), cat salivary gland (Druckrey 

 and Loch, 1943), and the Earle strain of mammalian cells in culture (Siegel 

 and Cailleau, 1956). Sometimes this stimulation is maintained but usually 

 it is temporary and passes over into an inhibition. The stimulation may be 

 as much as 100%, as in Chlorella at around 0.1 mM iodoacetate (Kohn, 



