52 1. lODOACETATE AND lODOACETAMIDE 



These facts make it evident that intracelkilar inhibition of an enzyme may 

 be quite different from that of the isolated and purified enzyme. 



The question of specificity of inhibition by iodoacetate must be postponed 

 until the various metabolic pathways have been discussed, but we can see 

 from the few enzymes mentioned that several are readily inhibited. If we 

 arbitrarily define a sensitivity to iodoacetate as implying a significant inhi- 

 bition at concentrations below 1 mM, since this blocks glycolysis effectively 

 in most cases, we note that certain alcohol dehydrogenases, succinate dehy- 

 drogenases, and proteolytic enzymes must be classed as sensitive. The re- 

 sults with most of the important enzymes will be given in later sections, 

 but data on a number of enzymes of interest are summarized in Table 1-9, 

 since these enzymes do not fall readily into any general category. It is 

 clear that several of them are inhibited markedly at concentrations below 

 1 mikf . It is already established that a specific action on 3-PGDH or the 

 glycolytic pathway in the broadest sense is impossible. This does not mean 

 that under certain circumstances an inhibition of glycolysis for practical 

 purposes is impossible, since many of the sensitive enzymes will not be in- 

 volved in what is measured, but it should sound a note of caution to facile 

 interpretation of results on cells or tissues. By no means all the enzymes 

 tested with iodoacetate have been included in Table 1-9, or in the later 

 tables, and we note only in passing that the following enzymes seem to be 

 inhibited very little or not at all: acetyl phosphatase, 5'-adenylate deami- 

 nase, amylase, amylo-l,6-glucosidase, arsenite dehydrogenase, cellulase, 

 chlorophyllase, diamine oxidase, inositol dehydrogenase, laccase, maltase, 

 mucinase, nucleotidases, nucleotide pyrophosphorylase, oxalate decarbox- 

 ylase, peroxidase, a-phenylmannosidase, phosphatases, polygalacturonase, 

 thiamine kinase, thioesterase, urease, uricase, uridine diphosphoglucose de- 

 hydrogenase, and vitamin K reductase. 



EFFECT ON ELECTRON TRANSPORT SYSTEMS 



Several NAD- and NADP-dependent dehydrogenases are inhibited by 

 iodoacetate in reasonably low concentration. The iodoacetate is not acting 

 on the nucleotides but on the apodehydrogenase. This was shown for the 

 inhibition of 3-PGDH by Adler et al. (1938). Incubation of NAD with iodo- 

 acetate and starting the reaction by addition of enzyme resulted in no initial 

 inhibition, whereas incubation of the enzyme and iodoacetate gave initial 

 inhibition. It is likely that in all these cases the inhibition is due to car- 

 boxymethylation of SH groups on the dehydrogenases. The various compo- 

 nents conducting electrons from NADH or NADPH to oxygen seem in 

 general to be fairly resistant to iodoacetate. The cytochrome system is not 

 inhibited significantly by iodoacetate up to 10 niM, at least as determined 

 with substrates (hydroquinone, ascorbate, p-phenylenediamine) feeding 

 electrons directly into this portion of the chain (Table 1-10). The observa- 



