98 1. lODOACETATE AND lODOACETAMIDE 



is strongly inhibited by iodoacetate but this is probably due to a block of 

 the EM pathway (Damodaran and Bangachari, 1951), while the moderate 

 inhibition of citrate formation anaerobically in rat testis is presumably re- 

 lated to an action on the formation of acetyl-CoA (Paul et at., 1954). 



The inhibition of the endogenous respiration of rat liver mitochondria by 

 iodoacetate (25% by 0.2 vaM and 70% by 1 raM) may be taken as evi- 

 dence that iodoacetate exerts a definite action on the cycle (Minnaert, 1960), 

 but it is also possible, not knowing the nature of the endogenous substrate, 

 that part of the inhibition is on a pathway leading to the cycle (e.g., the 

 fatty acid helix). Another effect of iodoacetamide on mitochondria must be 

 considered, namely, the marked swelling produced by concentrations of 

 5-10 vciM (Tapley, 1956; Hunter, 1961). Since other SH reagents produce 

 swelling, it has been postulated that SH groups are involved in the mito- 

 chondrial structure, but if so this is probably mediated through metabolic 

 disturbances. Whether such effects occur with lower concentrations of these 

 alkylating agents is not known. 



PHOSPHORUS METABOLISM 



One more aspect of metabolism must be considered before api^roaching 

 the effects of iodoacetate on carbohydrate oxidation, namely, the changes 

 in the various phosphate fractions, inasmuch as this is intimately connected 

 with the control of carbohydrate metabolism. One would expect iodoacetate 

 to depress the uptake of P^ in synthesizing and growing cells, since the for- 

 mation of ATP and consequently the synthesis of nucleotides, nucleic acids, 

 phospholipids, and other phosphorus-containing substances would be inhib- 

 ited. We have seen that the hexose mono- and diphosphates may increase 

 in response to iodoacetate (although even their formation will be eventually 

 stopped), but these are probably the only phosphates accumulating. Inhi- 

 bition of P^ uptake (i.e., total net phosphorylation) by iodoacetate or iodo- 

 acetamide has been demonstrated in Micrococcus aureus (Hotchkiss, 1956), 

 yeast (Kamen and Spiegelman, 1948; Hoffmann-Ostenhof and Kriz, 1950; 

 Stickland, 1956 c), sea urchin eggs (Lindberg and Ernster, 1948), human 

 erythrocytes (Gourley, 1951), intestine (Naito, 1944), rabbit lens (Midler 

 and Kleifeld, 1953), rat thymus nuclei (Ord and Stocken, 1961), and partic- 

 ulate suspensions of rat liver, brain, and tumor (Clowes and Keltch, 1952). 

 In most cases this is probably due to an inhibition of the EM pathway. 

 Iodoacetate actually causes a loss of cell P, under certain conditions. The 

 changes in P^ may be quite complex because of the many phosphorylated 

 compounds and the multitude of reactions involving phosphate (e.g., oxi- 

 dative phosphorylation, ATPases, phosphatases, transphosphorylases, and 

 various phosphorylations), even though the inhibition is only on 3-PGDH. 

 The changes will often depend on the initial state of the tissue when iodo- 



