148 1. lODOACETATE AND lODOACETAMIDE 



transport may be blocked either directly or through ATP depletion. Thus 

 in Staphylococcus aureus the uptake of glutamate requires energy and occurs 

 when glucose is fermented (Gale, 1951). The inhibition by iodoacetate here 

 could be indirect. The uptakes into bacteria and plant tissue (Birt and Hird, 

 1958) are well depressed by 2,4-dinitrophenol, indicating the importance of 

 ATP. On this basis one would expect the incorporation of amino acids into 

 proteins to be strongly inhibited by iodoacetate, but in only one case, that 

 of pea stems where synthesis of plasma proteins is reduced almost com- 

 pletely by 0.6 mM iodoacetate (Christiansen and Thimann, 1950 c), has 

 this been found to be true. In reticulocytes the incorporation of several 

 amino acids is inhibited only moderately by 2 mM iodoacetate (Borsook 

 et al., 1952), and in bull spermatozoa there is essentially no effect of 5 mM 

 iodoacetate (Bhargava et al., 1959). Moderate inhibition is seen in the sper- 

 matozoa anaerobically, which seems to point to an interference with the 

 supply of energy. The incorporation at 90 min is depressed 28% aerobically 

 and 52% anaerobically (Abraham and Bhargava, 1963). The incorporation 

 of C^*-labeled amino acids into protein anaerobically in rat liver homogenate 

 in the presence of crcatine-P to generate ATP is inhibited 7% by 0.05 mM, 

 54% by 0.5 mM, and 94% by 5 mM iodoacetate (Zamecnik and Keller, 

 1954) and this certainly points to a direct action on some step in the syn- 

 thesis. The activation of amino acids by ATP and the appropriate liver en- 

 zyme, preparatory to protein synthesis, is inhibited only 23% by 1 mM 

 iodoacetate, so that this is probably not an important site of action. 



The effects of iodoacetate on some of the enzymes involved in amino 

 acid metabolism are shown in Table 1-24 and, with certain exceptions, it 

 may be noted that as a class these enzymes are fairly resistant. Exceptions 

 are mammalian L-amino acid oxidase, certain decarboxylases, prolidase, and 

 serine deaminase. It is interesting that the transaminases, which are gen- 

 erally SH enzymes, are not readily inhibited by iodoacetate. 



In connection with amino acid metabolism, it may be mentioned that 

 the urea cycle can be depressed by iodoacetate through ATP depletion, as 

 postulated by Cohen and Hayano (1946) to explain the moderate inhibi- 

 tion (74% by 10 mM) on the conversion of citrulline to arginine in liver 

 homogenate. On the other hand, the reactions arginine -^ citrulline (Kor- 

 zenovsky, 1955) and citrulline ^- ornithine (Korzenovsky and Werkman, 

 1953) are quite resistant to iodoacetate. The arsenolysis of citrulhne to 

 form ornithine catalyzed by an enzyme from Streptococcus faecalis is, how- 

 ever, inhibited almost completely by 1.67 mM iodoacetate (Slade, 1955). 

 Modifications in urea formation by iodoacetate might also occur through 

 effects on amino acid metabolism. There is little evidence of direct potent 

 inhibition by iodoacetate on the urea cycle itself. 



Several enzymes involved in nitrogen fixation and nitrate reduction have 

 been examined (Table 1-25), but no studies on the general effects of iodo- 



