60 1. lODOACETATE AND lODOACETAMIDE 



tion that iodoacetate increased some 100-fold the rate of autoxidation of 

 cytochrome c (Boeri and Tosi, 1954) is interesting, but probably in metabolic 

 or cellular systems this would not alter the high rate of cytochrome c oxida- 

 tion via the oxidase. 



Evidence that there is little effect on the components from the initial 

 dehydrogenase to the cytochrome c is indirect and mainly on the basis 

 that the inhibition of the entire system can be fuUy attributed to the ac- 

 tion on the dehydrogenase. It is true that the oxidation of NADH and 

 NADPH by various preparations is occasionally inhibited moderately by 

 iodoacetate (Table 1-10), but in such systems it is difficult to know if the 

 normal electron transport sequence involving bound endogenous nucleotides 

 is being tested. The resistance of several metalloflavin oxidases to iodoace- 

 tate indicates little if any inhibition of these components. One can conclude 

 from the available data that inhibitions of most oxidations are located pri- 

 marily on the enzyme initially attacking the substrate when iodoacetate is 

 1 mM or lower, with possible minor contributions in certain cases from ef- 

 fects on electron transport between NADH or NADPH and cytochrome c, 

 particularly when higher concentrations are used. 



EFFECTS ON OXIDATIVE PHOSPHORYLATION 



It was known over 30 years ago that iodoacetate inhibits the formation 

 of ATP and creatine-P in tissues because of the depression of the oxidations 

 supplying the energy. We shall now try to deterriiine whether iodoacetate 

 can affect the generation of high-energy bonds more directly, namely, 

 through an uncoupling of oxidative phosphorylation. Most of the earlier 

 work involved fairly complex systems and the results are not easy to in- 

 terpret. Thus Kalckar (1937) found that phosphorylation is increased in cat 

 renal cortex mince when glucose is added, and that 1.25 raM iodoacetate 

 inhibits this completely, although depressing oxygen uptake only 37%. The 

 phosphorylation here was not determined by ATP formation but mainly as 

 accumulation of hexose phosphates, so that numerous sites of action might 

 be postulated. On the other hand, cyanide inhibits phosphorylation and 

 respiration equally. Ochoa (1941) measured phosphorylation (again as hex- 

 ose phosphates) associated with p>Tuvate oxidation in pigeon brain dis- 

 persions and stated that 1 mM iodoacetate has no effect on the P : ratio; 

 however, it seems to be reduced from 2.02 to 1.73 (when fluoride is present 

 to preserve the phosphate esters). The respiration of sea urchin egg homog- 

 enate in the presence of glucose is actually stimulated by 5 mM iodoace- 

 tate, but phosphorylation is simultaneously inhibited 14% (Lindberg and 

 Ernster, 1948). This early work does not prove an uncoupling action but 

 suggests it. 



Oxidative phosphorylation could be investigated directly only when 



