356 3. .V-ETHYLMALEIMIDE 



is inhibited more rapidly, but eventually the latter is more depressed (see 

 accompanying tabulation). ATP protects against iV-ethylmaleimide when 



Time (min) 



% Inhibition 



Mg++-activated Mg++ — Na+ — K+-activated 



all the ions are present but does not protect the Mg++-activated ATPase. 

 This preparation also contains NADH : cytochrome c oxidoreductase ac- 

 tivity and this in inhibited strongly by iV-ethylmaleimide. It was concluded 

 that there are at least three different types of SH group: (1) those required 

 for Na+-K+ activation, these reacting slowly, (2) those required for oxido- 

 reductase activity, these reacting rapidly, and (3) those required for Mg++ 

 activation, these reacting rapidly with A'-ethylmaleimide and not being 

 protected by ATP or NADH. 



EFFECTS ON METABOLISM 



Only superficial information for the most part is available on the meta- 

 bolic effects of iV-ethylmaleimide, and in no case has the site of action 

 been localized at a particular enzyme. Furthermore, many of the critical 

 enzymes in the major metabolic pathways have not been studied, as in- 

 spection of Table 3-1 makes evident. We find, for example, that respiration 

 of E. coli is inhibited 60% by 1 mM (Bridges, 1961), of sunflower stems 

 around 10% by 1 mM (Niedergang-Kamien and Leopold, 1957), and of 

 human erythrocytes some 50% by 1.3 mM iV-ethylmaleimide (Sheets 

 and Hamilton, 1958) but one has no basis for attributing these inhibitions 

 to any enzyme or group of enzymes. At these concentrations certain 

 dehydrogenases, electron transport systems, glycolytic and cycle enzymes 

 are inhibited, and the over-all respiratory inhibition may be related to all 

 these sites; on the other hand, it is possible, in view of our ignorance, that 

 a particular enzyme is selectively affected. A^-Ethylmaleimide has the po- 

 tentiality for being a selective inhibitor but we do not know if it is. One 

 of the most likely points of attack might be the a-keto acid oxidases, since 

 the oxidation of a-ketoglutarate by liver and kidney mitochondria is quite 

 potently inhibited (Table 3-1) (Rogulski et al., 1962). Another likely site 

 is the glycolytic pathway, since iV-ethylmaleimide has been shown to 

 depress glycolysis in stomach (Davenport et al., 1955), yeast (Weitzel and 



