EFFECTS ON METABOLISM 357 



Buddecke, 1959), and duck erythrocytes (Tosteson and Johnson, 1957). 

 This inhibition is quite potent, anaerobic glycolysis in yeast being inhibited 

 50% by 0.13 mlf A^-ethylmaleimide (about twice as potent as iodoacetate). 

 In erythrocytes the uptake of glucose is depressed even more than gly- 

 colysis, being essentially stopped at 1 mM. The synthesis of cellulose by 

 Acetobacter xylinum is completely blocked by 2 mM-ethylmaleimide, but 

 whether this is an action on glucose uptake or on the polymerization is 

 not known (Schramm et at., 1957). 



Lipid biosynthesis seems to be quite sensitive to A-ethylmaleimide. 

 The formation of fatty acids from acetyl-CoA and malonyl-CoA by an 

 extract of pigeon liver is reduced 56% by 0.012 mM and 93% by 0.025 mM 

 (Bressler and Wakil, 1962). It would be worthwhile to know how readily 

 A'-ethylmaleimide reacts with coenzyme A. Sterol synthesis can be blocked 

 at two sites at least, the formation of squalene from farnesyl-PP (Goodman 

 and Popjak, 1960) and the conversion of squalene to sterols (Goodman, 

 1961), both pathways being inhibited around 50% by 0.3-1 mM A"-ethyl- 

 maleimide. Much less is known of the effects of A' -ethylmaleimide on protein 

 synthesis, but in rabbit reticulocytes even 1 mM has scarcely any effect 

 on the incorporation of labeled leucine into protein (Borsook et al., 1957). 

 The uptake of cytidine into HeLa cell KNA is moderately reduced by 

 A^-ethylmaleimide at 0.02-0.2 mM (Srinivasan et al, 1964). The exact 

 site of the inhibition is not known but the RNA polymerase was suggested. 

 Another site in nucleoside metabolism was located by Younathan et al. 

 (1964) in the synthesis of UTP from uracil by extracts of rat brain, this 

 being 99% suppressed by 0.1 mM A^-ethylmalaimide. They showed that 

 the mechanism does not involve depletion of ATP or complexing with 

 Mg++, and since uridine and UMP accumulate it was assumed that the 

 primary attack is on the ATP:UMP phosphotransferase. This is substan- 

 tiated by the lack of inhibition on uridine phosphorylase and uridine kinase. 

 The incorporation of ^9]?^++ [^^q ferritin in liver slices is inhibited 90% 

 by 5 mM A^-ethylmaleimide (Mazur et al, 1960). Bioluminescence in the 

 sea pansy Renilla reniformis is rather potently depressed by A^-ethylmalei- 

 mide, 31% inhibition occurring at a concentration of 0.1 mM (Cormier, 

 1960). Out of such isolated strands no significant pattern can be woven. 



Insulin facilitates the uptake of glucose by tissues and it has been pos- 

 tulated that insulin is bound to the cell membranes by a disulfide link. 

 Cadenas et al. (1961) used A'-ethylmaleimide to block the SH groups at 

 the cell surface which may be involved in insulin binding. The isolated rat 

 heart takes up glucose at a slow rate and the addition of insulin to the per- 

 fusion fluid augments this uptake. Perfusion for 30 sec with 1 mM A^-ethyl- 

 maleimide does not affect the basal glucose uptake but inhibits the insulin- 

 stimulated uptake by 80%. If the hearts are pretreated with insulin, the 

 inhibition by A^-ethylmaleimide is prevented, which may be interpreted 



