358 3. A'-ETHYLMALEIMIDE 



as receptor occupancy by the insulin. Finally, the binding of insulin-I^^^ 

 to hearts is reduced to half by iV-ethylmaleimide. All of these results could 

 be adequately explained by assuming that iV-ethylmaleimide reacts with 

 those membrane SH groups involved in insulin binding, although they 

 would not prove that a disulfide bond is formed. The studies of Mirsky 

 and Perisutti (1962) on rat epididymal adipose tissue gave very different 

 results. iV-Ethylmaleimide at 1 mM inhibits the basal glucose oxidation 

 70% and the insulin-stimulated oxidation 81%. It may well be that a 

 true differential effect on uptake is obscured by the fact that measurements 

 of the C^^Og produced from labeled glucose were made. Furthermore, 

 binding of insulin-I^=^^ is not affected by 1 mM A' -ethylmaleimide in either 

 adipose tissue or diaphragm. Ott and Recant (1963) confirmed the results 

 on adipose tissue; on the other hand, one sees in the data that the effect 

 of insulin is progressively suppressed as the concentration of A-ethyl- 

 maleimide rises, being a complete inhibition at 1 mM; e. g., normally 

 500 //units/ml of insulin stimulate glucose oxidation 790% whereas in the 

 presence of 1 mM A-ethylmaleimide there is only a 60% increase. They 

 could also detect no effect of A-ethylmaleimide on the binding of insulin. 

 The problem thus rests in this unsatisfactory state and one does not know 

 whether to credit the discrepancies to tissue differences or to variations in 

 the techniques. 



EFFECTS ON TISSUE FUNCTIONS 



The use of A-ethylmaleimide to investigate the metabolic basis for 

 tissue function has been very limited, but the results obtained suggest that 

 it deserves more extensive consideration as a possibly selective SH reagent, 

 especially for the study of active transport. 



Gastric Acid Secretion 



A^-Ethylmaleimide, in common with other SH reagents, effectively in- 

 hibits the secretion of acid by the parietal cells (Davenport et al., 1955). 

 Indeed, A-ethylmaleimide is about 4 times more potent than iodoacetamide. 

 Stimulation of the secretion is observed at low concentrations, as with 

 iodoacetamide (Fig. 3-5). Both the resting and the carbachol-stimulated 

 secretion are depressed. Hollander (1956) showed that these in vitro results 

 can also be obtained in vivo by applying 1.5-2 mM solutions of A'-ethyl- 

 maleimide topically to Heidenhain pouches in the dog. The histamine- 

 stimulated acid secretion is blocked for long periods but recovery eventually 

 occurs after 12-24 hr. 



Glycolysis in the stomach is inhibited by A'-ethylmaleimide and it ap- 

 pears superficially that this may be correlated with the secretory suppres- 



