The effect of group selective reagentsN-ethylmaleimide and dithiothreitol on histamine H1-receptor binding sites in the vascular smooth muscle membranes

The chemical nature of the histamine H₁-receptors of beef aortic membranes has been elucidated by introducing two group selective reagents in the ³H]-mepyramine binding studies: dithiothreitol (DTT), a protein-disulphide group reducing reagent, andN-ethylmaleimide (NEM), a proteinthiol group alkylating agent.In the binding experiments, NEM independently inhibits ³H]-mepyramine binding. The inhibition is time and concentration dependent. DTT on the other hand potentiates the binding of the radioligand to its receptor and changes the affinity of histamine in competing for ³H]-mepyramine binding site. In the DTT-pretreated membranes (100 M), histamine shows a higher affinity for ³H]-mepyramine binding (K _i 0.35 M) than in the untreated membranes (K _i 3.7 M). Comparison of the pharmacological studies on the DTT-treated rabbit aortic strips and above binding studies, revealed a good correlation between the changes in the affinity of histamine for its receptor, when DTT was present. The results suggest an important role of the S-S and SH groups in the function of aortic histamine H₁-receptor.