Modifications of the binding properties of the human VIP receptor of IGR39 cells by sulfhydryl reagents.

The effects of specific sulfhydryl reagents, N-ethylmaleimide (NEM), p-chloromercuribenzoic acid (PCMB) and 5-5'-dithiobis(2-nitrobenzoic acid) (DTNB), were tested on the vasoactive intestinal peptide (VIP) receptor binding capacity of the human superficial melanoma-derived IGR39 cells. On intact cell monolayers NEM and PCMB inhibit the specific [125I]VIP binding in a time and dose-dependent manner while DTNB has no effect at any concentration tested. Inhibitory effects of NEM and PCMB on high and low affinity VIP receptor are not identical. With NEM-treated cells, only low affinity sites remained accessible to the ligand. Their affinity constant is not modified. With PCMB-treated cells, the binding capacity of high affinity sites is reduced by 56% while the binding capacity of low affinity sites is not significantly affected. For both types of binding sites, the affinity constants remain in the same range of that of untreated cells. On cells made permeable by lysophosphatidylcholine, DTNB is able to inhibit the specific [125I]VIP binding in a time and dose-dependent manner. The three sulfhydryl reagents stabilize the preformed [125I]VIP receptor complex whose dissociation in the presence of native VIP is significantly reduced. Labeling of free SH groups with tritiated NEM after preincubation of cells with DTNB and VIP made possible the characterization of reacting SH groups which probably belong to the receptor. Taken together, these data allow us to define three classes of sulfhydryl groups. In addition, it is shown that high and low affinity sites have different sensibility to sulfhydryl reagents.