Differential effects of suramin on the coupling of receptors to individual species of pertussis-toxin-sensitive guanine-nucleotide-binding proteins

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Abstract
The anti-helminthic drug suramin inhibited the basal high-affinity GTPase activity of both C6 BU1 glioma and NG 108-15 neuroblastoma .times. glioma hybrid-cell membranes with an IC50 (concentration causing half-maximal inhibition) value close to 30 .mu.g/ml. This effect was shown to occur via a non-competitive mechanism in which the binding affinity of the G-proteins for GTP was not altered, but the maximal velocity of the subsequent hydrolysis was reduced. In NG 108-15 membranes, both opioid peptides and foetal-calf serum stimulated high-affinity GTPase activity in a pertussis-toxin-sensitive manner. These effects have previously been shown to be mediated by different G-proteins [McKenzie, Kelly, Unson, Spiegel and Milligan (1988) Biochem. J. 249, 653-659]. Suramin completely prevented the opioid-peptide-stimulated increase in GTP hydrolysis, but did not prevent the opioid peptide from binding to its receptor. Suramin, however, did not block the foetal-calf-serum-stimulated GTPase response. This selective action of suramin provides further evidence for distinct roles for two separate pertussis-toxin-sensitive G-proteins in signal transduction in NG 108-15 membranes and provides the first evidence for a selective effect of a drug on the functions of different G-proteins.