Inhibition of [3H]Gpp(NH)p release from human platelet membranes by GDP

Previous reports have described hormone-stimulated release of radiolabeled guanine nucleotides from prelabeled membranes. In this paper, we report the inhibition by GDP of both hormone-independent and hormone-specific stimulation of [3H]Gpp(NH)p release from human platelet membranes. This inhibition was shown to be concentration specific and suggests a more complex mechanism of [3H]Gpp(NH)p release than was previously thought. A model is proposed in which guanosin triphosphates cause dissociation of the subunits of the guanine nucleotide binding protein, whereas guanosine diphosphate prevents this dissociation resulting respectively in stimulation or inhibition of the release of bound [3H]Gpp(NH)p.