Two affinity states for [3H]imipramine binding to the human platelet 5-hydroxytryptamine carrier: an explanation for the allosteric interaction between 5-hydroxytryptamine and imipramine.
5-Hydroxytryptamine (5-HT) showed a biphasic effect on the dissociation rate of [3H]imipramine from human platelet membranes: At low concentrations (EC50, approximately 2.5 microM), 5-HT stimulated the rate, as expected for mutually exclusive binding of 5-HT and imipramine; at higher concentrations (EC50, approximately 40 microM), 5-HT reduced this stimulated rate, a result consistent with 5-HT binding at a site that is physically distinct from both the imipramine binding site and the 5-HT transport recognition site of the 5-HT carrier. This modulatory effect could be mimicked by tryptamine, was saturable and independent of Na+ concentration, and could also be demonstrated for detergent-solubilized carriers. Monophasic association kinetics for [3H]imipramine binding were found. Heat stability experiments showed biphasic thermal inactivation curves. These results are consistent with [3H]imipramine binding to two classes of binding sites at the 5-HT carrier on human platelet membranes, with affinities three- to fivefold different. 5-HT can convert the lower-affinity state into the higher-affinity state.
O'Riordan
C
C
Department of Biochemistry, Trinity College, University of Dublin, Ireland.
Phillips
O M
OM
Williams
D C
DC
D001665:Binding Sites; D001792:Blood Platelets / Q000378:metabolism*; D002352:Carrier Proteins / Q000097:blood*; D006358:Hot Temperature; D006801:Humans; D007099:Imipramine / Q000378:metabolism*; D007700:Kinetics; D008566:Membranes / Q000378:metabolism; D012701:Serotonin / Q000378:metabolism*; D012995:Solubility; D014316:Tritium
D002352:Carrier Proteins; D014316:Tritium; D012701:Serotonin; D007099:Imipramine