Properties of small molecules affecting insulin receptor function

Small molecules with insulin mimetic effects: and oral availability are of interest for potential substitution of insulin injections in the treatment of diabetes. We have searched databases for compounds capable of mimicking one epitope of the insulin molecule known to be involved in binding to the insulin receptor (IR). This approach identifies thymolphthalein, which is a n apparent weak agonist that displaces insulin from its receptor, stimulate s auto- and substrate phosphorylation of IR, and potentiates lipogenesis in adipocytes in the presence of submaximal concentrations of insulin. The va rious effects are observed in the 10(-5)-10(-3) M range of ligand concentra tion and result in partial insulin activity. Furthermore, analogues of the related phenol red and fluorescein molecules fully displace insulin from th e IR ectodomain, however, without insulin agonistic effects. The interactio ns are further characterized by NMR, UV-vis, and fluorescence spectroscopie s. It is shown that both fluorescence and UV-vis changes in the ligand spec tra induced by IR fragments occur with Kd values similar to those obtained in the displacement assay. Nevertheless, insulin itself cannot completely a bolish binding of the small molecules. Determination of the binding stoichi ometry reveals multiple binding sites for ligands of which one overlaps wit h the insulin binding site on the receptor.