Contribution of the C-terminal dipeptide of transforming growth factor-alpha to its activity: biochemical and...

We have used a matrix of biological (two distinct guinea-pig stomach contractile smooth muscle preparations) and biochemical (human placental membrane receptor binding and phosphorylation) assays to evaluate the activity profiles of epidermal growth factor-urogastrone (EGF-URO, mouse and human), transforming growth factor-alpha (TGF-alpha, human) and the TGF-alpha derivative lacking the C-terminal dipeptide, Leu49-Ala50, TGF-alpha-(1-48). In the longitudinal muscle (LM) bioassay, the relative potencies of the peptides were: EGF-URO greater than TGF-alpha greater than TGF-alpha-(1-48), with relative activity ratios (EC50S) of approximately 1:3:16. In the LM assay system, TGF-alpha-(1-48) was a partial agonist. In the circular muscle (CM) bioassay, the relative order of potencies was: TGF-alpha- greater than EGF-URO greater than TGF-alpha-(1-48), with EC50S of about 1:2:7. In the CM assay, all three peptides were full agonists, even though EGF-URO caused an intense desensitization of the tissue whereas TGF-alpha and TGF-alpha-(1-48) did not. The relative affinities of the peptides in the placenta membrane binding assay, EGF-URO greater than TGF-alpha greater than TGF-alpha-(1-48), were in good qualitative and quantitative agreement with the LM (but not the CM) bioassay, with relative KDS in the proportions of about 1:3:17. In the phosphorylation assay, using either the phosphorylated EGF-URO receptor or calpactin-II as an index of receptor kinase activation, the relative potencies of the peptides, EGF-URO greater than TGF-alpha greater than TGF-alpha-(1-48), were also qualitatively in accord with the relative potencies measured in the LM and ligand binding assays (but not in the CM preparation); however, quantitatively, the relative potency ratios (EC50S) observed in the phosphorylation assay (1:2:3) were somewhat out of keeping with the relative values observed in the LM and ligand binding assays. All three peptides were full agonists in the phosphorylation assay. Our data point to the importance of the C-terminal dipeptide, Leu49-Ala50 of TGF-alpha in terms of the binding affinity and intrinsic activity of this polypeptide; and our work provides further evidence for the distinct nature of the EGF-URO/TGF-alpha receptor system present in the CM bioassay preparation. The biological/biochemical activity profiles documented for the three polypeptides can serve as a basis for the further evaluation of other synthetic and naturally occurring members of the EGF-URO/TGF-alpha family of polypeptides.