Differential effects of endomorphin-1, endomorphin-2, and Tyr-W-MIF-1 on activation of G-proteins in SH-SY5Y human neuroblastoma membranes.

Endomorphin-1 (Tyr-Pro-Trp-Phe-NH2) and endomorphin-2 (Tyr-Pro-Phe-Phe-NH2), peptides recently isolated from bovine and human brain, have high affinity and selectivity for mu opiate receptors. They share sequence similarity with the endogenous opiate-modulating peptide Tyr-W-MIF-1 (Tyr-Pro-Trp-Gly-NH2). The efficacies of these endogenous peptides and of the enkephalin analog DAMGO were compared by measuring their effects on the binding of guanosine-5'-O-(-gamma-[35S]thio)triphosphate ([35S]GTPgammaS) to G-proteins in membranes from SH-SYSY human neuroblastoma cells. DAMGO, endomorphin-1, and endomorphin-2 stimulated [35S]GTPgammaS binding dose dependently, with maximal effects of 60 +/- 9%, 47 +/- 9%, and 43 +/- 6% stimulation above basal and ED50 of 49 +/- 8 nM, 38 +/- 8 nM, and 64 +/- 13 nM, respectively. Tyr-W-MIF-1 showed only a small stimulation of binding (5% stimulation above basal, ED50 = 2 microM). When given in combination with the other opioids, however, Tyr-W-MIF-1 attenuated their ability to activate G-proteins. Thus, the endogenous opioids endomorphin-1 and endomorphin-2 activate G-proteins similarly to the synthetic agonist DAMGO, but the structurally similar peptide Tyr-W-MIF-1 produces only minimal stimulation of G-proteins.