Peptide transport system-1 (PTS-1) for Tyr-MTF-1 and Met-enkephalin differs from the receptors for either

Tyr-MIF-1 (Tyr-Pro-Leu-Gly-NH2) and Met-enkephalin share a saturable transp ort system (peptide transport system-1, PTS-1) across the blood-brain barri er but do not readily bind to each other's receptors. This information allo ws the unique opportunity to differentiate the transport protein(s) from th e receptors for either peptide in brain endothelial cells. PTS-1 was studie d in vitro by allowing radiolabeled Tyr-MIF-1 (I-125-Tyr-MIF-1) to bind to the solubilized proteins of isolated murine brain microvessels in the prese nce or absence of potential inhibitors. Sephadex chromatography separated b ound from free labeled peptide. The binding was saturable as shown by inhib ition with increasing concentrations of unlabeled Tyr-MIF-l. I-125-Tyr-MIF- 1 binding was not inhibited by an unrelated peptide or iodo-tyrosine. D-Tyr -MIF-1 had no effect, demonstrating the stereospecificity of the system. Me t-enkephalin decreased the binding of I-125-Tyr-MIF-1 to 84.4% of total, wh ereas Leu-enkephalin was without effect. Agonists for the mu, delta, and ka ppa opiate receptors did not change the binding, indicating that the protei ns which bound to I-125-Tyr-MIF-1 were not endogenous opiate receptors. The results indicate that, in vitro, Tyr-MIF-1 binds to brain microvessel prot eins with characteristics similar to PTS-1. (C) 1999 Published by Elsevier Science B.V. All rights reserved.