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

Tyr-MIF-1 (Tyr-Pro-Leu-Gly-NH2) and Met-enkephalin share a saturable transport system (peptide transport system-1, PTS-1) across the blood-brain barrier but do not readily bind to each other's receptors. This information allows the unique opportunity to differentiate the transport protein(s) from the receptors for either peptide in brain endothelial cells. PTS-1 was studied in vitro by allowing radiolabeled Tyr-MIF-1 (125I-Tyr-MIF-1) to bind to the solubilized proteins of isolated murine brain microvessels in the presence or absence of potential inhibitors. Sephadex chromatography separated bound from free labeled peptide. The binding was saturable as shown by inhibition with increasing concentrations of unlabeled Tyr-MIF-1. 125I-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. Met-enkephalin decreased the binding of 125I-Tyr-MIF-1 to 84.4% of total, whereas Leu-enkephalin was without effect. Agonists for the mu, delta, and kappa opiate receptors did not change the binding, indicating that the proteins which bound to 125I-Tyr-MIF-1 were not endogenous opiate receptors. The results indicate that, in vitro, Tyr-MIF-1 binds to brain microvessel proteins with characteristics similar to PTS-1.