Binding of Tyr-MIF-1 to isolated brain capillaries

Tyr-MIF-1 (Tyr-Pro-Leu-Gly-amide) and the enkephalins have previously been shown with in vivo studies to be transported out of the brain by a saturable, carrier-mediated system. The possibility that this transport system occurs at the level of the endothelial cells was tested by measuring the ability of isolated brain capillaries to bind radioiodinated Tyr-MIF-1. The binding of labeled Tyr-MIF-1 was displaced by unlabeled Tyr-MIF-1 (50% inhibition at 1.5 X 10(-6) M) and by nonradioactive (I127) iodinated Tyr-MIF-1, but not by tyrosine, MIF-1 (Pro-Leu-Gly-amide), Tyr-Pro, or D-Tyr-MIF-1. This pattern of inhibition is similar to that found for the in vivo transport of Tyr-MIF-1 out of the brain. Tyr-MIF-1 did not appear to be bound by albumin or degraded by cells aged 3-7 days at 4 degrees C. These results suggest that the transport system previously described in vivo for Tyr-MIF-1 and the enkephalins may be associated with the brain endothelial cells.