Phe(13),Tyr(19)-melanin-concentrating hormone and the blood-brain barrier:Role of protein binding

Melanin-concentrating hormone (MCH), found both peripherally and centrally, is involved in food ingestion. Although its expression in brain is increas ed by fasting, it is not known whether it crosses the blood-brain barrier ( BBB). Use of the sensitive method of multiple-time regression analysis has shown that almost all of the peptides and polypeptides tested cross the BBB at a rate faster than the vascular marker albumin. With this same method, however, we found that the 19-amino acid I-125-Phe(13),Tyr(19)-MCH did not cross faster than Tc-99m-albumin, Several mechanisms were excluded as possi ble explanations for the slow rate of influx. These included degradation, a ssociation with capillary endothelial cells, and transport from brain to bl ood. When Phe(13),Tyr(19)-MCH was perfused in blood-free buffer, however, i t entered the brain significantly faster than albumin. This suggested prote in binding as an explanation for the slow rate of influx when the MCH was a dministered in blood. Protein binding was confirmed by capillary zone elect rophoresis, which showed that almost all of the Phe(13),Tyr(19)-MCH added t o blood migrated with a large-molecular-weight substance. Sodium dodecyl su lfate-capillary gel electrophoresis of Phe(13),Tyr(19)-MCH in buffer additi onally showed that the MCH aggregated as a trimer, a factor not preventing its influx by blood-free per fusion. Thus, the results show that blood-born e Phe(13),Tyr(19)-MCH does not significantly cross the BBB, probably becaus e of its binding to serum proteins.