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 increased 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 possible explanations for the slow rate of influx. These included degradation, association with capillary endothelial cells, and transport from brain to blood. When Phe(13),Tyr(19)-MCH was perfused in blood-free buffer, however, it entered the brain significantly faster than albumin. This suggested protein binding as an explanation for the slow rate of influx when the MCH was administered in blood. Protein binding was confirmed by capillary zone electrophoresis, which showed that almost all of the Phe(13),Tyr(19)-MCH added to blood migrated with a large-molecular-weight substance. Sodium dodecyl sulfate-capillary gel electrophoresis of Phe(13),Tyr(19)-MCH in buffer additionally 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-borne Phe(13),Tyr(19)-MCH does not significantly cross the BBB, probably because of its binding to serum proteins.