Effect of peptidases at the blood brain barrier on the permeability of enkephalin

The blood brain barrier (BBB) presents an enzymatic barrier to the passage of peptides from blood to brain. The studies presented here used a well established in vitro model of the BBB to measure the presence of peptidases and the permeability of two opioid peptides. The in vitro BBB model consisted of confluent monolayers of bovine brain microvessel endothelial cells (BMECs). Enkephalin metabolizing enzymes, total aminopeptidase, aminopeptidase M (APM), angiotensin converting enzyme (ACE) and neutral endopeptidase (NEP) activities were measured in BMEC monolayers. The effect of specific inhibitors of APM, ACE and NEP on the permeability of [Met⁵]enkephalin (Met-Enk) and a conformationally constrained and enzymatically stable analog, DPDPE, also was determined. High levels of membrane-associated enzyme activity were measured for total aminopeptidase, APM and ACE. Interestingly, the permeability coefficient of Met-Enk was increased 4-fold in the presence of specific inhibitors of APM and ACE. Low levels of NEP activity were measured in BMEC monolayers and inhibition of NEP had no effect on Met-Enk permeability. The permeability coefficient for DPDPE was not increased with enzyme inhibitors but was 4-fold greater than Met-Enk alone. In the presence of APM or ACE inhibitors, there was no difference in the permeability of DPDPE and Met-Enk. These experiments demonstrate the presence of specific peptidases in BMECs and that the presence of inhibitors to Met-Enk inactivating peptidases significantly increased permeability of this biologically active peptide.