Beta-endorphin chimeric peptides: transport through the blood-brain barrier in vivo and cleavage of disulfide linkage by brain.

Water soluble peptides are normally not transported through the brain capillary wall, i.e. the blood-brain barrier (BBB). Chimeric peptides may be transportable through the BBB and are formed by the covalent coupling of a nontransportable peptide, e.g. beta-endorphin, to a transportable peptide vector, e.g. cationized albumin, using disulfide-based coupling reagents such as N-succinimidyl 3-[2-pyridyldithio(propionate)] (SPDP). The transcytosis of peptide into brain parenchyma, as opposed to vascular sequestration of blood-borne peptide, was quantified using an internal carotid artery perfusion/capillary depletion method. It is shown that [125I]beta-endorphin is not transported through the BBB, but is rapidly cleaved to free [125I] tyrosine via capillary peptidase. Therefore, chimeric peptide was prepared using [125I] [D-Ala2]beta-endorphin (DABE), owing to the resistance of this analogue to peptidase degradation. The [125I] DABE-cationized albumin chimeric peptide is shown to enter brain parenchyma at a rate comparable to that reported previously for unconjugated cationized albumin. When the [125I] DABE-cationized albumin chimeric peptide was incubated with rat brain homogenate at 37 C, the free [125I] DABE was liberated from the cationized albumin conjugate prior to its subsequent degradation into free [125I] tyrosine. Approximately 50% of the chimeric peptide was cleaved within 60 sec of incubation at 37 C. These studies demonstrate that 1) [125I]beta-endorphin is not transported through the BBB in its unconjugated form, 2) a [125I] DABE-cationized albumin chimeric peptide is transported through the BBB into brain parenchyma at a rate comparable to the unconjugated cationized albumin, and 3) brain contains the necessary disulfide reductases for rapid cleavage of the chimeric peptide into free beta-endorphin and this cleavage occurs before degradation of the [125I] DABE into [125I] tyrosine.