The contribution of helical potential to the in vitro receptor binding activity of a neuropeptide Y N-terminal deletion fragment.

In its dimeric form neuropeptide Y (NPY) folds into a compact structure in which the antiparallel oriented proline and alpha-helices apparently associate to form a primitive hydrophobic core. To investigate the contribution of helical stability to the receptor binding activity of NPY and its N-terminal deletion fragments, we synthesized and studied the solution conformational properties and in vitro activities of NPY, N alpha-acetyl-NPY2-36, NPY15-36, N alpha-propionyl-NPY15-36, and N alpha-succinyl-NPY15-36. NPY15-36 is significantly less helical than both NPY and N alpha-acetyl-NPY2-36, and this decreased helical potential is attributed to the absence of the intramolecular stabilizing interaction afforded by the proline helix in the latter analogues. However, in accord with the helix dipole model, the helical potential of NPY15-36 is significantly increased by N-terminal succinylation, whereas propionylation has no effect. In addition to an increase in helical potential, N alpha-succinyl-NPY15-36 is 2.5 and 4.6 times more active than NPY15-36 and N alpha-propionyl-NPY15-36, respectively, and is equipotent with N alpha-acetyl-NPY2-36 in displacing 1 nM [3H]-NPY from specific binding sites in rat brain membranes. The demonstration of a positive correlation between % alpha-helix content and in vitro binding activity suggests that the helical potential of N-terminal NPY deletion fragments contributes to their in vitro activity in the rat brain, and that a second role of the proline helix might be to stabilize the receptor-active conformation of the NPY alpha-helix.