Binding affinities to rat brain synaptosomes--synthesis of biotinylated analogues of Substance P.

The synthesis of four biotinylated analogues of Substance P is described. The affinities of these analogues and of their complexes with avidin for the 125I-Bolton Hunter Substance P binding sites on rat brain synaptosomes were determined. While these biotinylated peptides complexed to avidin retain a good biological activity on the guinea-pig ileum bioassay, we observe a net decrease in their binding affinities in the central nervous system. The present study confirms that in the central nervous system the higher affinity is related to the N-terminal tetrapeptide and establishes that the free amino group (N-alpha-Arg and N-epsilon-Lys) are not essential in the binding.