Synthesis and Biological Properties of a Biotinylated Derivative of Acth_1-17 for MSH Receptor Studies

Abstract

A biotinylated derivative of [β-Ala¹, Lys¹⁷]-ACTH_1-17-NH-(CH₂)₄-NH₂ (ACTH_1-17) was synthesized and biologically characterized. The heptadecapeptide with free N-terminus and blocked side-chains was prepared by the solid-phase method using TentaGel resin and a 4-aminobutylamide linker. Biotinyl-β-Ala-OH was then coupled to the terminal amino group and the resulting [Nα-(biotinyl-β-alanyl)-β-Ala¹, Lys¹⁷]-ACTH_1-17-NH-(CH₂)₄-NH₂ (Bio-ACTH_1-17) cleaved from the resin, purified and analyzed. Competition binding assays with mouse B16-F1 and human D10 and HBL melanoma cells using [¹²⁵I]-α-MSH as radioligand gave dissociation constants for Bio-ACTH_1-17 of 1.67 ± 0.07 nM (B16-F1), 0.02 ± 0.005 nM (D10) and 0.21 ± 0.02 nM (HBL). The EC₅₀ for Bio-ACTH_1-17 in the B16 melanin assay was 4.15 ± 1.0 nM. Analysis of the binding characteristics of [¹²⁵I]-Bio-ACTH_1-17 demonstrated that in human melanoma cells this radioligand was displaced by ACTH_1-17 as well as α-MSH whereas in B16-F1 cells the tracer was only displaced from the binding site by ACTH_1-17. Studies of Bio-ACTH_1-17 with streptavidin showed that the peptide is to a large