Re-evaluation of Cyclic Peptide Binding to Neurotensin Receptor 1 by Shifting the Peptide Register during Synthesis.

The head-to-tail cyclic peptide [Arg-Lys-Pro-Tyr-Tle-Leu] (peptide , where Tle is l--Leu) has previously been reported to bind to neurotensin receptor 1 (NTS1) (pKi = 5.97). Upon seeking to reproduce this finding, we found that peptide did not have a measurable affinity for NTS1. However, a semipurified preparation of peptide appeared to bind to NTS1 with pKi = 5.83 ± 0.25 SEM. Resynthesis of peptide using a shifted peptide register gave linear and cyclic forms of peptide that were both unable to bind to NTS1. We observe that the previously reported activity of peptide may be due to the presence of high affinity linear contaminants. Approximately 3% contamination with the linear variant would explain the apparent binding of the semipure peptide sample. From this study, we propose that shifting the peptide register during synthesis as a strategy to minimize the presence of potent precursor contaminants.