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Fluorescent-labeled bioconjugates of the opioid peptides biphalin and DPDPE incorporating fluorescein-maleimide linkers.

AIM
The conjugation of fluorescent labels to opioid peptides is an extremely challenging task, which needs to be overcome to create new classes of probes for biological assays.


MATERIALS & METHODS
Three opioid peptide analogs of biphalin and [D-Pen2,5]-Enkephalin (DPDPE) containing a fluorescein-maleimide motif were synthesized.


RESULTS & DISCUSSION
The biphalin analog 17 binds to opioid receptors with Kiμ = 530 ± 90 nM and Kiδ = 69.8 ± 16.4 nM. We then tested the ability of the compounds to stimulate G-protein-coupling, 17 activated μ-receptor expressing cells (EC50 = 16.7 ± 6.7 nM, EMax = 76 ± 4%) as well as δ-receptor expressing cells (EC50 = 42 ± 10 nM, EMax = 34 ± 8%). However, 17 was not able to fluorescently label receptor in live or fixed cells.


CONCLUSION
Our data suggest that the biphalin scaffold could be employed to develop fluorescent ligands with the appropriate fluorescent motif, and suggest a means for further probe development.