Development of a biotin-avidin probe for detecting opioid receptors.

Biotinylated derivatives of beta h-endorphin (beta h-EP) with C6 spacer arm, inserted between biotin and beta h-EP, were synthesized and isolated by HPLC. Liquid secondary ion mass spectrometry (LSIMS) indicated the presence of 1 to 4 biotin substituents per beta h-EP molecule, and in combination with the analysis of tryptic peptide fragments, specified the location of the biotinylated lysine residue. Affinities to mu receptors decreased with increasing biotinylation number. Association of the biotinylated ligands with avidin retained or even enhanced IC50 values at the mu site, thus, matching the relative binding affinity of underivatized beta h-EP with the monobiotinylated derivatives. Hence, monobiotinylated beta h-EP represents a versatile opioid receptor probe.