Synthesis of tritium labelled endomorphin II and its stability in the radioreceptor assay

Endomorphine I (Tyr-Pro-Trp-Phe-NH2) and endomorphin II (Tyr-Pro-Phe-Phe-NH 2) are recently isolated neuropeptides. They have the highest specificity a nd affinity fbr the mu-opiate receptor among all the endogenous substances so far described, and they may be natural ligands for this receptor [1]. We prepared the [H-3]endomorphin II with high specific radioactivity (53.4 Ci /mmol) by catalytic dehalotritiation. The precursor [(3,5-I-2)Tyr(1)]-endom orphin II was synthesized by solid phase peptide synthesis using Boc chemis try. Labelled endomorphin II was used to investigate its binding properties in rat brain membrane. The stability of [3H]endomorphin II toward enzymati c degradation in membrane preparation was examined by RP-HPLC and by using a radioactivity detector.