Preparation of specifically tritiated endomorphins

Endomorphin-1 (EM1, Tyr-Pro-Trp-Phe-NH2) and endomorphin-2 (EM2, Tyr-ProPhe -Phe-NH2) are natural tetrapeptide ligands of mu -opioid receptors involved in the modulation and attenuation of pain. For a detailed examination of t heir receptor-binding properties and their metabolic stability, tritium-lab elled EM1 and EM2 radioisotopomers were synthesized by catalytic dehalogena tion or saturation of the precursor peptides with tritium gas. Amino acid a nalysis revealed that the tritium labelling was specific and the specifical ly labelled radioligands possessed high specific activity, ranging from 0.7 7 TBq/mmol to 2.35 TBq/mmol. The biological half-lives of the peptides in t he biological matrix (295 min and 230 min for EM1 and EM2, respectively) in dicate that these radioligands are appropriate for binding assays in rat br ain membrane preparations. The radioisotopomers of EM2 are not statisticall y different in the receptor-ligand interaction, and they are excellent tool s for further comparative biochemical studies.