Phylogenetic diversity and functional efficacy of the C-terminally expressed heptapeptide unit in the opioid precursor polypeptide proenkephalin A.

The heptapeptide Met-enkephalin-Arg6-Phe7 (MERF) with the sequence of YGGFMRF is a potent endogenous opioid located at the C-terminus of proenkephalin-A (PENK), the common polypeptide precursor of Met- and Leu-enkephalin. Our systematic bioinformatic survey revealed considerable sequence polymorphism at the heptapeptide region of different PENK prepropeptides among 56 vertebrate animals. Four orthologous heptapeptides with single or double amino acid replacements were identified among 15 animals, such as YGGFMGY (zebrafish), YGGFMRY (newt), YGGFMKF (hedgehog tenrek) and YGGFMRI (mudpuppy). Each novel heptapeptide, together with the mammalian consensus MERF and Met-enkephalin, were chemically synthesized and subjected to functionality studies, using radioligand binding competition and G-protein activation assays in rat brain membranes. Equilibrium binding affinities changed from good to modest as measured by receptor type selective [3H]opioid radioligands. The relative affinities of the heptapeptides reveal slight mu-receptor (MOP) preference over the delta-receptors (DOP). [35S]GTPγS assay, which measures the agonist-mediated G-protein activation, has demonstrated that all the novel heptapeptides were also potent in stimulating the regulatory G-proteins. All peptides were effective in promoting the agonist induced internalization of the green fluorescence protein-tagged human mu-opioid receptor (hMOP-EGFP) stably expressed in HEK293 cells. Thus, the C-terminally processed PENK heptapeptide orthologs exhibited satisfactory bioactivities, moreover they represent further members of the so-called "natural combinatorial neuropeptide library" emerged by evolution.