Reptilian enkephalins: implications for the evolution of proenkephalin

To shed light on the evolution of the opioid peptide precursor proenkephalin, we have investigated the profile of immunoreactive enkephalins in reptilian brain using six antisera directed toward different portions of the proenkephalin molecule. Three different orders of reptiles--turtles, alligators, and lizards--were studied; these orders represent lineages diverging more than 250 million years ago. Reptilian brain was found to contain large quantities of Met5-enkephalin (Met5-enk), Leu5-enkephalin (Leu5-enk), and Met5-enk-Arg6-Phe7 (MERF); gel filtration and reverse-phase chromatography of acid extracts prepared from turtle brain indicated that nearly all of the immunoreactivity corresponding to these peptides could be ascribed to the authentic penta- to heptapeptides. Immunoreactive metorphamide, but not amidorphin, was present in all three species. Unlike mammalian brain, reptilian brain does not contain immunoreactive Met5-enk-Arg6-Gly7-Leu8 (MERGL). These results indicate that reptilian proenkephalin, while similar to mammalian enkephalin in containing Met5-enk, Leu5-enk, MERF, and metorphamide, nevertheless differs from mammalian proenkephalin. Reptilian proenkephalin either may contain the MERGL sequence in a form not recognized by the MERGL antibody, or may lack the MERGL sequence entirely.