The phylogeny of Met-enkephalin and Leu-enkephalin: Studies on the holostean fish Lepisosteus platyrhincus and the Australian lungfish, Neoceratodus forsteri

Acid extracts of the brain of the holostean fish Lepisosteus platyrhincus and the forebrain of the dipnoan fish Neoceratodus forsteri were separately fractionated by Sephadex G-50 column chromatography. For both species, Met-enkephalin-related immunoreactivity was detected coeluting with the total volume internal standard. Higher-molecular-weight Met-enkephalin-containing immunoreactive peaks were not detected in these chromatographs. Furthermore, immunoreactive forms with antigenic determinants identical to mammalian dynorphin A(1-17), dynorphin A(1-8), α-neo-endorphin, or dynorphin B(1-13) were not detected in either species. Reverse-phase HPLC analysis of enkephalin-sized immunoreactive material indicated the presence of authentic Met-enkephalin and Leu-enkephalin in the extracts of both species. In the brain of L. platyrhincus the molar ratio of Met-enkephalin to Leu-enkephalin was approximately 3:1, whereas, the molar ratio of these enkephalins in the forebrain of N. forsteri was approximately 14:1. C-terminally extended forms of Met-enkephalin were also detected in the extracts of both species. These results suggest that the ancestral proenkephalin gene of both actinopterygian and sarcopterygian fish contained both the Met-enkephalin and Leu-enkephalin sequences.