Demonstration of a cell-specific isomerization of invertebrate neuropeptides

Neurohemal organs of the lobster Homarus americanus contain isoforms of the crustacean hyperglycemic hormone, which differ by the third amino acid (phenylalanyl) residue that is either in the L- or in the D-configuration. Polyclonal antisera have been raised in rabbit against synthetic octapeptides with the sequence corresponding to the N-terminal part of the L- or D-phenylalanine-containing isoforms. Their specificity was shown by immunoassays, indicating that they discriminate the isoforms of the lobster hyperglycemic neuropeptides. It was demonstrated that the two major forms of the crayfish Orconectes limosus hyperglycemic hormone also correspond to peptide isomers containing the L- or D-phenylalanyl residue. The cellular distribution of the isoforms among the neurosecreting cells of the major neuroendocrine complex in lobster and crayfish has been studied by immunohistochemistry. Every hyperglycemic hormone-containing cell was labelled with the anti-L antisera while only some of them were visualized with the anti-D antisera. These results constitute the first observation of peptide isomerization at the cellular level and suggest that the isomerization process occurs in specialized neuroendocrine cells.