Detection of Met-enkephalin in the pars intermedia of the lampreys, Ichthyomyzon castaneus and Petromyzon marinus.

Acid extracts of the brain and pars intermedia of the chestnut lamprey, Ichthyomyzon castaneus, were fractionated by a combination of gel filtration chromatography and reversed-phase HPLC, and screened with RIAs specific for Met-enkephalin and Leu-enkephalin, respectively. In the brain extract, both Met-enkephalin and Leu-enkephalin were detected in a molar ratio of approximately 4:1. These results would suggest that these two enkephalins were derived from a lamprey Proenkephalin precursor. However, reversed-phase HPLC analysis of the pars intermedia of this species revealed the presence of Met-enkephalin, but not Leu-enkephalin. Analysis of the pars intermedia of the marine lamprey, Petromyzon marinus, also indicated the presence of Met-enkephalin but not Leu-enkephalin. These results would suggest that the Met-enkephalin present in the pars intermedia of lampreys may not be derived from Proenkephalin, but may originate from another opioid precursor. This possibility and alternative hypotheses to explain these observations are discussed.