Identification of an acidic dipeptide, beta-aspartylglycine, in the CNS of Aplysia.

A novel dipeptide, beta-aspartylglycine (beta-DG), has been isolated from tissues of the marine gastropod mollusc Aplysia californica. This compound was detected only in Aplysia and not in other molluscs, such as Helix or Mercenaria, or in lobster or frog. Among the Aplysia tissues, the highest levels of beta-DG were in nervous tissue and in the reproductive tract. beta-DG was assayed by HPLC as the o-phthaldialdehyde derivative and found to be present in all individual, identified neurons at a concentration of approximately 40 pmol/microgram protein. The peptide was identified as beta-DG by gas chromatography-mass spectrometry (GCMS) using trimethylsilyl derivatives prepared before and after acid hydrolysis. It was further characterized as the beta-isomer by TLC, including Rf, atypical blue-gray color with ninhydrin, and a violet color with Cu2+-ninhydrin. A fractionation scheme is described whereby acid-soluble tissue constituents can be divided into acidic, neutral, and basic components using mini ion-exchange columns. This partial purification prior to TLC analysis was necessary to remove compounds that interfered with the isolation of beta-DG.