α,N-Acetyl β-Endorphin [1-8] Is the Terminal Product of Processing of Endorphins in the Melanotrope Cells of Xenopus laevis, as Demonstrated by FAB Tandem Mass Spectrometry

Abstract

The major N-terminal acetylated endorphin of the pars intermedia of Xenopus laevis was purified and submitted to fast-atom bombardment tandem mass spectroscopy. The collisionally induced dissociation MS/MS spectrum of the [M+H]+ ion revealed sufficient fragment ions to determine unambiguously the identity of the peptide as α,N-acetyl β-endorphin [1-8], the sequence of which was predicted on the basis of the nucleotide sequence of Xenopus POMC cDNA. The determination was confirmed by showing that the synthetic peptide of this structure had identical FAB tandem mass spectrometric characteristics as the endogenous endorphin. We conclude that α,N-acetyl β-endorphin [1-8] is the terminal product of processing of endorphins in the melanotrope cell of Xenopus laevis.