Sequence determination of a peptide fragment from electric eel acetylcholinesterase, involved in the binding of quaternary ammonium

Specific photoaffinity labelling of purified electric eel acetylcholinesterase by 3 H‐labelled p ‐( N , N ‐dimethyl‐amino) benzenediazonium fluoroborate allows the identification of a labelled peptide fragment which is described as being involved in the binding of quaternary ammonium ions on this enzyme. Denaturation and proteolytic cleavage of the inactivated enzyme gave a mixture of peptide fragments. The purification of one labelled fragment, containing over 15% of the radioactivity incorporated in the enzyme, led to the following sequence: Gly‐Ser‐X‐Phe. The relatively low amount of this tetrapeptide did not allow us to determine the nature of the labelled residue X.