Rabbit muscle extracts catalyze the specific removal of N-acetylmethionine from acetylated peptides.

Rabbit muscle has been found to contain an activity that catalyzes the specific removal of Ac-Met from acetylated peptides. The activity is associated with free ribosomes and microsomes in the rabbit muscle extract but can be removed from these subcellular fractions by exposure to 0.5 M NaCl in the presence of 2 mM MgCl2; only partial removal was achieved with microsomes, but complete removal with ribosomes. A nearly 200-fold enrichment of the activity was achieved by this simple succession of differential centrifugation and salt extraction. Eighteen 14C-acetylpeptides have been tested as substrates for the partially purified activity assaying for the production of free 14C-acetylamino acid by high performance liquid chromatography. None of the peptides containing N-terminal acetylated Ala, Asp, Ser, or Gly were cleaved at a significant rate. Six of a total of eight peptides containing N-terminal Ac-Met were cleaved by the ribosomal extract at different rates. The active substrates varied in length from tri- to undecapeptides. The activity is inhibited by high concentrations of the protease inhibitor phenylmethylsulfonyl fluoride. Based on these observations, we tentatively conclude that the activity satisfy the criteria of a general N-terminal protein processing enzyme: it can remove Ac-Met from most, but not all, N-terminal sequences and appears to be inactive toward the N-terminal acetylamino acids most commonly found in eukaryotic proteins.