N-bromoacetyl-D-leucylglycine: An affinity label for neutral endopeptidase 24.11

Neutral endopeptidase 24.11 is rapidly inactivated by N-bromoacetyl-D-leucylglycine in a reaction which follows first-order kinetics at pH 8 and 37°C. The concentration dependence of inactivation revealed saturation kinetics with an apparent K_i of 10 mM and k_inact of 0.4 min^-1 at saturating inhibitor concentration. Enzyme can be protected from inactivation by either the substrate Leu⁵-enkephalin or the competitive inhibitors Phe-Gly or Phe-Ala. Inactivation of enzyme by N-bromo-[¹⁴C]acetyl-D-leucylglycine proceeds with the incorporation of a stoichiometric amount of labeled inhibitor. Tryptic digestion of the radioactively labeled enzyme followed by high performance liquid chromatography allowed the isolation of a modified peptide with the sequence T-D-V-H-S-P-G-N-F-R in which histidine (His⁷⁰⁴) is the modified residue. Site-directed mutagenesis was used to generate a mutant form of the enzyme in which histidine 704 was converted to a glutamine residue. This mutant enzyme retained less than 0.1% of the activity of the native enzyme. These results demonstrate that His⁷⁰⁴ is at the active site of neutral endopeptidase 24.11 and suggest a catalytic role for this residue.