Expression of Neutral Endopeptidase (enkephalinase) in Heterologous COS-1 Cells. Characterization of the Recombinant Enzyme and Evidence for a Glutamic Acid Residue at the Active Site

Neutral endopeptidase (EC 3.4.24.11) is an integral membrane protein found in the plasma membrane of many cell types. The cDNA coding for the complete primary structure of neutral endopeptidase has recently been cloned and sequenced (Devault, A. Lazure, C., Nault, C., Le Moual, H., Seidah, N. G., Chretien, M., Kahn, P., Powell, J., Mallet, J., Beaumont, A., Roques, B. P., Crine, P., and Boileau, G. (1987) EMBO J. 6, 1317-1322). Comparison of the sequence of neutral endopeptidase with that of thermolysin, a bacterial Zn-metalloendopeptidase, suggests that Glu-584 in neutral endopeptidase probably corresponds to Glu-143 in thermolysin, which is an essential amino acid involved in catalysis. To test directly the importance of Glu-584 in the catalytic activity of neutral endopeptidase by site-directed metagenesis, we have constructed an expression vector in which the rabbit kidney cDNA encoding the entire neutral endopeptidase sequence is introduced downstream from the SV40 virus early promotor. After transfection in COS-1 monkey kidney cells, this vector was found to promote the expression of a protein with biochemical and catalytic properties identical to kidney neutral endopeptidase. Oligonucleotide-directed mutagenesis of Glu-584 to either valine or aspartic acid completely abolished the enzymatic activity of the recombinant protein without changing its affinity for the substrate-related tritiated inhibitor [3H]N-[(2R,2S)-3-hydroxyamino-carbonyl-2-benzyl-1-oxopropyl]-glycine. This observation clearly identifies Glu-584 as one of the important residues responsible for the catalytic activity of the enzyme.