A useful fluorescence resonance energy transfer (FRET) substrate for assaying ECE-1 and ECE-2, ACE1 and ACE2. neprilysin, thermolysin (TLN), MMP-2 and MMP9, thimet oligopeptidase, cathepsin A, cathepsin X/Z, BACE, and insulinase, but not MMP1. Fluorescence of Mca (donor) is quenched by Dnp (quencher) in the intact peptide. Proteolytic cleavage of the FRET peptide separates the donor and the quencher, recovering the quenched fluorescence of Mca that is directly related to the enzyme activities and can be monitored at Ex/Em: 328 nm/393 nm. MW: 1388.46. Purity ≥95% by HPLC