Fluorogenic tri-peptide substrate for measuring the trypsin-like peptidase activity of the 20S proteasome. The 20S complex is composed of 28 subunits, arranged in an α₇β₇β7α₇ stoichiometry. Each of the two internal β-type rings harbors three different proteolytically active sites, provided by the amino-terminal residues of three constitutive subunits: β1 (post-glutamyl peptide hydrolase site), β2 (trypsin-like site) and β5 (chymotrypsin-like site). Ac-Arg-Leu-Arg-AMC is particularly useful due to the low Km, high specific activity and the fact that it is cleaved exclusively at the amide R-AMC bond to release fluorescent product.