Systems, including compositions, kits, and methods, suitable for performing enzyme assays, such as protease assays. The composition may include derivatives of rhodamine 110 that are substitued at the xanthylium nitrogens by a peptide moiety and a morpholine-4-carbonyl moiety, respectively. These compositions may be luminogenic substrates for a large variety of protease enzymes, with utility in a variety of sensitive protease enzyme assays.