Kits and peptide substrates for detecting transferase activity of a sample are provided. The kits include a substrate including a reporter compound, at least one of a phosphate group donor and a phosphate group acceptor, a buffer that supports enzymatic activity of the transferase, and a peptidase that cleaves a non-phosphorylated peptide substrate at a first rate and a phosphorylated peptide substrate at a second rate. The peptide substrates include a reporter compound and a first transferase substrate linked to the reporter compound on a first side of the reporter compound.