Surprisingly, neither peptide showed any inhibition of PADI4 activity as measured using the COLDER assays (Table , Figure ). Arg4 did, however, bind reversibly to PADI4 with an affinity of 2.0 µM, as measured by SPR, whilst Cit4 showed negligible binding at the concentrations tested (Figure ). This is consistent with Arg4 acting as a substrate of PADI4;