Biotin-labeled norbornene derivative 1 was also evaluated on papain. In the event, papain (15 µM) was incubated with 1 (10 mM) both with and without H 2 O 2 (2 mM) for 1 h at room temperature. These reactions were carried out under an atmosphere of N 2 or in the presence of DTT to prevent air oxidation of Cys25 (S4, S12-15). These reactions were analyzed by SDS-PAGE and western blotting. Probe 1 only labeled papain in the presence of H 2 O 2 (Fig. ).