Effect of various protease inhibitors on the activity of ΔN54SppATk. The inhibitors examined were leupeptin (a), antipain (b), chymostatin (c), diisopropyl fluorophosphate (d), phenylmethylsulfonyl fluoride (e), pepstatin (f), elastatinal (g), and EDTA (h). Activity measurements were performed at 60°C with the indicated inhibitor concentrations.