5-Oxo-L-prolylglycyl-L-arginine 4-nitroanilide 5-Oxo-pro-gly-arg-4-nitroanilide 5-oxo-prolyl-glycyl-arginine-4-nitroanilide 74661-31-7 Chromogenic substrate S-2444 L-Argininamide, 5-oxo-L-prolylglycyl-N-(4-nitrophenyl)- OPGAN S-2444