Hydrolysis of 60 μmol/L H-D-Trp-Arg-Arg-p-nitroanilide (•, ▪) or S2266 (○, □) by 1 nmol/L human APC (•, ○) or human thrombin (▪, □). Data points depict the absorbance at 405 nm due to the released p-nitroaniline (ε₄₀₅ = 9,920 mol/L⁻¹cm⁻¹) from the cleaved substrate. Experimental conditions are 5 mmol/L Tris, 0.1% PEG, 200 mmol/L NaCl, pH 8.0, at 25°C. When using H-D-Trp-Arg-Arg-p-nitroanilide, APC at a concentration of 1 nmol/L can easily be detected and gives an absorbance signal of released p-nitroaniline after 10 minutes that is 160-fold the noise level of ±0.0001 OD units (or ±10 nmol/L p-nitroaniline) of a Cary3 dual-beam spectrophotometer. On the other hand, thrombin present at the same concentration generates an absorbance signal of released p-nitroaniline after 10 minutes that is indistinguishable from noise. When using S2266, the signal due to substrate cleavage by thrombin far exceeds that due to cleavage by APC over the same time scale.