Inhibition of murine thrombin by rHCII variants. Murine thrombin (14 nM) was incubated for 60 seconds with 185 nM human rHCII (WT, ●; R189H, ○; K173Q, □) and either DS or heparin at the final concentrations shown. Residual thrombin activity was then assayed with a chromogenic substrate (ΔA405/min). No HCII/GAG indicates thrombin activity in the absence of HCII and glycosaminoglycan; IC₅₀, 50% of initial thrombin activity.