Formation of SDS-stable complexes between fusion proteins and thrombin. Either 1.0 μM API M358R (Panel A) or 1.0 μM HV3API M358R (Panel B) was combined with 0.2 μM thrombin (IIa) for the time in seconds shown above the lanes. Reactions were stopped with SDS and aliquots were electrophoresed on reduced 10% SDS polyacrylamide gels, and stained with Coomassie Brilliant Blue. Thrombin-dependent serpin-enzyme complexes with retarded mobility (either API M358R-IIa or HV3API M358R-IIa), unreacted serpins, cleaved serpins (cl API M358R or cl HV3API M358R), or the B chain of thrombin (IIa (B)) are labelled, at right. The position of molecular mass markers is shown, in kDa, to the left of each panel.