Reciprocal activation of PK and FXII. (A) Reciprocal activation of PK (60 nM) and FXII (25 nM) in the presence of HK-Met³⁷⁹ (magenta; 60 nM), HK-Lys³⁷⁹ (red; 60 nM) or in the absence of HK (light blue) was assessed with a chromogenic substrate assay. Cleavage of S-2302 (200 mM) was followed by continuous changes in optical density (OD; 405 nm) on a spectrophotometer. Control reactions that contained only PK (steel blue) or FXII (gold) are also shown. (B) Reciprocal activation of PK and FXII was performed as described earlier but reactions also contained silica nanospheres (25 mg/mL; 100 nm average diameter).