Endogenous VKOR and GGCX activity in HEK293 and AV12 cells as measured by in vitro enzymatic activity assay. (A) Endogenous GGCX activity of 1 × 106 HEK293 or AV12 cells was determined as described in “GGCX activity assay” using FLEEL as a substrate in the presence of propeptide. (B) Endogenous VKOR activity of 1 × 107 HEK293 or AV12 cells was determined as described in “VKOR activity assay” using K1(25) as the HPLC internal standard.