The submicrosomal site for the conversion of prothrombin precursor to biologically active prothrombin in rat liver

Abstract

The submicrosomal site for the conversion of prothrombin precursor to prothrombin in rat lever has been investigated by subcellular fractionation techniques. Prothrombin precursor activity could be detected in the luminal as well as the membrane fraction of the rough microsomes. The corresponding fractions from smooth microsomes did not exhibit any activity. After warfarin treatment of the rats, the concentration of prothrombin precursor in rough microsomes increased rapidly from approx. 2 to 6–8 h, when a plateau was reached. In smooth microsomes, prothrombin precursor activity appeared 1 h after injection of warfarin, and increased to a plateau reached after about 4 h. The total activity of prothrombin precursor at the plateau obtained after warfarin treatment was 4–5 times higher in the rough luminal fraction than in the corresponding smooth fraction. The vitamin K-dependent carboxylase activity was localized to the rough microsomes. The enzyme system was associated with the membrane, mainly at the luminal side, whereas the substrate appeared to be localized in both the luminal and membrane fraction. The results indicate that the conversion of prothrombin precursor to biologically active prothrombin occurs at a late stage in the rough endoplasmic reticulum or at a transition between rough and smooth endoplasmic reticulum.