Early Processing of Prothrombin and Factor X by the Vitamin K-dependent Carboxylase

Binding interactions between the membrane-associated vitamin K-dependent carboxylase and its prothrombin and factor X substrates have been investigated in liver microsomes. Both substrates are firmly attached to microsomal membrane fragments which also harbor the carboxylase. In vitro 14CO2 gamma-carboxylation of these substrates, triggered by reduced vitamin K1H2, resulted in release of 14C-labeled prothrombin precursors from the membrane fragments, but no release of 14C-labeled factor X precursors could be demonstrated, which suggested a difference in early processing of these substrates by the carboxylase. Warfarin treatment of rats resulted in a 3-fold increase in the membrane concentration of factor X antigens and a 20-fold increase in 14C gamma-carboxylation of the membrane pool of factor X carboxylase substrates. There was a dose-response relationship between the amount of drug administered to the rats and 14C labeling of the membrane pool of factor X carboxylase substrates. On the other hand, the membrane concentration of prothrombin antigens did not increase in response to the drug, and 14CO2 gamma-carboxylation of the membrane pool of prothrombin carboxylase substrates was the same in warfarin and saline-treated rats. The results demonstrate significant differences in the interaction between the carboxylase and its prothrombin and factor X substrates. It appears that the different interactions result from binding of the prothrombin and the factor X precursors to separate microsomal membrane proteins that are involved in the gamma-carboxylation reaction. Warfarin appears to induce the factor X precursor-specific but not the prothrombin precursor-specific binding proteins, which suggests a new mechanism for the action of warfarin. These binding proteins may be under different genetic control. Treatment of the prothrombin and the factor X carboxylase substrates with endonuclease H showed that the rat prothrombin and the human factor X carboxylase substrates are high mannose glycoproteins. The human prothrombin and the rat factor X carboxylase substrates did not, on the other hand, change their migration in sodium dodecyl sulfate-polyacrylamide gel electrophoresis gels after endonuclease H treatment. The data demonstrate differences in the glycoprotein nature of the rat and the human carboxylase substrates.