Vitamin K epoxidase: dependence of epoxidase activity on substrates of the vitamin K-dependent carboxylation reaction

Abstract

Vitamin K catalyzes the post-translational carboxylation of specific glutamyl residues in precursor proteins to form ycarboxyglutamyl residues which are present in prothrombin and other vitamin K-dependent proteins. These precursors increase in the livers of vitamin K-deficient rats, and the carboxylation of both endogenous protein precursors and low molecular weight peptide substrates can be demonstrated in detergent-solubilized microsomal preparations [1,21. The same microsomal preparations which carry out this O2 dependent carboxylation reaction will also catalyze the conversion of vitamin K to its 2,3epoxide (epoxidase activity). Observations that the enzymatic formation of the vitamin K epoxide is elevated in livers of vitamin K-deficient rats, which also contain high concentrations of prothrombin precursors, led to the proposal that epoxidation of vitamin K is associated with the action of the viatamin in promoting prothrombin synthesis [3]. This hypothesis has been strengthened by studies of the effects of various anticoagulants in normal and Warfarinresistant rats [4-71 by observations that many of the requirements for the epoxidation and carboxylation reactions are similar [8], by observations of the specificity and distribution of this activity [9,10], and by evidence of a common vitamin K intermediate required for both reactions [ 111. These studies (reviewed [ 121) support the concept that the formation of vitamin K epoxide is coupled to the vitamin K-dependent carboxylation event; but they are not conclusive. This report presents additional evidence that epoxidation of vitamin K is associated with the vitamin K-dependent carboxylation reaction. A syn-