Assessment of vitamin K status in human subjects administered "minidose" warfarin

Vitamin K is required to convert specific glutamyl residues in a limited number of proteins to gamma-carboxyglutamyl residues. The response of various measures of vitamin K insufficiency to the administration of 1 mg/d of the vitamin K antagonist warfarin was studied in two groups of nine older (55–75 y) or younger (20–28 y) subjects. The most consistent and extensive alteration was an increase in the concentration of serum under-gamma-carboxylated osteocalcin followed by increased immunochemical detection of plasma under-gamma-carboxylated prothrombin (PIVKA-II), and by a decreased urinary excretion of gamma-carboxyglutamic acid. Plasma concentrations of prothrombin were altered by this treatment but prothrombin times, factor VII activity, prothrombin F-1 x 2 concentrations, and a less sensitive assay for under-gamma-carboxylated prothrombine were not. The concentration of serum under-gamma-carboxylated osteocalcin was lower when subjects consumed 1 mg vitamin K/d than when they consumed their normal diet.