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.