ABSTRACT
Vitamin K activity is exhibited by phylloquinone, found in green plants, and by a series of menaquinones, which are synthesized by a limited number of anaerobic bacteria. The metabolic role of this vitamin is as a substrate for an enzyme, the vitamin K-dependent carboxylase, which mediates a posttranslational modification of a small number of proteins by converting specific glutamyl residues to g-carboxyglutamyl (Gla) residues. These proteins include a number that regulate hemostasis: prothrombin, factor VII, factor IX, factor X, and proteins C, S, and Z. The bone protein, osteocalcin, and matrix Gla protein, several found in bone and other tissues, also require vitamin K for their synthesis as do a small number of less well-characterized proteins. The human requirement for vitamin K is low, and the adequate intake for adult men and women is currently set at 120 and 90 mg=day, based on median intakes of the U.S. population. The classical symptom of a vitamin K deficiency, a hemorrhagic event, is essentially impossible to produce in adults without some underlying factor influencing absorption of the vitamin. However, newborn infants are routinely supplemented with vitamin K to prevent a condition called hemorrhagic disease of the newborn. A small amount of the protein osteocalcin circulates in plasma, and this protein is not maximally g-carboxylated at normal levels of intake. There is currently a great deal of interest in a possible role of vitamin K in promoting skeletal health. Supplementation of the diet with 45mg of menaquinone-4 is a widely used treatment for osteoporosis in Japan and other parts of Asia. The efficacy of this treatment in North America or Europe has not yet been established but remains a question of significant research interest.
