Vitamin K forms and function:There are a number of structurally similar, naturally occurring compounds which have vitamin K activity. The major plant form is phylloquinone (vitamin K1; usually abbreviated to K1). The menaquinones (vitamins K2; usually abbreviated to MK) are predominately of bacterial origin. While K1 has a 20-carbon phytyl side chain, the MKs have multiple prenyl side chains, their number being indicated by a suffix (i.e. MK-n). In the typical Western diet, K1 and MK-n account for 90% and 10% of the vitamin K intake, respectively . Dietary MK-n consists of MK-4, MK-7, MK-8, and MK-9 , although MKs with longer side chains up to MK-13 can be found in the human liver [3,4]. Menadione (usually abbreviated to K3) is a synthetic vitamin K homolog which, despite toxicity concerns and restricted biological activity, is available in some countries as a pharmaceutical vitamin K preparation . The biological activity of K3 in vivo depends entirely on its conversion to MK-4 [6,7]. At the cellular level, the co-factor role of vitamin K for the conversion of peptide-bound glutamate to gamma-carboxyglutamate (Gla) is well established, as is the associated metabolic cycle whereby the vitamin K 2,3-epoxide metabolite generated during gamma-glutamyl carboxylation is salvaged and re-cycled to active vitamin K [8, 9]. The liver synthesises seven vitamin K-dependent proteins that have a crucial role in blood coagulation (factors II, VII, IX and X, proteins C, S, and Z). Other vitamin K-dependent proteins (Gla proteins), with a widespread tissue distribution, have also been identified; this has led to a re-evaluation of the general physiological function of vitamin K and its role in human health. Putative roles of Gla proteins now extend to a diversity of functions, such as the regulation of bone turnover and calcification [10,11], inhibition of vascular calcification , and roles in vascular repair processes , cell cycle regulation, cell-cell adhesion and signal transduction . Of particular note is the accumulating body of evidence that has linked sub-optimal vitamin K reserves in bone to an increased risk of osteoporotic fracture [15,16] or to reduced bone mineral density . Vitamin K deficiency Vitamin K1 is found chiefly in green leafy vegetables, while MK is present in animal tissues. Like other fat-soluble vitamins, vitamin K is absorbed from the duodenum, where it is dependent on bile and pancre-atic secretions for solubilisation. Any condition causing the intestinal malabsorption of fat will therefore lead to a secondary deficiency of fat-soluble vitamins, including vitamin K. Deficient absorption of vitamin K leads to the depletion of its tissue stores, which is indicated by a decrease in circulating levels of the vitamin long before pathological changes develop. Vitamin K deficiency is more prevalent than generally supposed. Cases are often missed, or detected late, due to the use of inappropriate laboratory markers of vitamin K status - commonly the International Normalised Ratio (INR). The INR, which is based on the prothrombin time, is designed to detect bleeding tendencies and is a very insensitive marker of vitamin K status. Vitamin K status assessment: Tissue stores of vitamin K are evaluated by the direct measurement of circulating K1. This is supported by the analysis of PIVKA-II (under-carboxylated prothrombin - an abnormal species of Factor II that is only detectable in the circulation of patients with suboptimal vitamin K status). By running these two assays in tandem we are able to monitor the two most important determinants of vitamin K status - availability and utilisation. The serum concentration of K1 reflects its storage and transport. K1 is measured using a modified HPLC method with postcolumn chemical reduction and fluorescence detection . In healthy, normolipaemic adults the non-fasting reference range for K1 is 0.15-1.55 ug/L. Serum PIVKA-II is determined using a monoclonal antibody (C4B6) to PIVKA-II in an ELISA [19,20]. The C4B6 MAb used in this assay is conformation-specific such that, in the presence of calcium ions, it binds only to the under-carboxylated species of prothrombin and does not cross-react with fully carboxylated prothrombin. Results are expressed as arbitrary units (AU) per millilitre, where 1 AU is equivalent to 1 ug of multiple PIVKA-II species purified from patients treated with vitamin K antagonists. The upper limit of the reference range for PIVKA-II in adults is 0.2 AU/ml (200 ng/ml).
Patients should fast for 12 hours before taking blood for the assessment of vitamin K status and should have refrained from taking nutritional supplements for 24 hours prior to venipuncture. Please note that vitamin K cannot be measured without the PIVKAII test.
Cardiovascular disease and osteoporosis.
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