Martin J. Shearer

Metabolism and cell biology of vitamin K.

Naturally occurring vitamin K compounds comprise a plant form, phylloquinone (vitamin K(1)) and a series of bacterial menaquinones (MKs) (vitamin K(2)). Structural differences in the isoprenoid side chain govern many facets of metabolism of K vitamins including the way they are transported, taken up by target tissues, and subsequently excreted. In the post-prandial state, phylloquinone is transported mainly by triglyceride-rich lipoproteins (TRL) and long-chain MKs mainly by low-density lipoproteins (LDL). TRL-borne phylloquinone uptake by osteoblasts is an apoE-mediated process with the LRP1 receptor playing a predominant role. One K(2) form, MK-4, has a highly specific tissue distribution suggestive of local synthesis from phylloquinone in which menadione is an intermediate. Both phylloquinone and MKs activate the steroid and xenobiotic receptor (SXR) that initiates their catabolism, but MK-4 specifically upregulates two genes suggesting a novel MK-4 signalling pathway. Many studies have shown specific clinical benefits of MK-4 at pharmacological doses for osteoporosis and cancer although the mechanism(s) are poorly understood. Other putative non-cofactor functions of vitamin K include the suppression of inflammation, prevention of brain oxidative damage and a role in sphingolipid synthesis. Anticoagulant drugs block vitamin K recycling and thereby the availability of reduced vitamin K. Under extreme blockade, vitamin K can bypass the inhibition of Gla synthesis in the liver but not in the bone and the vessel wall. In humans, MK-7 has a greater efficacy than phylloquinone in carboxylating both liver and bone Gla proteins. A daily supplement of phylloquinone has shown potential for improving anticoagulation control.

Vitamin K metabolism and nutriture

Vitamin K functions as a co-factor for the post-translational carboxylation of specific glutamate residues to gamma-carboxyglutamate (Gla) residues in several blood coagulation factors (II, VII, IX and X) and coagulation inhibitors (proteins C and S) in the liver; as well as a variety of extrahepatic proteins such as the bone protein osteocalcin. This review outlines some recent advances in our understanding of the metabolism of vitamin K and its role in human nutriture. The introduction of new methodologies to measure the low endogenous tissue concentrations of K vitamins and circulating plasma levels of des-gamma-carboxyprothrombin (PIVKA-II) have provided correspondingly more refined indices for the assessment of human vitamin K status. The assays for vitamin K have also been used to study the sources, intestinal absorption, plasma transport, storage and transplacental transfer of K vitamins and the importance of phylloquinone (vitamin K1) versus menaquinones (vitamins K2) to human needs. The ability to biochemically monitor subclinical vitamin K deficiency has reaffirmed the precarious vitamin K status of the newborn and led to an increased appreciation of the risk factors leading to haemorrhagic disease of the newborn and how this may be prevented. Biochemical studies are leading to an increased knowledge of the mode of action of traditional coumarin anticoagulants and how some unrelated compounds (e.g. antibiotics) may also antagonize vitamin K and cause bleeding. There is also an awareness of the possible deleterious effects of vitamin K antagonism or deficiency on non-hepatic Gla-proteins which may play some subtle role in calcium homeostasis.

Studies on the Absorption and Metabolism of Phylloquinone (Vitamin K1) in Man

Publisher Summary This chapter presents studies on the absorption and metabolism of phylloquinone (vitamin K1) in man. The advances in synthetic methods have provided tritium or carbon-14 labeled vitamins K of high specific activity that have made it possible to carry out metabolic studies in man using acceptable levels of radioactivity and amounts of vitamin K approaching those believed to be within the physiological range. The polar metabolites in the feces, after either oral or intravenous administration of labeled phylloquinone, had the same chromatographic characteristics, and after intravenous injection, only 2-470 of the fecal radioactivity could be attributed to unchanged phylloquinone. It is suggested that an estimate of the absorption of phylloquinone in man is more accurately calculated from the amount of unchanged vitamin recovered from the feces. The disturbance of fat absorption resulting from pancreatic insufficiency, in contrast to that in bile-deficiency states, is characterized by a reduced generation of the solutes of mixed micelles, namely 2-mono-glycerides and fatty acids.

Depressed levels of circulating menaquinones in patients with osteoporotic fractures of the spine and femoral neck

Vitamin K1 functions in the conversion of glutamate residues, present in certain bone peptides, into the putatively active gamma-carboxyglutamate form. We have shown previously that the circulating levels of vitamin K1 are depressed in osteoporotic patients. However, it is known that menaquinones (vitamin K2:MK) may be more effective than vitamin K1 in this conversion of the inactive to active form of glutamate residues. A procedure for measuring such menaquinones has now demonstrated a marked deficiency of MK-7 and MK-8 in patients with osteoporotic fractures. It is suggested that estimates of circulating levels of K1, MK-7, and MK-8 might provide a biochemical risk marker of osteoporotic fractures.

Two-year randomized controlled trial of vitamin K1 (phylloquinone) and vitamin D3 plus calcium on the bone health of older women.

Dietary supplementation with vitamin K1, with vitamin D3 and calcium or their combination, was examined in healthy older women during a 2‐year, double‐blind, placebo‐controlled trial. Combined vitamin K with vitamin D plus calcium was associated with a modest but significant increase in BMC at the ultradistal radius but not at other sites in the hip or radius.

Compilation of a provisional UK database for the phylloquinone (vitamin K1) content of foods.

This paper reports the compilation of a food composition database for phylloquinone (vitamin K1) derived from the direct analysis of foods, recipe calculation and the assignment of values based on food similarities. All the basic and other food items used in these calculations had been analysed by HPLC and about 170 of the items had been obtained and assayed in the UK. Recipe calculations took account of the cooking method and changes in water and fat content. Currently, approximately 1501 food items with Royal Society of Chemistry/Ministry of Agriculture, Fisheries and Food food codes have been allocated a vitamin K1 value, and a further 282 new recipe codes are included in the database. Representative values from each food group are reported together with an indication of the potential variation. Detailed examples of some recipe calculations are included, and also the impact of changing the type of fat in recipes. Vitamin K1 is associated with, and most abundant in, photosynthetic tissues of plants. Accordingly, the highest concentrations (3000-6000 micrograms/kg) are found in dark-green leafy vegetables and herbs, such as kale, parsley, spinach and green cabbage. Intermediate concentrations (1000-2000 micrograms/kg) are found in plants with paler leaves such as white cabbage and lettuce or in green, non-leafy vegetables such as broccoli and brussel sprouts. Fats and oils contain variable amounts of vitamin K1 with the highest concentrations (300-1300 micrograms/kg) in soyabean, rapeseed and olive oils and the margarines based on them. Other foods such as dairy products, meat dishes and cereal-based foods (bread, biscuits, cakes, desserts etc.), although not in themselves particularly rich in vitamin K1 (< 200 micrograms/kg), may contribute significantly to intakes when consumption of green vegetables is poor. Within the scope of this present study, it has not been possible to address issues such as inter-sample variability, losses during storage or the bioavailability from different foods and further work on these aspects is needed.

Pharmacodynamic resistance to warfarin associated with a Val66Met substitution in vitamin K epoxide reductase complex subunit 1

The gene encoding vitamin K epoxide reductase complex subunit 1 (VKORC1), a component of the enzyme that is the therapeutic target site for warfarin, has recently been identified. In order to investigate the relationship betweenVKORC1 and warfarin dose response, we studied the VKORC1 gene (VKORC1) in patients with warfarin resistance. From a study group of 820 patients, we identified 4 individuals who required more than 25 mg of warfarin daily for therapeutic anticoagulation. Three of these had serum warfarin concentrations within the therapeutic range of 0.7-2.3 mg/l and showed wild-type VKORC1 sequence. The fourth warfarin resistant individual had consistently high (> or =5.7 mg/l) serum warfarin concentrations, yet had no clinically discernible cause for warfarin resistance. VKORC1 showed a heterozygous 196G-->A transition that predicted aVal66Met substitution in the VKORC1 polypeptide. This transition was also identified in 2 asymptomatic family members who had never received warfarin. These individuals had normal vitamin-K dependent coagulation factor activities and undetectable serum PIVKAII and vitamin K1 2,3 epoxide suggesting that their basal vitamin K epoxide reductase activity was not adversely affected by the VKORC1 Val66Met substitution. The association between a nucleotide transition in VKORC1 and pharmacodynamic warfarin resistance supports the hypothesis that VKORC1 is the site of action of warfarin and indicates thatVKORC1 sequence is an important determinant of the warfarin dose response.

Vitamin K deficiency bleeding (VKDB) in early infancy

Vitamin K deficiency bleeding (VKDB) is a rare and potentially life-threatening bleeding disorder of early infancy. Vitamin K stores are low at birth; thereafter breast-fed infants are at risk because of low concentrations in human milk. Classical VKDB occurs in the first week of life, is related to delayed or inadequate feeding and is readily prevented by small doses of vitamin K at birth. Late VKDB peaks at 3-8 weeks, typically presents with intracranial haemorrhage often due to undiagnosed cholestasis with resultant malabsorption of vitamin K. Diagnosis can be difficult but PIVKA-II measurements can provide confirmation even several days post-treatment. Without vitamin K prophylaxis, the incidence of late VKDB in Europe is 4-7 cases per 10(5) births; it is higher in SE Asia where in rural, low-income areas some 0.1% of affected infants may suffer intracranial bleeding. Late VKDB is largely preventable with parenteral vitamin K providing the best protection. The efficacy of oral prophylaxis is related to the dose and frequency of administration. Most multi-dose oral regimens provide protection for all except a small reservoir of infants with undetected hepatobiliary disease. Targeted surveillance of high-risk groups (e.g. biliary atresia) offers a novel approach to assess efficacy of prophylaxis.

Minerals and vitamins in bone health: the potential value of dietary enhancement.

Nutrition is important to bone health, and a number of minerals and vitamins have been identified as playing a potential role in the prevention of bone diseases, particularly osteoporosis. Despite this, there is currently no consensus on maximum levels to allow in food or as dietary supplements. The benefits of supplementation of populations at risk of osteoporosis with Ca and vitamin D are well established. Prolonged supplementation of Ca and vitamin D in elderly has been shown to prevent bone loss, and in some intervention studies to prevent fragility fractures. Although P is essential to bone health, the average intake is considered to be more than sufficient and supplementation could raise intake to adverse levels. The role of vitamin K in bone health is less well defined, though it may enhance the actions of Ca and vitamin D. Sr administered in pharmacological doses as the ranelate salt was shown to prevent fragility fractures in postmenopausal osteoporosis. However, there is no hard evidence that supplementation with Sr salts would be beneficial in the general population. Mg is a nutrient implicated in bone quality, but the benefit of supplementation via foodstuffs remains to be established. A consensus on dietary supplementation for bone health should balance the risks, for example, exposure of vulnerable populations to values close to maximal tolerated doses, against evidence for benefits from randomised clinical trials, such as those for Ca and vitamin D. Feedback from community studies should direct further investigations and help formulate a consensus on dietary supplementation for bone health.

Vitamin k status among children with cystic fibrosis and its relationship to bone mineral density and bone turnover

Objective. The aim of this study was to assess vitamin K status in an unselected population of children with cystic fibrosis (CF) and to investigate any vitamin K effect on bone turnover and bone mineral status. Methods. Children ≥5 years of age who were attending the CF unit were invited to enter the study. Fasting blood samples were analyzed for levels of vitamin K1 and prothrombin produced in vitamin K absence; total, undercarboxylated, and carboxylated osteocalcin (OC); and bone-specific alkaline phosphatase and procollagen I carboxy-terminal propeptide (bone formation markers). Levels of N-telopeptide and free pyridinoline and deoxypyridinoline (bone breakdown products) were measured in urine samples. Bone mineral density and bone mineral content were measured at the lumbar spine and for the total body with a GE Lunar Prodigy densitometer. Statistical analyses were performed with Minitab version 9.1. Results. One hundred six children entered the study. Sixty-five of 93 children (70%) from whom blood samples were obtained showed suboptimal vitamin K status, on the basis of low serum vitamin K1 levels, increased prothrombin produced in vitamin K absence levels, or both abnormalities. Vitamin K1 levels showed a significant negative correlation with undercarboxylated OC levels but showed no significant correlation with any marker of bone turnover or measurement of bone mineral status. Undercarboxylated OC levels were correlated significantly with bone turnover markers, which themselves showed a significant negative correlation with measurements of bone mineral density and content. There were no significant correlations between carboxylated or undercarboxylated OC levels and bone density measurements. Conclusions. Vitamin K1 deficiency is common among children with CF, and routine supplements should be considered. Through its role in the carboxylation of OC, vitamin K deficiency may be associated with an uncoupling of the balance between bone resorption and bone formation. A cause-effect relationship between vitamin K deficiency and low bone mass has not been proved.