Analysis of isotope ratios in vitamin K1 (phylloquinone) from human plasma by gas chromatography/mass spectrometry

Development of a liquid chromatography mass spectrometry method for the determination of vitamin K1, menaquinone-4, menaquinone-7 and vitamin K1-2,3 epoxide in serum of individuals without vitamin K supplements

Objectives

Vitamin K and metabolites have a beneficial role in blood coagulation, bone metabolism and growth. However, the determination of vitamin K concentrations in the blood in patients consuming a diet with naturally occurring vitamin K is currently challenging. We aim to develop a cost-effective and rapid method to measure vitamin K metabolites with potential application for clinics and research.

Methods

We developed a simple liquid chromatography-tandem mass spectrometric (LC-MS/MS) method for the determination of vitamin K1, menaquinone-4 (MK-4), menaquinone-7 (MK-7) and vitamin K1-2,3 epoxide in human serum and validated the method in a study cohort of 162 patients tested for carbohydrate malabsorption and in 20 patients with oral phenprocoumon intake.

Results

The overall precision (CVs) ranged between 4.8 and 17.7% in the specified working range (0.06–9.0 nmol/L for all analytes except for MK-7 with 0.04–6.16 nmol/L). In the malabsorption cohort samples, measured values were obtained for all different vitamin K metabolites except for vitamin K1-2,3 epoxide. This metabolite could be detected only in patients with phenprocoumon intake. The good performance of the method is especially achieved by the interaction of three factors: the use of lipase in the sample preparation, the use of an atypical fluorinated reversed phase column, and a logarithmic methanol gradient.

Conclusions

The described method is able to determine the concentration of four vitamin K metabolites in a time-efficient, simple and cost-effective manner. It can be suitable for both routine clinics and research.

Quantifying dietary vitamin K and its link to cardiovascular health: a narrative review

Cardiovascular disease is the leading cause of death and disability worldwide. Recent work suggests a link between vitamin K insufficiency and deficiency with vascular calcification, a marker of advanced atherosclerosis. Vitamin K refers to a group of fat-soluble vitamins important for blood coagulation, reducing inflammation, regulating blood calcium metabolism, as well as bone metabolism, all of which may play a role in promoting cardiovascular health. Presently, there is a lack of a comprehensive vitamin K database on individual foods, which are required to accurately calculate vitamin K1 and K2 intake for examination in epidemiological studies. This has likely contributed to ambiguity regarding the recommended daily intake of vitamin K, including whether vitamin K1 and K2 may have separate, partly overlapping functions. This review will discuss the presence of: (i) vitamin K1 and K2 in the diet; (ii) the methods of quantitating vitamin K compounds in foods; and (iii) provide an overview of the evidence for the cardiovascular health benefits of vitamin K in observational and clinical trials.

A concise review of quantification methods for determination of vitamin K in various biological matrices

Vitamin K is an essential nutrient in the body and involved in numerous physiological and pathophysiological functions. Both the lack and surplus of vitamin K can put human health at risk. Therefore, it becomes necessary to monitor vitamin K concentrations in different biomatrices through establishing sensitive and specific analytical methods. This review collectively describes an updated overview of the sample pretreatment methodologies and methods for quantitative determination of vitamin K that have been used in last two decades. High Performance Liquid Chromatography (HPLC) is commonly utilized as a standard for separation of vitamin K in combination with different detection including spectroscopic, spectrometric, fluorometric and mass spectroscopy. Recent progress in sample pretreatment technologies and quantitation methodologies have enhanced the ability to identify and quantitate vitamin K in biomatrices to further advance our understanding of the role of this vitamin in human health and disease.

The external quality assurance of phylloquinone (vitamin K(1)) analysis in human serum

The vitamin K external quality assurance scheme (KEQAS) aims to assist in the harmonization of phylloquinone (vitamin K(1)) analysis in order to improve the comparability of clinical and nutritional studies. Serum samples were despatched to 17 groups from eight countries during 2000-2006. Using pilot data (1996-1999), an analytical performance target of 20% absolute difference from the all-laboratory trimmed mean (ALTM) was assigned and formed the basis for interlaboratory comparison. Assay specificity, analytical bias and assay performance were evaluated. From 21 batches of samples distributed, 414 results were reported of which 2.7% were outliers. The mean interlaboratory absolute difference from the ALTM was 21.7% with 47% of groups consistently meeting the performance target. The mean interlaboratory coefficient of variation was 29.6%. The false positive rate for phylloquinone depleted samples was high at 35%. Bias was found to be independent of HPLC-detector type (fluorescence vs electrochemical). Assay characteristics for the measurement of phylloquinone in human serum compare favourably with methods for analytes at equivalent concentrations. The high proportion of false positive results suggest that poor assay specificity at low phylloquinone concentrations is a common problem, which in the clinical setting could lead to underreporting of vitamin K deficiency.

The effect of different meals on the absorption of stable isotope-labelled phylloquinone

Few studies have investigated the absorption of phylloquinone (vitamin K1). We recruited twelve healthy, non-obese adults. On each study day, fasted subjects took a capsule containing 20 microg of 13C-labelled phylloquinone with one of three meals, defined as convenience, cosmopolitan and animal-oriented, in a three-way crossover design. The meals were formulated from the characteristics of clusters identified in dietary pattern analysis of data from the National Diet and Nutrition Survey conducted in 2000-1. Plasma phylloquinone concentration and isotopic enrichment were measured over 8 h. Significantly more phylloquinone tracer was absorbed when consumed with the cosmopolitan and animal-oriented meals than with the convenience meal (P = 0.001 and 0.035, respectively). Estimates of the relative availability of phylloquinone from the meals were: convenience meal = 1.00; cosmopolitan meal = 0.31; animal-oriented meal = 0.23. Combining the tracer data with availability estimates for phylloquinone from the meals provides overall relative bioavailability values of convenience = 1.00, cosmopolitan = 0.46 and animal-oriented = 0.29. Stable isotopes provide a useful tool to investigate further the bioavailability of low doses of phylloquinone. Different meals can affect the absorption of free phylloquinone. The meal-based study design used in the present work provides an approach that reflects more closely the way foods are eaten in a free-living population.

Measurement of deuterium-labeled phylloquinone in plasma by high-performance liquid chromatography/mass spectrometry

Phylloquinone (vitamin K(1)) is a lipophilic compound present in plasma at low concentrations, which presents technical challenges for determining its bioavailability or metabolic fate using stable isotopes. We developed a method to simultaneously measure unlabeled and deuterium-labeled phylloquinone concentrations in plasma specimens using high-performance liquid chromatography/mass spectrometry with atmospheric pressure chemical ionization (LC-APCI/MS). Phylloquinone was extracted from plasma using hexane, further purified by solid-phase extraction, and then quantified using high-performance liquid chromatography with an APCI/MS as a detector. Plotting the expected versus the measured amount of serial dilutions of either unlabeled or labeled phylloquinone gave correlation coefficients (R) of 0.999 for both compounds. The minimum detectable concentrations of unlabeled and labeled phylloquinone were 0.05 and 0.08 pmol/injection, respectively. Pooled plasma samples spiked with between 0.5 and 32 nmol phylloquinone/L gave average recoveries of 96.7% with 5.4% relative standard deviation (RSD) for unlabeled phylloquinone and 96.2% with 6.6% RSD for labeled phylloquinone. Plasma phylloquinone concentrations determined by LC-fluorescence and LC-APCI/MS methods from healthy subjects (n = 17) were not statistically different (P = 0.13). The LC-APCI/MS method is a sensitive technique for simultaneous determination of both unlabeled and labeled phylloquinone and can be applied to bioavailability studies.

A possible ethanol-catalyzed rearrangement of vitamin K(1) detected by gas chromatography/mass spectrometry

We studied vitamin K(1)(20), vitamin K(1)(25), and vitamin K(1) epoxide in n-hexane and ethanol solutions by gas chromatography/mass spectrometry (GC/MS) utilizing a DB-5 MS fused-silica capillary column. In ethanol solutions of K(1), we observed an extra peak eluting from the GC column with somewhat longer retention time than K(1)(20). A similar peak following K(1)(25) was also found. These peaks were not found in n-hexane solutions of K(1). A close examination of the mass spectra of these peaks indicated that they were vitamin K(1) variants containing a base peak at m/z 225 characteristic of the methylnaphthoquinone ring with a four-carbon side chain. In addition, they contained the molecular ions of K(1)(20) and K(1)(25), respectively. We conclude that K(1)(20) and K(1)(25), but not K(1) epoxide, might undergo rearrangements in ethanol involving an intramolecular proton transfer and a shift of the beta,gamma-double bond on the phytyl side chain toward the ring. The conjugation of the phytyl double bond with the quinone ring is probably the driving force of the rearrangement. We emphasize, however, that our conclusion is based only on mass spectral analysis and would require further investigation by other spectroscopic methods.

Nutrition and bone health projects funded by the UK Food Standards Agency: have they helped to inform public health policy?

The UK Food Standards Agency convened an international group of expert scientists to review the Agency-funded projects on diet and bone health in the context of developments in the field as a whole. The potential benefits of fruit and vegetables, vitamin K, early-life nutrition and vitamin D on bone health were presented and reviewed. The workshop reached two conclusions which have public health implications. First, that promoting a diet rich in fruit and vegetable intakes might be beneficial to bone health and would be very unlikely to produce adverse consequences on bone health. The mechanism(s) for any effect of fruit and vegetables remains unknown, but the results from these projects did not support the postulated acid-base balance hypothesis. Secondly, increased dietary consumption of vitamin K may contribute to bone health, possibly through its ability to increase the gamma-carboxylation status of bone proteins such as osteocalcin. A supplementation trial comparing vitamin K supplementation with Ca and vitamin D showed an additional effect of vitamin K against baseline levels of bone mineral density, but the benefit was only seen at one bone site. The major research gap identified was the need to investigate vitamin D status to define deficiency, insufficiency and depletion across age and ethnic groups in relation to bone health.

Liquid chromatography combined with mass spectrometry for 13C isotopic analysis in life science research

Among the different disciplines covered by mass spectrometry, measurement of (13)C/(12)C isotopic ratio crosses a large section of disciplines from a tool revealing the origin of compounds to more recent approaches such as metabolomics and proteomics. Isotope ratio mass spectrometry (IRMS) and molecular mass spectrometry (MS) are the two most mature techniques for (13)C isotopic analysis of compounds, respectively, for high and low-isotopic precision. For the sample introduction, the coupling of gas chromatography (GC) to either IRMS or MS is state of the art technique for targeted isotopic analysis of volatile analytes. However, liquid chromatography (LC) also needs to be considered as a tool for the sample introduction into IRMS or MS for (13)C isotopic analyses of non-volatile analytes at natural abundance as well as for (13)C-labeled compounds. This review presents the past and the current processes used to perform (13)C isotopic analysis in combination with LC. It gives particular attention to the combination of LC with IRMS which started in the 1990's with the moving wire transport, then subsequently moved to the chemical reaction interface (CRI) and was made commercially available in 2004 with the wet chemical oxidation interface (LC-IRMS). The LC-IRMS method development is also discussed in this review, including the possible approaches for increasing selectivity and efficiency, for example, using a 100% aqueous mobile phase for the LC separation. In addition, applications for measuring (13)C isotopic enrichments using atmospheric pressure LC-MS instruments with a quadrupole, a time-of-flight, and an ion trap analyzer are also discussed as well as a LC-ICPMS using a prototype instrument with two quadrupoles.

A stable isotope method for the simultaneous measurement of vitamin K1 (phylloquinone) kinetics and absorption

OBJECTIVES

To measure uptake and disposal kinetics and absolute absorption of vitamin K(1) using two stable isotope-labelled forms of vitamin K(1).

SUBJECTS

Ten subjects (nine women and one man) aged between 22 and 31 years, with a mean (+/-standard deviation) body mass index of 22.5+/-2.4 kg/m(2). Subjects took capsules containing 3 microg of methyl-(13)C vitamin K(1), three times a day for six days to reach a steady state for plasma vitamin K(1) isotopic enrichment. On day seven, subjects were given an intravenous dose of Konakion MM to measure disposal kinetics and at the same time, a capsule containing 4 microg of ring-D(4) vitamin K(1) to measure absorption. Plasma vitamin K(1) concentration was measured by high-performance liquid chromatography and isotopic composition by gas chromatography mass spectrometry.

RESULTS

The disposal kinetics of the intravenous dose of vitamin K(1) were resolved into two exponentials with half-times of 0.22 (+/-0.14) and 2.66 (+/-1.69) h. Absorption of oral, deuterated vitamin K(1) was 13 (+/-9)%.

CONCLUSIONS

Two-compartmental kinetic parameters observed in this study are similar to those obtained previously using radioactive tracers, but there may be additional slow-turnover body pools acting as stores of vitamin K(1). The kinetic parameters determined from the intravenous dose allowed determination of the absolute absorption of vitamin K(1) from a bolus oral dose.