Rapid assay for gamma-carboxyglutamic acid in urine and bone by precolumn derivatization and reversed-phase liquid chromatography

Laboratory assessment of vitamin K status

Vitamin K is required for the ɣ-carboxylation of specific glutamic acid residues within the Gla domain of the 17 vitamin K-dependent proteins (VKDPs). The timely detection and correction of vitamin K deficiency can protect against bleeding. Vitamin K also plays a role in bone metabolism and vascular calcification. Patients at increased risk of vitamin K deficiency include those with a restricted diet or malnutrition, lipid malabsorption, cancer, renal disease, neonates and the elderly. Coagulation assays such as the prothrombin time have been used erroneously as indicators of vitamin K status, lacking sufficient sensitivity and specificity for this application. The measurement of phylloquinone (K1) in serum is the most commonly used marker of vitamin K status and reflects abundance of the vitamin. Concentrations <0.15 µg/L are indicative of deficiency. Disadvantages of this approach include exclusion of the other vitamin K homologues and interference from recent dietary intake. The cellular utilisation of vitamin K is determined through measurement of the prevalence of undercarboxylated VKDPs. Most commonly, undercarboxylated prothrombin (Protein Induced by Vitamin K Absence/antagonism, PIVKA-II) is used (reference range 17.4–50.9 mAU/mL (Abbott Architect), providing a retrospective indicator of hepatic vitamin K status. Current clinical applications of PIVKA-II include supporting the diagnosis of vitamin K deficiency bleeding of the newborn, monitoring exposure to vitamin K antagonists, and when used in combination with α-fetoprotein, as a diagnostic marker of hepatocellular carcinoma. Using K1 and PIVKA-II in tandem is an approach that can be used successfully for many patient cohorts, providing insight into both abundance and utilisation of the vitamin.

Age group and sex do not influence responses of vitamin K biomarkers to changes in dietary vitamin K

Inadequate vitamin K intake has been associated with abnormal soft tissue calcification. Older adults may have insufficient intakes of vitamin K and respond less to vitamin K supplementation compared with younger adults. However, little is known about the determinants that influence the response to vitamin K supplementation. Our primary objective was to assess dietary and nondietary determinants of vitamin K status in healthy younger and older adults. In a nonrandomized, nonmasked study, 21 younger (18-40 y) and 21 older (55-80 y) men and women consumed a baseline diet (200 μg phylloquinone/d) for 5 d, a phylloquinone-restricted diet (10 μg phylloquinone/d) for 28 d, and a phylloquinone-supplemented diet (500 μg phylloquinone/d) for 28 d. Changes in vitamin K status markers in response to vitamin K depletion and repletion were studied and the influences of BMI, body fat, and circulating TG were assessed by including them as covariates in the model. Despite baseline differences in measures of vitamin K status, plasma phylloquinone tended to increase (P = 0.07) and the percentage of uncarboxylated osteocalcin and uncarboxylated prothrombin both improved with phylloquinone supplementation (P < 0.007), regardless of age group or sex. Only the excretion of urinary menadione, a vitamin K metabolite, was greater among younger adults in response to depletion than in older adults (P = 0.012), regardless of sex. Adiposity measures and circulating TG did not predict response of any measures. In conclusion, poor vitamin K status can be similarly improved with vitamin K supplementation, regardless of age group or sex.

Dietary phylloquinone depletion and repletion in older women

Biological markers indicative of poor vitamin K status have been associated with a greater risk for hip fracture in older men and women. However, the dietary phylloquinone intake required to achieve maximal carboxylation of hepatic and extrahepatic vitamin K-dependent proteins is not known. In an 84-d study in a metabolic unit, 21 older (60-80 y) women were fed a phylloquinone-restricted diet (18 micro g/d) for 28 d, followed by stepwise repletion of 86, 200 and 450 micro g/d of phylloquinone. Plasma phylloquinone, urinary gamma-carboxyglutamic acid excretion and gamma-carboxylation of hepatic (prothrombin) and extrahepatic proteins (osteocalcin) decreased in response to phylloquinone restriction (P < 0.001), demonstrating the production of subclinical vitamin K deficiency. The gamma-carboxylation of prothrombin was restored to normal levels in response to phylloquinone supplementation at 200 micro g/d. In contrast, all other biochemical markers of vitamin K status remained below normal levels after short-term supplementation of up to 450 micro g/d of phylloquinone. These data support previous observations in rats that hepatic vitamin K-dependent proteins have preferential utilization of phylloquinone in response to phylloquinone dietary restriction. Moreover, our findings suggest that the current recommended Adequate Intake levels of vitamin K (90 micro g/d) in women do not support maximal osteocalcin gamma-carboxylation in older women.

Effects of a hydrogenated form of vitamin K on bone formation and resorption

BACKGROUND

Hydrogenation of vegetable oils affects blood lipid and lipoprotein concentrations. However, little is known about the effects of hydrogenation on other components, such as vitamin K. Low phylloquinone (vitamin K1) intake is a potential risk factor for bone fracture, although the mechanisms of this are unknown.

OBJECTIVE

The objective was to compare the biological effects of phylloquinone and its hydrogenated form, dihydrophylloquinone, on vitamin K status and markers of bone formation and resorption.

DESIGN

In a randomized crossover study in a metabolic unit, 15 young adults were fed a phylloquinone-restricted diet (10 microg/d) for 15 d followed by 10 d of repletion (200 microg/d) with either phylloquinone or dihydrophylloquinone.

RESULTS

There was an increase and subsequent decrease in measures of bone formation (P = 0.002) and resorption (P = 0.08) after dietary phylloquinone restriction and repletion, respectively. In comparison with phylloquinone, dihydrophylloquinone was less absorbed and had no measurable biological effect on measures of bone formation and resorption.

CONCLUSION

Hydrogenation of plant oils appears to decrease the absorption and biological effect of vitamin K in bone.

Response of vitamin K status to different intakes and sources of phylloquinone-rich foods: comparison of younger and older adults

BACKGROUND

Phylloquinone, found in dark-green vegetables and certain plant oils, is the primary dietary source of the fat-soluble vitamin K. Limited data suggest that the relative bioavailability of phylloquinone from vegetables is lower than that from a supplement. This finding is relevant to the maintenance of optimal vitamin K status.

OBJECTIVE

The objective of this study was to compare, in younger and older adults, the relative bioavailability of phylloquinone from a vegetable with that of a fortified oil.

DESIGN

In a crossover design with three 15-d residency periods in a metabolic unit, younger and older men and women (n = 36) consumed a mixed diet containing 100 microg phylloquinone/d. During 2 residency periods, the mixed diet was supplemented for 5 d with either broccoli (377 microg phylloquinone/d; broccoli diet) or phylloquinone-fortified oil (417 microg/d; oil diet). The relative bioavailability of phylloquinone was defined by the difference in plasma phylloquinone, percentage serum undercarboxylated osteocalcin (%ucOC), and urinary gamma-carboxyglutamic acid in response to 5 d of supplementation.

RESULTS

For both younger and older adults, plasma phylloquinone concentrations were higher (P < 0.001) and %ucOC values were lower (P = 0.001) after the broccoli and oil diets than after the mixed diet only. Overall, the response to broccoli supplementation was not significantly different from the response to the fortified oil in either age group. Urinary gamma-carboxyglutamic acid did not change in response to supplementation.

CONCLUSIONS

There was no significant difference in the relative bioavailability of phylloquinone, as evidenced by the lack of a significant difference in plasma phylloquinone and %ucOC between the 2 groups after either the broccoli or oil diets for younger and older adults.

Analysis of gamma-carboxyglutamic acid content of protein, urine, and plasma by capillary electrophoresis and laser-induced fluorescence

When the properties of an analyte are known, the separation system can be designed to make the analyte of interest migrate at either a much faster or a much slower velocity compared to other molecules in the sample matrix. A simple and sensitive method to analyze the gamma-carboxyglutamic acid (Gla) content of protein, urine, and plasma was developed using capillary electrophoresis with laser-induced fluorescence detection (CE-LIF). The separation method is designed according to the specific properties of three amino acids of interest. The number of Gla residues from three vitamin K-dependent proteins were estimated by quantifying the amount of fluorescein thiocarbamyl derivative of Gla after alkaline hydrolysis and fluorescein isothiocyanate labeling. Human prothrombin, blood coagulation factor X, and bovine osteocalcin were calculated to have 10.0 +/- 0.7, 11.0 +/- 0.6, and 2.1 +/- 0.1 Gla residues per mole of protein, respectively, which agreed well with amino acid sequencing data. The analysis of free Gla content in urine and plasma was also demonstrated by this method. It was demonstrated that submicrograms of protein can be characterized by CE-LIF.

Olestra dose response on fat-soluble and water-soluble nutrients in humans

Ninety normal healthy adults were given 0, 8, 20 or 32 g/d olestra for 8 wk as part of a diet that provided 1 +/- 0.2 of the recommended dietary allowance (RDA) of vitamins A, D, E and K, folate zinc, calcium and iron. In addition, a 20 microg/d supplement of vitamin D was supplied. The diet provided 15% of energy from protein, 35% from fat and 55% from carbohydrate. The purpose of the study was to determine the dose response of olestra on vitamins D, E and K, carotenoids, vitamin B12, folate and zinc. Circulating concentrations of retinol, carotenoids, tocopherols, 25-hydroxy- and 1,25-dihydroxyvitamin D metabolites, phylloquinone, des-gamma-carboxyprothrombin, prothrombin, folate and hematological parameters were measured biweekly, as were urine concentrations of zinc and gamma-carboxyglutamic acid (Gla). Clinical chemistry, urinalysis and vitamin B12 absorption were measured at wk 0 and 8. Olestra reduced serum concentrations of carotenoids, alpha-tocopherol, 25-hydroxyergocalciferol and phylloquinone in a dose-responsive manner. Olestra did not affect Gla excretion, plasma des-gamma-carboxyprothrombin or prothrombin concentrations, prothrombin time, vitamin B12 absorption, overall vitamin D status or the status of folate or zinc. Laboratory evaluations showed no health-related effects of olestra. Subjects in all groups reported common gastrointestinal symptoms such as loose stools, fecal urgency and flatulence, which were transient and generally mild to moderate in severity. These symptoms did not affect protocol compliance or the ability to measure the potential for olestra to affect nutrient availability.

Characteristics and culture of osteoblasts derived from avian long bone

A method is presented for isolating primary osteoblasts from the periosteal surface of chick tibia. The culture system identified supports both cell proliferation and phenotype retention. Cell numbers increased 8-fold in Week 1 and 20-fold over a total of 12 days. Well-established osteoblast markers, alkaline phosphatase staining, gamma-carboxyglutamic acid, osteocalcin, type I collagen, and parathyroid hormone binding were detected. Osteocalcin, gamma-carboxyglutamic acid, and type I collagen were present on culture Day 4, and were increased in amount by Day 8, but were similar to the earlier level on Day 12, suggesting that the phenotype may revert to a less differentiated state by 12 days in culture. Alkaline phosphatase staining was intense at all three assay times, however. During the last 4 days of the 12-day culture period, proliferation rates were higher than in the previous 8 days.

Dietary induced subclinical vitamin K deficiency in normal human subjects

A subclinical vitamin K deficiency was induced in 32 healthy subjects (four groups of eight males and females) aged 20-40 and 60-80 yr residing in the Metabolic Research Unit of the Human Nutrition Research Center on Aging at Tufts University. Volunteers were initially fed (4 d) a baseline-period diet containing the recommended daily allowance for vitamin K which is equivalent to 80 micrograms/d of phylloquinone (vitamin K1). During the baseline period various parameters of vitamin K nutritional status were monitored. The baseline period was followed by a 13-d depletion period during which the subjects were fed a very low vitamin K1 diet (approximately 10 micrograms/d). After depletion, the subjects entered a 16-d repletion period (four stages lasting 4 d each) during which time they were repleted with 5, 15, 25, and 45 micrograms of vitamin K1 per day. Vitamin K1 depletion dramatically and significantly decreased plasma vitamin K1 levels (P < 0.0001) in both elderly and young groups to values 13-18% of day 1 (elderly 0.22 nM, young 0.14 nM). Repleting the subjects with up to 45 micrograms of vitamin K1 per day failed, in the case of the young subjects, to bring plasma vitamin K1 levels back into the normal range. Dietary vitamin K1 restriction induced different responses in the urinary excretion of gamma-carboxyglutamic acid between the young and the elderly subjects with values decreasing significantly (P < 0.03) in the young while remaining unchanged in the elderly. The vitamin K1 depletion period had no significant effect on either prothrombin and activated partial thromboplastin times, or Factor VII and protein C (as determined by antigenic and functional assays). By using a monoclonal antibody, decarboxy prothrombin was found to increase slightly but significantly in both groups (P < 0.05) as a consequence of the low vitamin K1 diet. This study clearly shows that a diet low in vitamin K1 can result in a functional subclinical deficiency of vitamin K (decreased urinary gamma-carboxyglutamic acid excretion) without affecting blood coagulation.

Determination of protein-bound urinary gamma-carboxyglutamic acid in calcium nephrolithiasis

Nephrocalcin is a urinary gamma-carboxyglutamic acid (Gla) containing protein that may be a physiological inhibitor of calcium oxalate nephrolithiasis. Nephrocalcin isolated from urine of stone formers seems to be abnormal in lacking Gla that is required for inhibitory activity. In order to study this hypothesis, we compared the protein-bound urinary Gla contents in 32 calcium oxalate stone formers and in 24 controls. Protein-bound Gla was resolved by reversed-phase high-performance liquid chromatography after elimination of free Gla, alkaline hydrolysis and precolumn derivatization with o-phthalaldehyde and mercaptoethanol. Protein-bound urinary Gla concentrations were similar in stone formers (0.83 +/- 0.38 mumol/l, mean +/- SD) and controls (0.81 +/- 0.27) and were less than 5% of free urinary Gla. However, excretion rates of free and protein-bound Gla (nmol/min) were higher in stone formers (P = 0.006 and P = 0.002). Positive correlations (P = 0.000) were observed between free and protein-bound Gla both in controls and in stone formers. These results do not support the hypothesis of a lacking Gla nephrocalcin in stone formers.

Evidence that free gamma carboxyglutamic acid circulates in serum

We report a rapid, sensitive and reproducible method to measure free gamma-carboxyglutamic acid (GLA) in serum using precolumn derivatization with O-phthalaldehyde, reversed-phase chromatography and deproteinization of serum by ultrafiltration. Serum free GLA level in 62 healthy adults was 167 +/- 46 pmol/ml and was increased (302 +/- 195 pmol/ml) in 14 patients with primary hyper-parathyroidism, a disease characterized by an increased bone turnover. Peptide bound GLA averaged 413 pmol/ml. In rabbits receiving massive doses of warfarin during 6 days there was a time- and dose-dependent decrease of serum free GLA but a significant fraction was still measurable. These data indicate that free GLA circulates and originates both from the metabolism of the vitamin K-dependent clotting factors and from bone metabolism.

Bone status in nonambulant, epileptic, institutionalized youth. Improvement with vitamin D therapy

This study was conducted to determine if the adverse effects of anticonvulsant drug therapy and nonambulancy on bone status could be overcome with vitamin D therapy in severely handicapped individuals. Six male and five female gastrostomy fed, nonambulant, epileptic, profoundly mentally retarded individuals ranging in age from 7 to 17 years were given vitamin D therapy at a dosage of 4,000 IU/m2 body surface area/day for 6 months. Photon absorptiometry and biochemical indices of bone status were measured to follow the effects of therapy. Bone mineral content expressed as a percentage of normal improved by 11 percent (p less than 0.01), from 59.6 to 66.1 percent. Tartrate-resistant acid phosphatase, total alkaline phosphatase, and the bone isoenzyme activities declined 11 percent, 18 percent, and 11 percent respectively. These reductions were not statistically significant but they were consistent with the improvements observed by photon absorptiometry. The results of our study suggest that a conservative supplement of vitamin D will improve the bone status of severely disabled youths.

Analysis of gamma-carboxyglutamic acid by reverse phase HPLC of its phenylthiocarbamyl derivative

A method is presented for analysis of gamma-carboxyglutamic acid based on its derivatization with phenylisothiocyanate and reverse phase HPLC analysis of the resulting phenylthiocarbamyl derivative. Proteins were hydrolyzed with sodium hydroxide and the hydrolysates were desalted on Dowex 50 eluted with ammonium hydroxide. The resulting amino acid mixtures were derivatized with phenylisothiocyanate and the phenylthiocarbamyl derivatives were separated under isocratic conditions on either C18 or C8 reverse phase columns using 0.14 M Tris, 0.05% triethylamine, titrated to pH 7.5 with glacial acetic acid, plus 2% acetonitrile, and detected by absorbance at 254 nm. The method is linear over the range from 10 to 1000 pmol of gamma-carboxyglutamic acid and the limit of detection is near 2 pmol. The utility of the method was verified for analysis of purified prothrombin yielding a value of 10.3 mol of gamma-carboxyglutamic acid per mole in agreement with sequence data. No gamma-carboxyglutamate was detectable for acid-hydrolyzed samples of prothrombin, nor in acid- or base-hydrolyzed samples of bovine serum albumin. Application of this method failed to corroborate the reported presence of gamma-carboxyglutamate in a putative mitochondrial gamma-carboxyglutamate-containing calcium-binding protein. The method was also tested for determination of beta-carboxyaspartate, beta-hydroxyaspartate, phosphoserine, phosphothreonine, and phosphotyrosine in an attempt to identify an unknown material which appeared in preparations of the mitochondrial protein.

Characterisation of two differently processed forms of human recombinant factor IX synthesised in CHO cells transformed with a polycistronic vector

A stable transformed cell line constitutively expressing human factor IX has been established. Wild-type Chinese hamster ovary cells (CHO cells) were transformed using a polycistronic expression vector carrying a previously isolated factor IX cDNA and a selection gene encoding the Escherichia coli xanthine-guanine phosphoribosyl transferase. One clone, CHO 622.4, contains a high number of genomically integrated plasmids and secretes 1-3 mg factor IX l-1 day-1 into the culture medium with a biological activity ranging from 25% to 40%. The recombinant molecule was purified either by conventional chromatography or by immunoaffinity chromatography using antibodies specific to a calcium-induced factor IX conformer. The purified recombinant protein migrates as a single band with the same mobility as that of natural factor IX on SDS/polyacrylamide gels. N-terminal sequencing shows tow differently processed forms of recombinant factor IX: whereas the majority of the zymogen is correctly processed, approximately 20% of the purified recombinant molecule contains an 18-amino-acid NH2-extension corresponding to the precursor form of factor IX. Analysis of the 4-carboxyglutamic acid content indicates a high but incomplete carboxylation (70%) of the recombinant molecule as compared to natural factor IX. The carbohydrate composition of both the natural and recombinant molecules has been determined. Both molecules have a N-glycan structure of similar complexity, indicating that factor IX contains all the information to direct the same glycosylation pattern in human liver cells and in an unrelated cell line such as CHO-K1.

Two proteins with gamma-carboxyglutamic acid in frog bone: isolation and comparative characterization

Two gamma-carboxyglutamic acid-containing proteins were purified from neutral (pH 7.5) EDTA-extract of frog, Rana catesbiana, cortical bone by Sephadex G-75 gel filtration, DEAE-Sephadex A-25 chromatography and successive hydroxyapatite column chromatography. These two bone gamma-carboxyglutamic acid-containing proteins, termed osteocalcin, P-1 and P-2, had molecular weights of about 5100 and 4900, respectively, based on their amino-acid composition. Both species of osteocalcin have two gamma-carboxyglutamic acid residues, one disulfide bond, but there was no 4-hydroxyproline in either molecule. Each N-terminus of both proteins was acetylated and each C-terminal amino acid was lysine. The isoelectric points of P-1 and P-2 are 4.02 and 3.91, respectively, and their pI values shifted to more neutral pH in the presence of calcium ions. Equilibrium dialysis has indicated that each of these two proteins binds specifically 2 mol Ca2+, and nonspecifically more, 4-5 mol, Ca2+ in 0.02 M Tris-HCl/0.15 M NaCl (pH 7.4), at 4 degrees C. By the best-fitted calculation, P-1 had one high affinity Ca2+-binding site (Kd1 = 0.17 mM) and one lower affinity site (Kd2 = 0.29 mM), and P-2 contained one high affinity site (Kd1 = 0.154 mM) and one lower affinity site (Kd2 = 0.67 mM).

Vitamin-K-dependent proteins in microsomes of primary Lewis lung tumors

Microsomes isolated from Lewis lung (LL) primary tumors raised in C57BL/6 mice have been shown to (i) contain a 4-hydroxycoumarin (warfarin)-sensitive cycle of vitamin K metabolism which is at least qualitatively similar to that of liver, and (ii) catalyze the incorporation of NaH14 CO3 into endogenous protein in a vitamin-K hydroquinone-dependent reaction to produce gamma-carboxyglutamate. As in liver microsomes, LL microsomal reduction of vitamin K 2,3-epoxide to vitamin K was greatly enhanced by exogenous dithiols such as dithiothreitol, but under identical conditions the former was 10-fold faster. The R(+) and S(-) warfarin enantiomers were highly and equally effective inhibitors of both the liver and tumor vitamin K 2,3-epoxide reductases-the average I50 against the tumor enzyme was 0.25 microM. Partially purified reductases isolated by centrifugation of sodium-cholate-treated liver and LL tumor microsomes over a discontinuous sucrose gradient were also inhibited by the sulfhydryl reagent N-ethylmaleimide following their reduction by dithiothreitol. Like the activity of the epoxide reductase, that of the gamma-carboxylase was much lower in tumor than in liver microsomes and was only detectable in microsomes isolated from tumor-bearing mice previously administered S(-) warfarin. In view of the reported inhibition of LL tumor metastasis by warfarin and diet-induced vitamin-K deficiency, vitamin-K-dependent proteins may play a role in the spread and/or subsequent growth of LL cells.