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Biomedical subjects

M H Knapen

Publications and source records attributed to M H Knapen.

At least 19 recordsLinked to original sources
PubMed2000

Evaluation of a bead-based enzyme immunoassay for the rapid detection of osteocalcin in human serum.

BACKGROUND: Circulating osteocalcin is a well-known marker for bone formation, but none of the commercial kits currently available can be used in automated systems. Here we present the first semiautomated assay for human serum osteocalcin. METHODS: Polystyrene beads were coated with antibodies against the COOH terminus of osteocalcin and used in the COBAS((R)) EIA System. Osteocalcin was detected with peroxidase-conjugated antibodies against the osteocalcin NH(2) terminus. RESULTS: The time required to analyze an unknown sample was 60 min, with a lower detection limit of 4.5 microg/L and a linear dose-response curve between 4.5 and 100 microg/L. The intraassay imprecision (CV) was 5-8% (n = 21); the interassay variation was 6-9% (n = 14). In samples from human volunteers and patients, data generated with the newly developed assay were comparable to those obtained with standard microtiter plate-based assays. CONCLUSIONS: The coated beads assay may be implemented on fully automated analyzers, which not only may further reduce imprecision but may also substantially increase the applicability of osteocalcin as a marker for bone metabolism in the routine clinical setting.

Adult↗
PubMed1998

Correlation of serum osteocalcin fractions with bone mineral density in women during the first 10 years after menopause.

Serum immunoreactive osteocalcin (irOC) consists of two fractions that differ from each other by their affinity for hydroxyapatite. The high and low affinity fractions are referred to as irOCbound and irOCfree, respectively. To evaluate whether these fractions are determinants for different characteristics of bone or bone metabolism, we have performed a cross-sectional study among 212 apparently healthy women between 20 and 90 years of age. Bone mineral density (BMD) was determined at the lumbar spine, and the right femur neck, trochanter, and Ward's triangle using dual-energy X-ray absorptiometry (DXA). Biochemical markers for bone formation and resorption were determined in serum and in urine. After classification according to menopausal age, an inverse correlation was found in the 1-10 years postmenopausal women between irOCfree and BMD, notably of the Ward's triangle and femur neck. It is concluded that in 1-10 years postmenopausal women, irOCfree is an independent marker for BMD, but that in other age groups the association is less clear or is absent.

Absorptiometry, Photon↗
PubMed1998

Improved bone metabolism in female elite athletes after vitamin K supplementation.

In female elite athletes strenuous exercise may result in hypoestrogenism and amenorrhoea. As a consequence a low peak bone mass and rapid bone loss are often seen in relatively young athletes. In postmenopausal women, increased intake of vitamin K may result in an increase of serum markers for bone formation, a decrease of urinary markers for bone resorption, and a decrease in urinary calcium loss. In the present paper we report an intervention study among eight female athletes, four of whom had been amenorrhoeic for more than one year, whereas the others had been using oral contraceptives. All participants received vitamin K supplementation (10 mg/day) during one month, and various bone markers were measured before and after treatment. At baseline the athletes not using oral contraceptives were biochemically vitamin K-deficient as deduced from the calcium binding capacity of the circulating bone protein osteocalcin. In all subjects increased vitamin K was associated with an increased calcium-binding capacity of osteocalcin. In the low-estrogen group vitamin K supplementation induced a 15-20% increase of bone formation markers and a parallel 20-25% decrease of bone resorption markers. This shift is suggestive for an improved balance between bone formation and resorption.

Adult↗
PubMed1998

Bone markers during a 6-month space flight: effects of vitamin K supplementation.

Rapid bone loss is a serious health problem for astronauts during long lasting missions in space. We have recorded the changes of biochemical markers for bone metabolism in one of the astronauts during the 6-month space flight of the EUROMIR-95 mission. Immediately after launch both bone resorption markers and urinary calcium excretion increased about two fold, whereas bone formation markers remained unchanged. After 12 1/2 weeks the astronaut received vitamin K1 (10 mg/day for 6 weeks). Vitamin K is known to be involved in the formation of gamma-carboxyglutamate (Gla) in proteins, such as the calcium-binding bone Gla-proteins osteocalcin and matrix Gla-protein. Concomitant with the start of vitamin K treatment, the calcium-binding capacity of osteocalcin increased, and so did the urinary excretion of free Gla. This is suggestive for a subclinical vitamin K-deficiency in the astronaut before vitamin K-supplementation. During periods of high vitamin K status markers for bone formation (osteocalcin and bone alkaline phosphatase) had increased as compared to the first part of the flight. The mean increases were 14 and 23%, respectively. Our data suggest that increased intake of vitamin K may contribute to counteracting microgravity-induced loss of bone mass during long lasting space missions, but need confirmation in more astronauts.

1-Carboxyglutamic Acid↗
PubMed1997

Strategies for developing human osteocalcin standards: a critical evaluation.

Osteocalcin is a small protein uniquely produced by osteoblasts and odontoblasts. Since about 30% of the de novo synthesized osteocalcin is set free in the blood stream, it is widely used as a marker for bone formation. However, circulating immunoreactive osteocalcin (irOC) consists of several fractions, which may differ from each other with respect to size and calcium binding properties. Whereas it is generally assumed that the fraction with high affinity for hydroxyapatite reflects bone formation, we have found a high correlation between bone mass and irOC with low affinity for hydroxyapatite. Since the antibodies used in various commercial test kits for osteocalcin have different affinities for the various irOC fractions, well-defined homogenous standards have to be prepared which may be used for standardization of the detection techniques used in various laboratories. In this paper we give a critical evaluation of the strategies which may be followed.

Amino Acid Sequence↗
PubMed1996

Osteocalcin detection in aging serum and whole blood: stability of different osteocalcin fractions.

Human serum osteocalcin is a well known bone formation marker. On the basis of their different affinities for hydroxyapatite, the total immunoreactive osteocalcin may be separated into two fractions. Six commercial test kits for osteocalcin were compared. All kits reacted with both osteocalcin fractions but the absolute amounts found in the same serum samples differed widely. During serum storage at room temperature, there was no significant loss of osteocalcin during the first 6 h. After longer storage periods, the recorded decrease of osteocalcin depended on the system used: with two kits, over 80% of the original immunoreactive antigen was left after 9 days. It is considered that the different osteocalcin fractions may become useful as markers for different metabolic bone processes. A more precise definition of the various circulating osteocalcin fractions, and the development of separate tests for each fraction, are requirements for the optimal use of osteocalcin as a diagnostic tool for metabolic bone disorders.

Adult↗
PubMed1996

Effects of vitamin K on bone mass and bone metabolism.

Vitamin K is involved in blood coagulation and in bone metabolism via the carboxylation of glutamate residues in (hepatic) blood coagulation factors and (osteoblastic) bone proteins. The bioavailability of nutritional vitamin K depends on the type of food, the dietary fat content, the length of the aliphatic side chain in the K-vitamer and probably also the genetically determined polymorphism of apolipoprotein E. Although undercarboxylation of blood coagulation factors is very rare, undercarboxylated osteocalcin (bone Gla-protein) is frequently found in postmenopausal women. Supplementation of these women with extra vitamin K causes the markers for bone formation to increase. In parallel, a decrease of the markers for bone resorption is frequently seen. Insufficient data are available to conclude that the regular administration of vitamin K concentrates will reduce the loss of bone mass in white women at risk for developing postmenopausal osteoporosis.

Animals↗
PubMed1995

Role of vitamin K in bone metabolism.

Vitamin K is a cofactor required for the formation of gamma-carboxyglutamate (Gla) residues in proteins. Osteoblasts produce at least three different Gla-containing proteins: osteocalcin, matrix Gla-protein, and protein S. After cellular secretion of these proteins, the main part of each remains bound to the hydroxyapatite matrix in bone, but their function remains unclear. Part of the newly synthesized osteocalcin is also set free into the bloodstream, where it may be used as a diagnostic marker for bone formation. Several studies have demonstrated that a poor vitamin K status is associated with an increased risk of osteoporotic bone fractures. Whether vitamin K supplementation will reduce the rate of bone loss in postmenopausal women remains a matter of debate.

Animals↗
PubMed1993

Vitamin K-induced changes in markers for osteoblast activity and urinary calcium loss.

The objective of this study was to identify subjects in whom vitamin K has an effect on markers for calcium and bone metabolism and to detect hitherto-unnoticed correlations between vitamin K-induced changes in these markers. Participants in our studies were apparently healthy women, in whom we measured serum-immunoreactive osteocalcin (irOC) before and after adsorption to hydroxylapatite; total serum alkaline phosphatase (T-AP) and bone-specific alkaline phosphatase (B-AP); and fasting urinary calcium and creatinine. We describe a trial among 145 women who were treated with vitamin K (1 mg/day) for 2 weeks, and a prospective placebo-controlled trial among two groups each of 70 postmenopausal women with a treatment period of 3 months. It turned out that in elderly women vitamin K induced increased levels of serum irOC with a high affinity for hydroxylapatite (irOCbound), whereas that with low affinity (irOCfree) remained unaffected. In placebo-treated women the ratio irOCfree/irOCbound shifted from 0.38 to 0.65 around the 50th year of age. This shift was not found in vitamin K-treated women. After 3 months of treatment the vitamin K-induced changes in irOCbound were correlated with changes in B-AP, whereas irOCfree was correlated to urinary calcium excretion. In fast losers of urinary calcium vitamin K induced a 30% decrease of calcium excretion. The hypothesis is put forward that irOCbound may be a marker for bone formation, that serum irOCfree may be a marker for bone resorption, and that the serum irOCfree/irOCbound ratio may become a marker for skeletal remodeling.(ABSTRACT TRUNCATED AT 250 WORDS)

Adult↗
PubMed1993

Effects of vitamin K and oral anticoagulants on urinary calcium excretion.

In a subgroup of postmenopausal women vitamin K induced a decrease of the urinary calcium loss. This effect was significant (P < 0.0001) in the so-called fast losers of calcium (calcium/creatinine ratio > 0.5). To find out whether vitamin K antagonists would have an opposite effect, a study was started among 141 persons on long-term oral anticoagulant therapy. In this population the number of fast losers was recorded, and compared to that in a group of age- and sex-matched non-treated controls. Notably in young men the fraction of fast losers was significantly higher in the anticoagulant-treated group than in the control group (25 v 0%, P < 0.02). Differences between treated and nontreated groups may also be found in other markers for calcium and bone metabolism, notably in serum osteocalcin concentration and in urinary hydroxyproline excretion. The conclusion of our study is that oral anticoagulant treatment must be regarded as a potential risk factor for a high loss of urinary calcium.

Adult↗
PubMed1989

The effect of vitamin K supplementation on circulating osteocalcin (bone Gla protein) and urinary calcium excretion.

STUDY OBJECTIVE: To determine whether vitamin K administration affects urinary calcium excretion in postmenopausal women. DESIGN: Before- and after-trials with a 2-week treatment period. SUBJECTS: Healthy postmenopausal women (55 to 75 years old) were recruited from the convents in and around Maastricht. Controls (25 to 40 years old) were healthy premenopausal volunteers. INTERVENTION: Daily administration of 1 mg of vitamin K for 2 weeks. MEASUREMENTS: Serum immunoreactive osteocalcin: hydroxylapatite binding (HAB) capacity of serum immunoreactive osteocalcin; excretion of calcium, hydroxyproline, and creatinine in the urine during the last 2 h of a 16-h fasting period. RESULTS: In premenopausal women, no effect of vitamin K administration was seen. In the postmenopausal group, vitamin K induced increased serum immunoreactive osteocalcin concentration; normalization of the HAB capacity of serum immunoreactive osteocalcin (this marker was less than 50% that of the controls in the pretreatment samples); a decrease in urinary calcium excretion, notably in the "fast losers" of calcium; and a parallel decrease in urinary hydroxyproline excretion in the fast losers of calcium. CONCLUSIONS: The serum immunoreactive osteocalcin level may vary with vitamin K status. This variance should be taken into consideration if osteocalcin is used as a marker for osteoblast activity. Vitamin K is one factor that may play a role in the loss of bone mass in postmenopausal osteoporosis.

Adult↗
PubMed1988

Circulating osteocalcin during oral anticoagulant therapy.

In this paper we present the following observations: 1) In sheep vitamin K-antagonists like phenprocoumon induce a decrease of the serum levels of osteocalcin (bone Gla-protein) and of the affinity of the circulating osteocalcin for hydroxyapatite. 2) In sheep vitamin K counteracts the effect of phenprocoumon on the blood coagulation system, but not that on the osteocalcin production. 3) In human subjects vitamin K-antagonists also lead to decreased levels of serum osteocalcin and a low affinity of the protein for hydroxyapatite. 4) These two variables reached steady-state levels within 24 h after the start of oral anticoagulant treatment and--at continuation of the therapy--they remained low for at least several years.

Acenocoumarol↗
PubMed1988

Vitamin K is no antagonist for the action of warfarin in rat osteosarcoma UMR 106.

The recycling of vitamin K in the liver occurs via one or two dithiol-dependent reductases, which are strongly inhibited by coumarin derivatives such as warfarin. This inhibition may be partly overcome by the action of a NADH-dependent reductase, which is relatively insensitive for warfarin. In this paper we demonstrate that the osteoblast-like osteosarcoma UMR 106 does not contain the NADH-dependent reductase. Assuming that a similar enzyme distribution occurs in normal osteoblasts this explains the observation of Price and Kaneda, that the administration of vitamin K to rats efficiently counteracted the effect of warfarin on blood coagulation, but that the vitamin had no effect on the Gla-content of serum osteocalcin.

Animals↗
PubMed1988

The quantification of gammacarboxyglutamic acid residues in plasma-osteocalcin.

In this paper we describe an assay procedure for determining the amount of gammacarboxyglutamic acid (Gla) residues in serum- or plasma-osteocalcin. The test includes removing by ethanol precipitation the majority of the proteins from the plasma (e.g., the Gla-containing coagulation factors) and the specific extraction of osteocalcin with the aid of immobilized immunopurified antibodies. It is demonstrated that the Gla-content of circulating osteocalcin from normal cows is similar to that of osteocalcin obtained from bone. Hence, the fact that part of the newly synthesized osteocalcin does not bind to the hydroxyapatite matrix in bone cannot be explained by an undercarboxylation of the molecule.

1-Carboxyglutamic Acid↗