Physical activity and biochemical markers of bone turnover in elderly patients

To assess the role of physical therapy in improving the remodelling processes of bone turnover, biochemical markers of bone formation and resorption (osteocalcin, parathormone, bone-specific alcaline phosphatase, somatotropic hormone, C-terminal propeptide type I procollagen, somatomedin, insulin-like growth factor bound protein, C-terminal telopeptide type I collagen) have been investigated in elderly patients before and after a cycle of physical therapy. Patients of both sexes, immobilized on hospital admission day because of fractures or neurologic diseases, underwent physiotherapy for one month. Following physical treatment we found significant increases of osteocalcin, bone-specific alcaline phosphatase and somatomedin in the female group, while no significant difference was detected in males. Our data support that the mechanical stimulus significantly improves the recovery of osteoblastic activity in women, while in men the increases in bone remodelling markers are not significant. Differences in life-style between male and female patients are proposed as determinants in the bone remodelling response to physical therapy.

Biochemical markers of bone formation in Thai children and adolescents

The measurement of biochemical markers of bone turnover is essential in the study of skeletal metabolism in health and diseases. Due to variations in the rate of bone growth in different age groups and possible ethnic differences, age-specific reference ranges for biochemical markers should be established in a particular pediatric population. In this study, biochemical markers of bone formation, bone-specific alkaline phosphatase (BAP), and osteocalcin (OC) in healthy Thai children and adolescents aged 9 to 18 years were evaluated in relation to their ages and pubertal development. Serum BAP levels in boys increased with age and peaked at about 12 to 13 years. In contrast, there was a progressive decline of serum BAP levels with advancing age in girls older than 9 years. Serum OC also increased with age and reached a peak at ages 12 and 13 years in girls and boys, respectively. In addition, both serum BAP and OC levels also varied with pubertal stages. The BAP levels in boys increased sharply at pubertal stage 3 and decreased at pubertal stage 5. In girls, the BAP levels showed a fairly constant high level up to stage 3, followed by a remarkable decrease thereafter. The OC levels in boys increased sharply at pubertal stage 4 and decreased thereafter. In girls, OC started to increase at pubertal stage 3 with no subsequent changes. The levels of serum BAP and OC were higher in boys than in girls at pubertal stages 3 to 5 and at stages 2, 4, and 5, respectively. Moreover, only serum BAP level showed significant positive correlation with height velocity in both genders. In multiple regression analyses, gender, age, and pubertal stage were consistently correlated with both serum BAP and OC levels. In summary, male and female adolescents have different patterns of changes in biochemical markers of bone formation.

Identification of novel proteolytic forms of osteocalcin in human urine

In this study, we report the isolation and characterization of osteocalcin in human urine using mass spectrometry and N-terminal sequencing. Multiple proteolytic forms of osteocalcin were found, which consisted of 16-27 residues from the middle region of the molecule. Several fragments had residue Gly7 at the N-terminus and the most predominant was fragment 7-31. Additional fragments starting from residue Asp14 were detected in the samples of children and young adults. Immunochemical detection of urine osteocalcin fragments had a statistically significant negative correlation to bone mineral density in evaluation of urine samples from 75-year-old women. Thus, the measurement of osteocalcin fragments in urine may have potential applications in diagnostics related to disorders of bone metabolism.

A new assay method that detects only intact osteocalcin. Two-step non-invasive diagnosis to predict adynamic bone disease in haemodialysed patients

BACKGROUND

We studied the usefulness of a new assay method that detects only the intact human osteocalcin molecule in haemodialysed patients.

METHODS

Iliac bone biopsy specimens obtained from 62 haemodialysed patients were analysed.

RESULTS

Bone formation rates (BFR/BS) correlated positively with serum intact osteocalcin concentrations (n=62), osteocalcin concentrations assayed by a conventional method (n=31), parathyroid hormone (PTH) concentrations (n=62), and total alkaline phosphatase concentrations (r=0.602, 0. 588, 0.650, and 0.401 respectively). Based on ROC curve and Youden index analysis, the optimal cut-off value to distinguish adynamic bone disease from a mild lesion was 195 pg/ml of serum PTH concentration (Youden index=0.233) or 30 ng/ml of serum intact osteocalcin concentration (Youden index=0.628). The optimal cut-off value to distinguish between hyperparathyroid bone and a mild lesion was 455 pg/ml of serum PTH level (Youden index=0.63) or 50 ng=ml of serum intact osteocalcin concentration (Youden index=0.634). Since both ROC curve and Youden index suggested that the serum PTH concentration was not a good marker to distinguish adynamic bone from a mild lesion or hyperparathyroid bone, we designed a two-step procedure. The first step was the diagnosis of adynamic bone (cut-off: 65 pg/ml) or hyperparathyroid bone (cut-off: 455 pg/ml) according to serum PTH concentration. In a second step, we assessed serum intact osteocalcin concentration in patients with serum PTH concentrations between 65 and 455 pg/ml. The cut-off values for adynamic and hyperparathyroid bone in this diagnostic approach were 30 and 70 ng/ml respectively. As a result, 49 out of 62 patients were diagnosed properly. The Youden index of this two-step diagnosis was 0.527 and 0.661 for adynamic bone and hyperparathyroid bone respectively. Sensitivity markedly improved to 94.4% and 96.2% for adynamic bone and hyperparathyroid bone respectively, without sacrificing specificity (84.0 and 88.8% respectively).

CONCLUSION

Measurement of serum intact osteocalcin concentration is useful for the diagnosis of adynamic bone in haemodialysed patients. A two-step procedure involving also simultaneous measurement of serum PTH concentration further improved the sensitivity of each individual marker while maintaining specificity.

Development and Evaluation of Three Immunofluorometric Assays That Measure Different Forms of Osteocalcin in Serum

Background: Circulating human osteocalcin (hOC) has been used as a marker of bone formation. Our aim was to validate three immunofluorometric assays (IFMAs), measuring different forms of hOC.

Methods: The two-site IFMAs were based on previously characterized monoclonal antibodies. Assay 2 recognized intact hOC, assays 4 and 9 measured the NH2-terminal mid-fragment and the intact hOC. In addition, assay 9 required hOC to be γ-carboxylated.

Results: A 76–79% increase of serum immunoreactive hOC was found in the postmenopausal group compared with the premenopausal group with all IFMAs. With EDTA-plasma samples, the observed increases were lower (49–65%). The hOC concentration in the postmenopausal group receiving hormone replacement therapy was 42–44% lower than that in the postmenopausal control group in both serum and EDTA-plasma samples. The depressed carboxylation in warfarin-treated patients was accompanied by lower results in assay 9. The ratio of assay 9 to assay 4 totally discriminated the warfarin-treated patients from the controls. Assay 9 showed the smallest decreases in measured hOC after storage of serum or plasma for 4 weeks at 4 °C, followed by assay 4 and assay 2. Results from the last assay were <17% of their initial values after 4 weeks of storage. No diurnal variation was observed with assay 9 as opposed to the two other IFMAs.

Conclusion: The three assays with their distinct specificity profiles (intact vs fragmented and carboxylated vs decarboxylated hOC) may provide valuable tools for investigating the significance of different hOC forms in various bone-related diseases.

Demonstration of the predominant urine osteocalcin fragments detectable by two-site immunoassays

We have isolated and characterized human osteocalcin (OC) fragments from pubertal urine. The fragments were isolated by immunoaffinity chromatography based on monoclonal antibody 6F9 and further purified by reverse phase chromatography. The major isolated forms, which were detectable with two-site immunofluorometric assays for serum OC, span residues 6-30 and 7-30 as determined by mass spectrometry and N-terminal amino acid sequencing. Full-length OC was not detectable in the supernatant fraction of urine but could be extracted with guanidinium hydrochloride from the sediment of urine samples. Urine samples from subjects with different menopausal status were measured by two different two-site assays. Urine OC (uOC) concentrations were 12- to 16-fold higher in the pubertal group than in the adult group. Also, the uOC concentration in a postmenopausal group was significantly higher than in a premenopausal group. The difference was 125% and 75% (values for p < 0.0001), respectively, when measured with the two assays. uOC concentrations in postmenopausal subjects on hormone replacement therapy were indistinguishable from the premenopausal subjects. The fact that uOC can be measured by a noncompetetive two-site assay design offers improved analytical sensitivity. Urine as the sample matrix is also especially interesting because the predominant markers of bone resorption, collagen type I peptides or cross-links, are performed on urine samples. Our results from the technical validation of two-site assays for uOC and from applying these to human pubertal and pre- and postmenopausal samples calls for more extensive clinical validation.

Basic principles and clinical applications of biochemical markers of bone metabolism: biochemical and technical aspects

The interest in and the need for effective measures to be used in the screening, diagnosis, and follow-up of disorders of connective tissue, bone, and mineral metabolism has markedly grown. Next to clinical and imaging techniques, indices of bone turnover have come to play an important role in the assessment of metabolic bone disease. In osteoporosis, recent research has shown that bone markers may also be used to predict future bone loss and hip fractures (in larger cohorts of older patients), identify individuals at risk for osteoporosis, select therapy, and predict and monitor the therapeutic response in individual patients. The development of new markers of bone metabolism has greatly enriched the spectrum of serum and urine analytes used in the assessment of skeletal pathologies. Besides total alkaline phosphatase, other markers such as bone-specific alkaline phosphatase, osteocalcin, or the collagen propeptides are being used to measure bone formation. Bone resorption, previously assessed only by the measurement of urinary calcium and hydroxyproline, may now be detected more precisely by a number of new serum and urine markers. Among these, the pyridinium crosslinks and the telopeptides of collagen type I are presently considered the most specific markers of bone resorption. More recently, bone sialoprotein has also been suggested as a marker of bone resorption in serum. Tartrate-resistant acid phosphatase is now measurable by immunoassay. This article surveys the biochemistry and relevant technical aspects of the currently available markers of bone metabolism.

Differential effects of inhaled budesonide on serum osteocalcin in children and adolescents with asthma

In recent years, measurement of serum osteocalcin has been introduced for assessment of bone turnover in patients treated with exogenous glucocorticoids. Studies in children with asthma on inhaled glucocorticoids, however, have shown inconsistent results. The aim of the present study is to assess bone turnover in prepubertal children and in adolescents with asthma treated with inhaled budesonide using three different osteocalcin assays: the Pharmacia Osteocalcin CAP FEIA, the CIS OSTK-PR and CIS IRMA ELSA-OSTEO assays. Two studies were conducted: 1) a randomised double blind two-period crossover study of 22 prepubertal children aged 5-12 years. In one period 800 microg budesonide was given once in the morning, in the other 400 microg was given twice daily; 2) a randomised double blind placebo controlled two period crossover study of inhaled budesonide 400 microg twice daily in fourteen 13-16 year old adolescents with pubertal stages II-V. In both studies, treatment periods were of four weeks duration, and blood samples were collected at the last day of each period. In the prepubertal children none of the osteocalcin assays detected any statistically significant differences between any of the periods. In the adolescent group reduced levels of osteocalcin were seen during budesonide treatment. The suppression reached statistical significance with the CAP FEIA (P = 0.03) and the OSTK-PR (P = 0.01) assays, but not with the ELSA-OSTEO assay (P = 0.06). Correlation analyses showed statistically significant correlation coefficients varying between 0.58 and 0.91 (P = 0.03 and P < 0.0001, respectively). The effect of inhaled glucocorticoids on serum osteocalcin may depend on the assay applied, and inhaled glucocorticoids have differential effects in children and adolescents.

A dual-label immunofluorometric assay for human osteocalcin

Circulating human osteocalcin (hOC) has been shown to be comprised of two main forms: the intact 1-49 form and the proteolytic N-terminal midfragment (N-mid) spanning amino acid residues 1-43 or 1-44. We used three monoclonal antibodies (MAbs) raised against hOC and bovine osteocalcin in developing a dual-label assay for the simultaneous measurement of the proportions of the intact and N-mid forms in serum samples. The assay is based on time-resolved fluorescence utilizing differently labeled trace MAbs. Biotinylated MAb 2H9 is used as a capture antibody for both the intact hOC and the N-mid. Tracer MAb 6F9 labeled with a Europium (III)-chelate binds to the intact the N-mid and the intact hOC, whereas tracer MAb 3G8 labeled with a Terbium (III)-chelate binds to the intact hOC only. The simultaneous binding of the antibodies was tested by comparing full-length hOC purified from human bone and hOC shortened from the C terminus by four amino acid residues with carboxypeptidase Y. Serum hOC measurements with the dual-label assay were in agreement with the corresponding single-label assays (r = 0.96 for intact + N-mid assay and r = 0.81 for intact assays, n = 91). The lower correlation between the intact assays was attributable to proteolytic susceptibility of the intact form due to one additional freezing and thawing cycle in carrying out the dual-label assay. As measured with the dual-label assay, the levels (mean +/- SD) of serum intact + N-mid OC were 6.2 +/- 2.1 ng/ml in the premenopausal group (n = 44), 13.9 +/- 4.9 ng/ml in the postmenopausal group without hormone replacement therapy (HRT; n = 13), and 7.5 +/- 3.4 ng/ml in the postmenopausal group with HRT (n = 13). The levels of intact hOC in the same groups were 4.8 +/- 1.4 ng/ml, 9.8 +/- 2.9 ng/ml, and 5.3 +/- 2.1 ng/ml, respectively. Whether the main forms of OC or their relative proportions in serum can be used for predicting bone diseases or for monitoring the progression and management of diseases awaits further investigations.

Immunoreactive osteocalcin forms in conditioned media from human osteoblast culture and in sera from healthy adult control subjects and patients with bone pathologies

BACKGROUND

The aim of this work was to study the immunoreactive forms of bone Gla protein (BGP) present in conditioned media of human osteoblast cultures (BGP released from osteoblast) and in the sera of healthy adult control subjects and patients with bone pathologies (chronic renal failure on haemodialysis, Paget's disease of bone and post-menopausal osteoporosis).

METHODS

The technical procedure used was a combination of high-performance liquid chromatography (HPLC) and different BGP assays with several specificities to analyse BGP levels in the different HPLC fractions. Aliquots of conditioned media or sera were purified through a Sephadex G-50m column and by HPLC (C4 reverse-phase column) in a 25-40% acetonitrile gradient. Two-minute fractions were collected and divided into three aliquots in order to determine osteocalcin content using three different assays: (a) ELSA-OST-NAT IRMA, which only detects intact osteocalcin; (b) ELSA-OSTEO IRMA, which detects intact osteocalcin and N-terminal fragments; and (c) OSCA Test RIA, which detects intact osteocalcin, C-terminal and other fragments.

RESULTS

We found different immunoreactive forms of osteocalcin in the culture medium of human osteoblasts and in sera from control subjects and patients for the bone pathologies studied. We did not find great qualitative differences between the immunoreactive osteocalcin profile found in the culture medium from human osteoblasts and the sera from healthy control subjects. However, the different bone pathologies show different characteristic patterns of immunoreactive forms of osteocalcin.

CONCLUSIONS

An interesting finding has been the detection, both in sera and in osteoblast culture media, of several immunoreactive forms of intact osteocalcin that eluted from HPLC at different acetonitrile percentages, and therefore correspond to different molecular forms.

Bone biomarkers as tools in osteoporosis management

Biochemical tests that can index bone turnover rate in the patient are increasingly being used in the study and management of osteoporosis. Markers of bone formation and resorption are reviewed here, including their molecular basis, relative strengths and weaknesses in clinical performance, and future potential. A bone mass measurement (e.g., by dual-energy x-ray absorptiometry) and a biochemical index of bone turnover provide different but complementary information that can aid in predicting risk of future bone loss and osteoporotic fracture. A specific and responsive bone resorption marker can also be used to monitor and establish the short-term effectiveness of an antiresorptive therapy in the patient. Bone-specific alkaline phosphatase (an osteoblast enzyme) and osteocalcin (a bone matrix protein) levels in serum are the best markers of bone formation. Collagen degradation products in urine, particularly cross-linked telopeptides and pyridinolines, have the highest specificity to bone resorption activity. The telopeptide markers (NTx and CTx) appear to be the most specific and responsive markers of systemic osteoclast activity.

Identification of peptide fragments generated by digestion of bovine and human osteocalcin with the lysosomal proteinases cathepsin B, D, L, H, and S

We have determined the primary cleavage sites in the bone Gla protein (BGP; osteocalcin) for several of the proteases that could act on the protein during bone resorption and turnover, cathepsins B, D, L, H, and S. The time course of BGP digestion by each cathepsin was first determined by sodium dodecyl sulfate polyacrylamide gel electrophoresis. We then incubated human and bovine BGP with each cathepsin for a sufficient time to reduce the level of intact protein by at least 20-fold, isolated the major cleavage peptides, and identified each by N-terminal sequence analysis and by amino acid analysis. Our results show that BGP has relatively few cathepsin-sensitive sites and that these sites are located at the N and C terminus of the 49-residue protein. Cathepsins B, L, H, and S readily cleave BGP at the G7-A8 bond; cathepsin L also cleaves at R43-R44; cathepsin B also cleaves at R44-F45; and cathepsin D cleaves only at A41-Y42. The immunoreactivity of the major peptides generated by cathepsin cleavage was evaluated using the original radioimmunoassay developed for the detection of BGP in human serum. The BGP 8-49 fragment cross-reacts identically with native BGP, while the 8-43 and the 1-44 fragments require 20- to 40-fold higher concentrations to achieve the same level of displacement as the native protein. The 1-41 and 8-41 fragments are unable to significantly displace the labeled native BGP tracer at any concentration tested. These results demonstrate the utility of peptides generated by cathepsin digestion in the mapping of the antigenic epitopes recognized by a given BGP immunoassay.

Epitope mapping of nine monoclonal antibodies against osteocalcin: combinations into two-site assays affect both assay specificity and sample stability

Nine monoclonal antibodies (Mabs) were raised against human recombinant osteocalcin fusion protein (rGST-hOC) or bovine osteocalcin (bOC) and selected to develop two-site immunoassays for human osteocalcin (hOC). The detection system was based on the time-resolved measurement of the fluorescence of europium chelates conjugated to the tracer Mabs. Based on the ability of the Mabs to recognize different forms of hOC (carboxypeptidase Y-digested, alkylated hOC, thermally decarboxylated hOC, recombinant forms of hOC, and tryptic peptides derived from hOC) and the information obtained from combinations of the Mabs in two-site assays, an epitope map was created. The epitope map was useful in understanding the behavior of the two-site combinations of the Mabs with serum samples. The two-site combinations could be divided into subgroups detecting either full-length hOC or full length+large NH2-terminal fragment as stimulated by the carboxypeptidase Y-digested form of hOC (it lacks four COOH-terminal residues), which with intact specific assays showed cross-reactivities ranging from 7 to 14% when compared with full-length hOC. In addition, differences were observed in the ability of the combinations to detect thermally decarboxylated hOC (lacks gamma-carboxylation at residues 17, 21, and 24) with cross-reactivities ranging from 8 to 85% when compared to gamma-carboxylated hOC. The analysis of human serum samples showed considerable differences in the concentration and stability of serum OC. This was attributed as the varying ability of the Mabs to detect different proteolytic fragments derived from hOC and/or differences in the degree of gamma-carboxylation of hOC. The in vitro generation of the large NH2-terminal fragment during incubation of the serum samples at room temperature (RT) and during prolonged storage at -20 degrees C in an undercooled state was detectable as loss of immunoreactivity (ranging from -42 +/- 17 to -50 +/- 15% in 16 h at RT, n = 3) with Mab combinations detecting only full-length hOC. Two-site combinations detecting full-length+large NH2-terminal fragment showed no loss of immunoreactivity after incubation of the serum samples at RT for 16 h. With all assays, an increase of serum OC ranging from 16 to 38% was found in postmenopausal samples (n = 24) when compared with premenopausal samples (n = 17), but the degree of statistical significance varied from not significant to p < 0.01.

Biological variation in bone-derived biochemical markers in serum

Analytical, within-subject and between-subject components of variation have been determined for total alkaline phosphatase (ALP) and its bone isoform, osteocalcin, and tartrate-resistant acid phosphatase (TR-ACP) in serum specimens from a cohort of 10 healthy subjects over a 1-month period. From these data, we have calculated the desirable analytical imprecisions, the indices of individuality, the critical differences for significant change detection, and the number of specimens required to estimate the homeostatic setpoint of an individual. For total ALP and its bone isoform, most of the observed variability was biologic, whereas osteocalcin and TR-ACP also had relatively high analytical variability. Total and bone ALP were also the parameters showing the lowest within-subject variability, whereas TR-ACP showed the lowest interindividual fluctuation. Osteocalcin and bone ALP had marked individuality, showing that the use of population-based reference limits is inadequate for their interpretation. The applicable differences required for two results to be significantly different (p < or = 0.05) are bone ALP: 20%; osteocalcin: 29%; and TR-ACP: 35%. The results demonstrate that the biochemical markers of bone turnover studied are of limited use to screen for metabolic bone disorders, but can be useful adjuncts for monitoring groups of patients in longitudinal studies.

Sandwich immunoassay specific for the N-terminal sequence of osteocalcin

A monoclonal antibody (10B) against the N-terminal sequence of human osteocalcin was selected to characterize its epitope and species specificity. The cross-reactivity of 10B with human and rat osteocalcin demonstrated that the reactivities of 10B with both human and rat osteocalcins were very similar. The pin technology method was used to determine the epitope and clearly demonstrated that the epitope recognized by 10B was localized to residues 12-16, the sequence of which is identical in rat and human osteocalcin molecules. This monoclonal antibody was found to be useful for designing region-specific sandwich immunoassay systems for human and rat N-terminal osteocalcin (N-OC) molecules using rabbit anti-hN20 and anti-rN20 polyclonal antibodies, respectively. The osteocalcin levels in serum determined by this N-OC method were stable during prolonged storage at 25 degrees C and the method could be usefully applied in the development of immunoassay systems for many osteocalcin molecules from many other species.