Assay of pyridinium crosslinks in serum using narrow-bore ion-paired reversed-phase high-performance liquid chromatography

Identification of pyridinoline, a collagen crosslink, as a novel intrinsic ligand for the receptor for advanced glycation end-products (RAGE)

Advanced glycation end-products (AGEs) elicit inflammatory responses via the receptor for AGEs (RAGE) and participate in the pathogenesis of diabetic complications. An earlier study showed that 3-hydroxypyridinium (3-HP), a common moiety of toxic AGEs such as glyceraldehyde-derived pyridinium (GLAP) and GA-pyridine, is essential for the interaction with RAGE. However, the physiological significance of 3-HP recognition by RAGE remains unclear. We hypothesized that pyridinoline (Pyr), a collagen crosslink containing the 3-HP moiety, could have agonist activity with RAGE. To test this hypothesis, we purified Pyr from bovine achilles tendons and examined its cytotoxicity to rat neuronal PC12 cells. Pyr elicited toxicity to PC12 cells in a concentration-dependent manner, and this effect was attenuated in the presence of either the anti-RAGE antibody or the soluble form of RAGE. Moreover, surface plasmon resonance-based analysis showed specific binding of Pyr to RAGE. These data indicate that Pyr is an intrinsic ligand for RAGE.

Abbreviations: AGEs: advanced glycation end-products; RAGE: receptor for advanced glycation end-products; DAMPs: damage-associated molecular patterns; PRR: pattern recognition receptor; TLR: toll-like receptor; GLAP: glyceraldehyde-derived pyridinium; 3-HP: 3-hydroxypyridinium; Pyr: pyridinoline; HFBA: heptafluorobutyric acid; GST: glutathione S-transferase; SPR: surface plasmon resonance; ECM: extracellular matrix; EMT: epithelial to mesenchymal transition

The antipsychotics haloperidol and chlorpromazine increase bone metabolism and induce osteopenia in female rats

The main objective of this study was to evaluate the effects of the antipsychotic drugs haloperidol (HAL) and chlorpromazine (CPZ) on bone mineral density (BMD) in female rats and to examine the relationship between the effects on bone and reproductive organs or hormone concentrations. Female rats were orally administered HAL (2 or 10 mg/kg) or CPZ (25 or 50 mg/kg) once daily (7 days/week) for 6 months resulting in a significant increase in prolactin. Hyperprolactinemia resulted in enlarged corpora lutea in the ovary, because prolactin has a luteotropic activity. Thus, atrophy in the uterus, epithelial mucification in the vagina and continuous diestrus stages were observed. These events in the reproductive organs induced a decrease in estradiol, elevation of biochemical markers of bone metabolism, significant reductions of BMD in trabecular bone of the femur and decreased trabecular bone in the femur. The bone loss is associated with an increase in bone resorption due to decreased estradiol derived from the luteotropic activity of prolactin. The mechanism of dopamine blockers to induce bone loss in female rats is considered to be rodent specific because the luteotropic effects of prolactin are confined primarily to rodents. Also, it appears that chronic hyperprolactinemia and maintained corpora lutea leading to bone loss are commonly inducible in female rats receiving long-term treatment with antipsychotic drugs possessing dopamine D2 receptor antagonist activity.

Online hemodiafiltration versus acetate-free biofiltration: a prospective crossover study

Online hemodiafiltration (online HDF) and acetate-free biofiltration (AFB) are 2 innovative renal replacement therapies. Convincing evidence has shown that both techniques are superior to conventional hemodialysis in many aspects. The aim of the present investigation was to compare online HDF and AFB in 12 stable maintenance hemodialysis patients in a prospective, randomized crossover trial. Twelve stable dialysis patients, age 49.7 +/- 11.3 years and on dialysis for 83.5 +/- 76.7 months, were treated prospectively and randomly by either AFB, predilution HDF (pre-HDF), or postdilution HDF (post-HDF) for a total of 36 weeks using exclusively F60S high-flux dialyzers. Routine blood biochemical tests, bone metabolism parameters, and clearance for both small and larger molecular weight substances were measured at defined intervals. During the trial period inter- and intradialysis symptoms, e.g., hypotensive episodes and intradialysis arterial blood gas analyses, were recorded. Both online HDF and AFB were well accepted by the overwhelming majority of patients and also by the dialysis staff. Pretreatment sodium, total and ionized calcium, chloride, bicarbonate, and urea did not differ within or between the 3 treatment groups. Potassium increased slightly in HDF patients while phosphate and beta2-microglobulin (beta2-M) decreased in all groups. After dialysis, AFB patients exhibited a significantly higher bicarbonate concentration and lower potassium level when identical potassium concentrations in dialysate were used. Patients receiving AFB manifested less intradialysis partial pressure of oxygen drop and partial pressure of carbon dioxide rise than those on HDF treatments. HDF treatments could afford higher single-pool and double-pool Kt/V, higher effective urea and beta2M clearance, and lower total interdialysis symptom scores than the AFB treatment method. While bone metabolism parameters did not differ between the 3 dialysis modalities, some parameters such as deoxypyridinoline in HDF and osteocalcin, pyridinoline, and deoxypyridinoline in AFB deteriorated at the end of the crossover study. Aluminum concentration decreased progressively to about one-third of prestudy values at the end of the study with all 3 treatments. AFB was associated with a lower predialysis mean arterial pressure (MAP), a smaller drop in MAP during treatment, and similar hypotension episodes compared with the 2 HDF treatments. Albumin concentration showed a trend to decrease during the first 2 months of the trial period followed by a slight increase thereafter but still significantly lower than initial value at the end of crossover. Both online HDF and AFB share most of the features of optimal renal replacement therapy. Online HDF is superior to AFB in such aspects as increased delivered dialysis dose both for small and larger molecular weight toxins and less interdialysis symptoms. On the other hand, AFB is associated with a smaller effect on arterial blood gas values and improved intradialysis hemodynamic tolerance. Some dialysis-related symptoms and complications in the case of our AFB practice could be attributable, at least in part, to low dialysate calcium level.

A Collection Method and High-Sensitivity Enzyme Immunoassay for Sweat Pyridinoline and Deoxypyridinoline Cross-Links

Background: Collagen cross-link molecules such as pyridinoline (PYD), deoxypyridinoline (DPD), and N-terminal cross-linked peptides (NTX) have been measured in urine as indices of bone resorption. However, very little is known regarding the excretion of pyridinolines into other biological fluids. We report a collection device, normalizing analyte, and high-sensitivity immunoassay for quantitative analysis of free pyridinoline cross-links in sweat.

Methods: Flame atomic emission and ion-selective electrode techniques were used to measure potassium as a sweat volume marker. The Pyrilinks immunoassay for urine free pyridinolines was optimized to increase sensitivity for measurements in sweat. The precision, accuracy, and detection limit of this assay were characterized. To assess values and variability of sweat pyridinolines in human subjects, a nonocclusive skin patch was used to collect sweat samples from a reference group and from a mixed group experiencing accelerated bone resorption, postmenopausal women and men receiving gonadotropin-releasing hormone for prostate cancer.

Results: The immunoassay intra- and interassay variations were ≤10% and <16%, respectively, with a detection limit of 309 pmol/L. Linearity upon dilution and analytical recovery ranged from 93% to 109% and 85% to 122%, respectively. Sweat PYD values normalized to potassium output yielded a weekly intraindividual biological variability of 14.7%. The mean increase in the population experiencing increased bone resorption vs the reference group was 36% (P <0.05) for sweat PYD/K vs 23–40% (P <0.05) for urinary PYD/Cr, DPD/Cr, and NTX/Cr.

Conclusion: We conclude that this new platform sweat collection technology and PYD immunoassay show potential as an indicator of bone resorption.

Serum collagen crosslinks as markers of bone turn-over during GH replacement therapy in growth hormone deficient adults

OBJECTIVES

Bone metabolism is an important target for GH replacement therapy. However, in adults, treatment periods exceeding 12 months are required for a positive effect of GH on bone mineral density. Thus, to detect an early effect of GH on bone, markers of bone turn-over are important. Pyridinoline (PYR) and deoxypyridinoline (DPYR) are well-defined sensitive markers of bone resorption, but to date only urinary assays have been available. We report the use of a novel assay to measure changes in serum PYR and DPYR in GH deficient (GHD) adults during GH replacement therapy.

STUDY DESIGN

The study consisted of a 6-month randomized, double-blind, placebo-controlled study of the administration of GH (Genotropin) (0.25 IU/Kg/week (0.125 IU/kg/week for the first four weeks)) followed by a 6-month open phase of GH therapy.

PATIENTS

Thirty-five GHD adults (17 women; mean age 39.8 years; range 21.1-59.9) on conventional hormone replacement therapy as required, were studied.

MEASUREMENTS

Bone formation was analysed using serum bone alkaline phosphatase (BAP) and serum osteocalcin (OC). Bone resorption was analysed using serum pyridinoline (PYR) and serum deoxypyridinoline (DPYR). Bone mineral density (BMD) was determined by dual energy X-ray absorptiometry (DEXA).

RESULTS

After 6 months placebo treatment there were no significant changes in any of the bone markers analysed, nor in BMD. In the active arm of the study there was a significant increase in serum OC, BAP, PYR and DPYR (P = 0.03, P = 0.004, P = 0.003 and P = 0.01, respectively), remaining significantly elevated over their baseline levels for the subsequent 6 months of treatment (P = 0.04, P = 0.009, P = 0.003 and P = 0.04, respectively). No changes were observed in BMD in any of the groups after 6 months GH treatment. In the active arm of the study, after 12 months GH treatment there was a significant increase in BMD at both the lumbar spine and femoral neck (P = 0.01 for both sites).

CONCLUSIONS

In summary, the present study confirms that administration of GH treatment to GHD adult patients significantly activates bone remodelling, with the effect of GH both in bone formation and bone resorption markers being maximal after 6 months of treatment. The serum assay for PYR and DPYR has a number of practical and theoretical advantages over the urine assay and gave similar results to those previously reported for the urine assay.

Automated on-line solid-phase extraction and high-performance liquid chromatographic analysis of total and free pyridinium crosslinks in serum

An automated on-line solid-phase extraction procedure for free and total pyridinium crosslinks in serum, with HPLC analysis, is described. The pyridinium crosslinks either following hydrolysis in 3 M HCl or free in neat serum were extracted using a Gilson Aspec XLi system onto extraction cartridges containing an octylsilane/cation exchanger sorbent. Up to 525 microliters of serum could be loaded onto the extraction cartridges. After washing, the crosslinks were eluted with 400 microliters of 100 mM sodium formate pH 5 and 380 microliters of this was concentrated on a RPB guard column eluted with 100 mM HFBA. The crosslinks were backflushed and separated on a 5-micron ODS analytical column eluted with 30 mM HFBA with 18% MeCN at 1 ml/min and detected by their native fluorescence (excitation 295 nm, emission 400 nm). The use of a high sensitivity fluorescence detector was essential. Recoveries were 95-100% with a limit of detection (S/N = 2) of 109 pmol/l (pM) for pyridinoline (Pyr) and 143 pM for deoxypyridinoline (dPyr). The inter-assay R.S.D. was 9% for pyridinoline and 10.8% for deoxypyridinoline. The throughput of the system was up to 50 samples per day.

Biochemical markers of bone metabolism: an overview

OBJECTIVES

An overview of biochemical markers of bone metabolism is presented along with indications for their clinical utilization.

DESIGN AND METHODS

The structure, cyclical metabolism, and hormone regulation of bone is reflected by markers of resorption, formation and/or turnover. Markers of resorption representing degradation of type 1 collagen, include N-telopeptides, C-telopeptides, hydroxyproline, and the collagen crosslinks pyridinoline and deoxypyridinoline; acid phosphatase, a marker of osteoclast activity, and urinary calcium are also indicators of bone resorption. Bone formation markers indicate osteoblast activity; bone-specific alkaline phosphatase and the N-terminal and C-terminal extension peptides of procollagen reflect formation of organic matrix in bone. Osteocalcin, produced by osteoblasts but also released during osteoclastic degradation, may indicate either formation when resorption and formation are coupled or turnover when they are uncoupled.

RESULTS

Bone markers respond to intervention more rapidly than techniques such bone mineral density. Resorption markers respond approximately 1 to 3 months after intervention; markers of formation respond later, after 6 to 9 months. Bone markers may add useful information for assessing fracture risk and for monitoring osteoporosis, Paget's disease of bone, cancer metastasis, and metabolic disease. Various therapeutic interventions may affect release of some bone markers.

CONCLUSION

Bone disease has high prevalence in adults so bone markers will become even more important for assessing fracture risk and monitoring therapy as populations age. Characteristics of bone markers are dependent on biology and the assay used. Substantial work remains in characterizing existing assays, identifying better markers and performing the clinical studies to define which bone markers should be measured and when.

Collagen crosslinks in fibromyalgia

OBJECTIVE

To determine if abnormal collagen metabolism is a characteristic of fibromyalgia.

METHODS

The diagnosis of fibromyalgia was made according to the American College of Rheumatology criteria. Skin biopsy samples were obtained from the trapezius region of 8 patients with fibromyalgia. Urine was collected under standardized conditions from 55 control subjects and 39 patients with fibromyalgia, and serum was obtained from 17 controls and 22 patients with fibromyalgia. Pyridinoline (Pyd), an indicator of connective tissue disease, and deoxypyridinoline (Dpyd), an indicator of bone degradation, both of which represent products of lysyl oxidase-mediated crosslinking in collagen, were analyzed by ion-paired and gradient high-performance liquid chromatography (HPLC) methods with fluorescence detection. Levels of hydroxyproline (Hyp), a collagen turnover marker, were also measured. The findings were related to creatinine levels, and the Pyd:Dpyd ratio was determined.

RESULTS

Highly ordered cuffs of collagen were observed around the terminal nerve fibers by electron microscopic examination of biopsy tissue from all 8 patients with fibromyalgia, but were not observed in any of the control skin samples. The Pyd:Dpyd ratios in the urine and serum and the Hyp levels in the urine were significantly lower in patients with fibromyalgia than in healthy controls.

CONCLUSION

Decreased levels of collagen crosslinking in fibromyalgia may contribute to remodeling of the extracellular matrix and collagen deposition around the nerve fibers, and may contribute to the lower pain threshold at the tender points. Analysis of altered collagen metabolism either by histologic examination on biopsy, or preferably, by HPLC analysis of collagen metabolites in urine or serum may aid in understanding more about the pathogenesis of fibromyalgia.

The determination of pyridinium crosslinks in urine and serum as a possible marker of cartilage degradation in rheumatoid arthritis

The determination of the collagen crosslinks pyridinoline (Pyd) and deoxypyridinoline (Dpyd) seems to be a successful way of characterizing topic destructive processes in rheumatoid arthritis (RA). Dpyd is a specific marker of collagen I resorption in bone, whereas Pyd is released from types I and II collagen in bone and cartilage. Both crosslinks were examined in 38 RA patients concurrently by RP-gradient-HPLC in urine and serum. A positive correlation was found between the inflammatory activity (measured by CrP) and the level of collagen crosslinks in urine. A correlation between serum and urine concentrations was demonstrable for Pyd, but not for Dpyd. Different elimination kinetics for fragments containing either Pyd or Dpyd are a possible explanation for this observation. The ratio of Pyd/Dpyd is known to be a useful marker to distinguish between destruction of cartilage and bone collagen. Because the Pyd/Dpyd ratio in urine does not necessarily correspond to that in serum, probably as a result of metabolic or elimination processes, the usefulness of the relationship between the crosslinks in urine as a method of differentiating between cartilage and bone degradation must be questioned.

A longitudinal study of markers of bone turnover in Graves' disease and their value in predicting bone mineral density

Whether biochemical markers can predict improvement in reduced bone mineral density (BMD) associated with thyrotoxicosis in unclear. We investigated the relationship between serum osteocalcin (OC), bone-specific alkaline phosphatase (b-ALP), serum deoxypyridinoline (Sdpd) and pyridinoline (Spyr), 24-hour urinary deoxypyridinoline (Udpd), and BMD in 17 thyrotoxic patients during 1 yr of treatment. Coinciding with euthyroidism at 4-8 weeks, there was a peak in b-ALP and OC and a prompt fall into the normal range in Udpd and Sdpd, but not Spyr, levels. Mean b-ALP continued to be raised at week 52 when it was inversely correlated with BMD. Mean BMD rose approximately 6%, P < 0.01, over 1 yr. Coupling indices were calculated as a measure of bone balance and, at diagnosis, was [minus4.26 in favor of bone resorption and rose with treatment in favor of bone formation: weeks 2: -0.23; 4: +4.01; 8: +4.37; 12: +4.44; 24: +2.32; and 52: +1.56. Bone turnover is balanced within 2 weeks of starting treatment for thyrotoxicosis. Udpd accurately indicates thyrotoxic bone resorption. Serum b-ALP indicates continuing bone formation and, at 1 yr, may provide a marker for low BMD. OC, Sdpd, and Spyr are less sensitive in documenting bone remodeling during treatment of thyrotoxicosis.

Determination of pyridinium crosslinks in serum an optimization of sample preparation

Urinary pyridinoline (UPD) and deoxypyridinoline (UDPD) are selective markers in kinetic studies of mature collagen degradation in connective tissue, especially in bone. In patients with renal dysfunction, the determination of UPD and UDPD is not entirely reliable, while in anuretic patients it is impossible. As renal dysfunction is considered a risk factor for bone diseases, it is essential to determine both markers directly in the serum (SPD and SDPD). Due to the high serum concentrations of proteins, which during acid hydrolysis are converted to amino acid hydrochlorides, the system butanol-water is sometimes separated into two phases during sample preparation. Should this fact not be taken into account, the usual sample processing on a cellulose sorbent could yield substantially lower false results. This calls for some preventive measures: to ensure the homogeneity of the system containing n-butanol it is recommended to add an appropriate third component, e.g. methanol.

Pyridinium crosslinks in the monitoring of patients with bone metastases from carcinoma of the breast

Assessing the response of bone metastases to systemic therapy remains a difficult clinical problem. The currently available markers of bone disease are limited by the length of time before the changes that accompany regression or progression become evident. The pyridinium crosslinks, pyridinoline (Pyr) and deoxypyridinoline (dPyr), are a recently described group of compounds formed by collagen breakdown. Elevated urinary crosslinks were demonstrated in patients with bone metastases when compared with controls (P<0.0001). Improvements in the sensitivity of the high performance liquid chromatography technique have enabled us to measure these compounds in serum for the first time; Pyr and dPyr were also significantly elevated when compared with controls (P<0.001). We found significant correlations between Pyr and dPyr in serum (p = 0.88; P<0.0001) as well as in urine (p = 0.94; P<0.0001). In addition, a significant relationship existed between serum Pyr and the percentage of bone involved (p = 0.78; P<0.0001). Here we describe or preliminary results using this new assay and consider the role of these markers in the clinical assessment of metastatic bone disease from breast cancer.

The renal clearance of free and conjugated pyridinium cross-links of collagen

We developed a sensitive reversed-phase high performance liquid chromatography (HPLC) assay to measure free and total Pyridinoline (Pyr) and Deoxypyridinoline (Dpyr) in serum. The method was used to measure total serum levels in seven premenopausal women (ages 52.2 +/- 2.4 years) and to investigate the renal clearance of free and conjugated Pyr and Dpyr in two groups of children (group 1: 13 girls, ages 11-13 years; group 2: 18 older children [2 male, 16 female], ages 13-18 years). Blood and 24-h urine samples were collected from the younger group to measure the renal clearance, and blood and 2-h morning urine samples from the older group were collected to investigate the fractional clearance. Total Pyr and Dpyr in the premenopausal women was 4.08 +/- 0.91 and 1.18 +/- 0.39 nmol/l, respectively. Free and total Pyr and Dpyr in serum and urine was elevated in both groups of children. The free serum levels were 16 and 18% in young and older children, respectively, compared with 40 and 46% in the urine. The percentage of free Dpyr in serum decreased with total urinary Pyr excretion (r = -0.56, p < 0.005, n = 31). The renal clearance of the free cross-link fraction in both groups was 4-fold higher than the conjugated fraction. The fractional clearance of the free fraction was greater than 1 (p < 0.001) and the conjugated fraction less than 1 (p < 0.001). The fractional excretion of free Dpyr increased with total urinary Pyr excretion (r = 0.66, p < 0.005, n = 13). We conclude that HPLC can be used to measure free and total Pyr and Dpyr in serum and that some free Pyr and Dpyr excreted in urine is produced by the kidney.

Biochemical Studies in the Evaluation and Management of Osteoporosis: Current Status and Future Prospects

OBJECTIVE

To examine the current status of biochemical markers of bone remodeling and to offer a perspective on their future clinical applications.

METHODS

We provide a comprehensive overview of the bone remodeling cycle, the hormonal control of bone remodeling, the specific biochemical markers of bone resorption and bone formation, and their current clinical applications.

RESULTS

Bone remodeling occurs in discrete packets, known as bone modeling units, on the surfaces of the skeleton. Osteoclasts are responsible for bone resorption, and osteoblasts are responsible for bone formation. Bone resorption and formation are normally coupled, and an imbalance in these processes can lead to metabolic bone diseases such as osteoporosis. Investigators have attempted to measure various biochemical markers of bone resorption (such as urine calcium and hydroxyproline) and formation (such as bone-specific alkaline phosphatase and osteocalcin) as a reflection of response to therapy. Although several markers of bone resorption and formation can be assayed, an ideal marker has yet to be discovered or proved.

CONCLUSION

Until a precise, inexpensive assay is developed, currently available biochemical markers, used in conjunction with measurements of bone density, are considered the most reasonable tools for classifying and directing the management of osteoporosis.

High-performance liquid chromatographic determination of pyridinium crosslinks in serum, urine and dialysate of patients in chronic renal failure

This study describes an isocratic reversed phase high-performance liquid chromatographic method for the analysis of pyridinium crosslinks in serum, urine and dialysates obtained from patients with chronic renal failure on haemodialysis. The mean (SD) urine pyridinoline (PYD) and deoxypyridinoline (DPD) to creatinine (Cr) ratio in 19 healthy volunteers was 28.9 (6.3) and 9.1 (3.6) mumol/mol, respectively. In the 22 patients the PYD/Cr and DPD/Cr ratio was 244.6 (436.5) and 66.5 (116.8) mumol/mol, respectively. The mean serum PYD concentration in 29 patients of 268.5 (334.4) nmol/L was significantly higher than that of 5.9 (1.5) nmol/L found in normal volunteers: the mean DPD concentration was 82.9 (93.7) nmol/L in the patients but was undetectable in the serum from the normal volunteers. The concentration of crosslinks in pre-dialysis serum samples was higher than those found post-dialysis reflecting a significant removal of the crosslinks during dialysis. The assay of pyridinium crosslinks in serum, urine and dialyses fluid could potentially provide evidence of bone collagen turnover in patients in renal failure. Their measurement in serum and dialysate could be particularly useful in anuric patients.

Quantitative analysis of pyridinium crosslinks of collagen in the synovial fluid of patients with rheumatoid arthritis using high-performance liquid chromatography

The pyridinium crosslinks are important and definite biomarkers of mature hard tissue collagen degradation. A gradient ion-paired reversed-phase high-performance liquid chromatographic method was used for the simultaneous determination of both crosslinks in synovial fluid (SF) samples of 25 patients with rheumatoid arthritis (RA). The mean +/- SD levels of pyridinoline (Pyd) and deoxypyridinoline (Dpyd) in SF were 107.7 +/- 182.3 nmol/l and 4.8 +/- 8.3 nmol/l, respectively. The Pyd/Dpyd ratio, which indicates the amount of Pyd released from cartilage rather than bone, amounted to 30.8 +/- 29.5. This value is significantly higher than in urine or serum of the same patients. These data suggest increased destruction of joint cartilage in patients with RA and the release of collagen II fragments in SF. In addition, the levels of the crosslinks in SF reflect considerable interindividual variation, indicating substantial individual differences in the amount of collagenous material that is degraded.

Detection of pyridinium cross-links in human bile

This study evaluated whether pyridinium cross-links, which are positively charged, besides renal clearance are also cleared by the liver into bile. In 13 human bile samples tested, we were able to detect both pyridinoline (PYD) and deoxypyridinoline (DPD) in small amounts which were estimated to be about 1-2% of the amount usually found in urine. To further evaluate the amount of pyridinium cross-links excreted through bile, we studied the stability of these compounds at the alkaline pH of bile. No effect on their stability was detected over a 6-hour incubation. The origin of these molecules in bile and the significance of this finding in the use of PYD and DPD as bone resorption markers are discussed.

Determination of pyridinium crosslinks in plasma and serum by high-performance liquid chromatography

A chromatographic method for the determination of pyridinoline (Pyr) and deoxypyridinoline (Dpyr) in serum and plasma is described. The analytical procedure involved plasma or serum purification by ultrafiltration (20,000 relative molecular mass cut-off) under centrifugation at 2500 g for 4 h, as an innovative step. Analysis was done by isocratic high-performance liquid chromatography with fluorescence detection. The linearity of the method was tested from 0.6 to 15 pmol/ml and 0.12 to 3 pmol/ml for Pyr and Dpyr, respectively. The detection limit was 60 fmol/ml for both crosslinks. Except for Dpyr in plasma (coefficient of variation 19.9%), intra-assay variation was always below 10% in serum and plasma. The method has been applied to the quantification of crosslinks in serum and plasma of healthy volunteers and also in mouse and rat plasma. Serum proved to be the most suitable biological fluid for the systemic measurement of these compounds in humans and under the experimental conditions used, contained an average of 3.62 +/- 0.65 and 0.7 +/- 0.18 pmol/ml Pyr and Dpyr, respectively.

Biochemical markers of bone metabolism

In this article we review the biochemical basis for markers of bone metabolism and comment on their bone specificity and representativeness for bone tissue. Major developments have recently taken place particularly with respect to markers of bone collagen metabolism; accordingly, they are in the focus of this review. We also attempt to relate the various collagenous and non-collagenous markers to each other and to the phases of the osteoblast phenotype.