Kinetic Studies of Newly Patented Aminoalkanol Derivatives with Potential Anticancer Activity as Competitive Inhibitors of Prostate Acid Phosphatase

BACKGROUND

Acid phosphatase and its regulation are important objects of biological and clinical research and play an important role in the development and treatment of prostate and bone diseases. The newly patented aminoalkanol (4-[2-hydroxy-3-(propan-2-ylamino)propyl]-1,7-dimethyl-8,9-diphenyl-4-azatricyclo[5.2.1.02,6]dec-8-ene-3,5,10-trione hydrochloride) (I) and (4-[3-(dimethylamino)-2-hydroxypropyl]-1,7-dimethyl-8,9-diphenyl-4-azatricyclo[5.2.1.02,6]dec-8-ene-3,5,10-trione hydrochloride) (II) derivatives have potential anticancer activity, and their influence on enzymatic activity can significantly impact the therapeutic effects of acid phosphatase against many diseases. Therefore, in this study, we investigated the action of compounds (I) and (II) on acid phosphatase.

METHODS

Capillary electrophoresis was used to evaluate the inhibition of acid phosphatase. Lineweaver-Burk plots were constructed to compare the Km of this enzyme in the presence of inhibitors (I) or (II) with the Km in solutions without these inhibitors.

RESULTS

Compound (I) showed a stronger competitive inhibition against acid phosphatase, whereas derivative (II) showed a weaker competitive type of inhibition. The detailed kinetic studies of these compounds showed that their type and strength of inhibition as well as affinity depend on the kind of substituent occurring in the main chemical molecule.

CONCLUSIONS

This study is of great importance because the disclosed inhibition of acid phosphatase by compounds (I) and (II) raises the question of whether these compounds could have any effect on the treatment possibilities of prostate diseases.

A Novel Sandwich ELISA for Tartrate-Resistant Acid Phosphatase 5a and 5b Protein Reveals that Both Isoforms are Secreted by Differentiating Osteoclasts and Correlate to the Type I Collagen Degradation Marker CTX-I In Vivo and In Vitro

Tartrate-resistant acid phosphatase type 5 (TRAP) exists as two isoforms, 5a and 5b. 5b is a marker of osteoclast number and 5a of chronic inflammation; however, its association with bone resorption is unknown. In this study, a double-TRAP 5a/5b sandwich ELISA measuring 5a and 5b protein in the same sample was developed. TRAP 5a and 5b protein levels were evaluated as osteoclast differentiation/activity markers in serum and in culture, and their correlation to the resorption marker CTX-I was examined. Serum TRAP 5a and 5b concentrations in healthy men were 4.4 ± 0.6 ng/ml and 1.3 ± 0.2 ng/ml, respectively, and they correlated moderately to each other suggesting that their secretion is coupled under healthy conditions. A correlation was also observed between serum TRAP 5a and 5b with CTX-I, suggesting that both TRAP isoforms associate with osteoclast number. During osteoclast differentiation on plastic/bone, predominantly 5b increased in media/lysate from M-CSF/RANKL-stimulated CD14+ PBMCs. However, substantial levels of 5a were detected at later stages suggesting that both isoforms are secreted from differentiating OCs. More TRAP 5b was released on bone indicating a connection to osteoclast resorptive activity, and a peak in TRAP 5b/5a-ratio coincided with rapid CTX-I release. At the end of the culture period of M-CSF + RANKL-stimulated CD14+ PBMCs, there was a correlation between the secretion of TRAP 5a and 5b proteins with CTX-I. The correlation of not only 5b but also 5a with collagen degradation, both in serum and osteoclast cultures indicates that a considerable proportion of the TRAP 5a originates from osteoclasts and may reflect a hitherto undisclosed regulatory mechanism during bone resorption and bone remodeling.

Biochemical Markers Reflecting Thyroid Function in Athyreotic Patients on Levothyroxine Monotherapy

BACKGROUND

Some investigators reported that among athyreotic patients on levothyroxine (LT4) monotherapy following total thyroidectomy, the patients with normal serum thyrotropin (TSH) levels had mildly low serum free triiodothyronine (fT3) levels, whereas the patients with mildly suppressed serum TSH levels had normal serum fT3 levels, and the patients with strongly suppressed serum TSH had elevated serum fT3 levels. The objective of the present study was to clarify which of these three patient groups is closer to their preoperative euthyroid condition.

METHODS

A total of 133 consecutive euthyroid patients with papillary thyroid carcinoma who underwent a total thyroidectomy were prospectively studied. The patients' serum levels of lipoproteins, sex hormone-binding globulin, and bone metabolic markers measured preoperatively were compared with the levels measured at postoperative LT4 therapy 12 months after the thyroidectomy.

RESULTS

The postoperative serum sex hormone-binding globulin (p < 0.001) and bone alkaline phosphatase (p < 0.01) levels were significantly increased in the patients with strongly suppressed TSH levels (≤0.03 μIU/mL). The postoperative serum low-density lipoprotein cholesterol levels were significantly increased (p < 0.05), and the serum tartrate-resistant acid phosphatase-5b levels were significantly decreased (p < 0.05) in the patients with normal TSH (0.3 < TSH ≤5 μIU/mL). In the patients with mildly suppressed TSH (0.03 < TSH ≤0.3 μIU/mL) and fT3 levels equivalent to their preoperative levels, all metabolic markers remained equivalent to their preoperative levels.

CONCLUSIONS

The serum biochemical markers of thyroid function in patients on LT4 following total thyroidectomy suggest that the patients with mildly suppressed TSH levels were closest to euthyroid, whereas those with normal TSH levels were mildly hypothyroid and those with strongly suppressed TSH levels were mildly hyperthyroid. These data may provide novel information on the management of patients following total thyroidectomy for thyroid cancer or benign thyroid disease.

Effects of 1α,25-(OH)2D3 on the formation and activity of osteoclasts in RAW264.7 cells

The hormonally active form of vitamin D3, 1α,25-(OH)2D3, has an important role in bone metabolism. This study examined the effects of 1α,25-(OH)2D3 on the ability of two cytokines, receptor activator of nuclear factor-κB ligand (RANKL) and macrophage-colony stimulating factor (M-CSF), to induce RAW 264.7 cells to form osteoclasts. A TRAP histochemical staining assay and bone resorption analysis were used to identify the rate of formation and activity of osteoclasts. The numbers of osteoclasts formed, and their bone resorption activity, was enhanced by the addition of 1α,25-(OH)2D3. The expression levels of osteoclast-specific proteins that are essential for bone resorption, integrin β3, V-ATPase, CAII, CTSK, TRAP and MMP-9, were detected by western blotting. During 48 h, the expression levels of all these proteins significantly increased. Quantitative real-time polymerase chain reaction was used to determine the expression levels of the transcription factors, c-Fos and NFATcl. The expression levels of c-Fos and NFATc1 also increased 24h after treatment with 1α,25-(OH)2D3. These results suggest that 1α,25-(OH)2D3 can regulate bone metabolism by directly enhancing the formation and maturation of osteoclasts.

Biology and clinical significance of tartrate-resistant acid phosphatases: new perspectives on an old enzyme

Type 5 tartrate-resistant acid phosphatase (TRAP) has been a clinically relevant biomarker for about 50 years. It has always been a reliable and specific cytochemical marker for hairy cell leukemia and for differentiated cells of monocytic lineage. Only recently has the test for serum TRAP activity been accepted as sensitive and specific enough for clinical use as a marker of osteoclasts and bone resorption. This has come about through steady advances in knowledge about TRAP enzymology, structure, function, and molecular regulation and a consequent appreciation that TRAP isoforms 5a and 5b have very different clinical significance. As a measure of osteoclast number and bone resorption, TRAP 5b has diagnostic and prognostic applications in osteoporosis, cancers with bone metastasis, chronic renal failure, and perhaps other metabolic and pathologic bone diseases. Serum TRAP 5a, on the other hand, has no relationship to bone metabolism but seems instead to be a measure of activated macrophages and chronic inflammation. Exploration of the real clinical usefulness of serum TRAP 5a for diagnosis and disease management in a wide variety of chronic inflammatory diseases is only now beginning. This perspective traces the important basic scientific developments that have led up to the refinement of serum TRAP isoform immunoassays and their validation as biomarkers of disease. Many unanswered questions remain, providing a wealth of opportunity for continued research of this multifaceted enzyme.

Clinical performance of immunoreactive tartrate-resistant acid phosphatase isoform 5b as a marker of bone resorption

Previous immunoassays developed for the measurement of serum tartrate-resistant acid phosphatase (TRACP) have lacked specificity for osteoclastic TRACP, TRACP 5b, or have not shown satisfactory clinical performance. The aim of this study was to evaluate the clinical performance of a novel immunocapture activity assay for TRACP 5b, in comparison to telopeptide fragments of type I collagen. Within-subject variability and the effect of feeding on TRACP 5b and telopeptides of type I collagen were assessed in 20 healthy premenopausal women. Diurnal variation of TRACP 5b and serum beta C-terminal cross-linked telopeptide of type I collagen (sbetaCTX) was assessed in 12 healthy postmenopausal women. Renal clearance was assessed in 19 end stage renal failure patients undergoing routine haemodialysis. Response to antiresorptive treatment and calcium supplementation was assessed in osteoporotic postmenopausal women treated with alendronate and calcium (n = 16) or with calcium alone (n = 7) for 24 weeks.Within-subject variability (CVi) of TRACP 5b was 6.6%, lower than CVi of urinary and serum telopeptides. TRACP 5b decreased by 2.4 +/- 0.8%, in response to feeding (P < 0.05) compared to 7.0 +/- 2.6% to 7.9 +/- 3.7% for urinary telopeptides (P < 0.05 to < 0.01) and 8.5 +/- 1.7% to 17.8 +/- 2.6% for serum telopeptides (P < 0.0001). The amplitude of the diurnal rhythm for TRACP 5b was small compared to that of sbetaCTX, 14 +/- 4% vs. 137 +/- 14%. Haemodialysis did not have a significant effect on TRACP 5b but reduced sbetaCTX by 46 +/- 4% (P < 0.0001). In response to alendronate, TRACP 5b decreased by 39 +/- 4% compared to 49 +/- 4% to 69 +/- 5% for urinary telopeptides and 75 +/- 8% for sbetaCTX. We conclude that TRACP 5b shows an attenuated response to antiresorptive therapy in comparison with other markers of bone resorption, but that this may be offset by lower biological variability. TRACP 5b may provide useful additional information about bone resorption.

Acid phosphatases as markers of bone metabolism

Various biochemical markers have been used to assess bone metabolism and to monitor the effects of treatments. Tartrate resistant acid phosphatase (TRAP; EC 3.1.3.2) has often been used to assess bone absorption. Although osteoclasts contain abundant TRAP and they are responsible for bone resorption, the total TRAP activities in the serum measured by colorimetric methods little reflect the bone turnover. TRAP 5 is further separated into 5a and 5b by electrophoresis. Type 5b is considered to be derived from the osteoclast, and therefore attempts are being made to measure exclusively serum TRAP 5b by kinetic methods, immunological methods, and chromatographic methods including ion-exchange and heparin column chromatography.

Bone metabolism in childhood rheumatic disease

There is an increasing appreciation among physicians about the importance of bone health in children as a potential modifiable risk factor for development of osteoporosis later in life. Many unanswered questions about normal pediatric bone health make the treatment and clinical investigation of bone disease in children with chronic rheumatic diseases challenging. Understanding normal bone biology along with the epidemiology and diagnosis of osteoporosis is the key to the understanding of the potential for abnormal bone metabolism in the juvenile rheumatic diseases.

Serum tartrate-resistant acid phosphatase isoforms in rheumatoid arthritis

OBJECTIVES

Our objective was to evaluate the significance and source of serum tartrate-resistant acid phosphatase (TRACP) in patients with rheumatoid arthritis (RA).

METHODS

Thirty-five RA, 32 osteoarthritis (OA) and 16 control subjects were studied. Serum TRACP-5b activity and total TRACP protein were determined by immunoassay. TRACP isoforms were analyzed by non-denaturing polyacrylamide gel electrophoresis (PAGE). Serum bone alkaline phosphatase (BAP), cross-linked N-terminal telopeptides (NTx), and C-terminal telopeptides (ICTP) of type I collagen were estimated as markers of bone turnover. C-reactive protein (CRP) was measured as a marker of chronic inflammation. Macrophages and dendritic cells (DC) were developed from peripheral blood monocytes. Cell lysates and culture supernatants were analyzed for TRACP isoforms by immunoassay and PAGE.

RESULTS

In RA, mean TRACP-5b activity was normal, but median total TRACP protein was increased twofold (p<0.001). In OA, TRACP-5b activity and protein were normal. In RA, TRACP-5b activity correlated weakly with ICTP (r=0.56) while TRACP protein levels correlated weakly with NTx (r=0.43). Additionally, TRACP protein, but not TRACP-5b activity correlated significantly with CRP (r=0.42). Macrophage and DC lysates contained TRACP-5b, while tissue culture supernatants contained TRACP-5a.

CONCLUSIONS

Increased total TRACP protein in RA sera was probably due to TRACP-5a and not derived from osteoclasts. Rather, it could be a secreted product of inflammatory macrophages and DC.

Biochemical markers of bone turnover

BACKGROUND

Osteoporosis in many countries has reached epidemic proportions. This has stimulated the development of biochemical markers to assist in the assessment of osteoporotic risk and in monitoring the efficacy of treatment. Biochemical markers of bone turnover are products released from osteoblasts and osteoclasts or collagen breakdown products.

MARKERS

Markers of bone formation include bone-specific alkaline phosphatase (BAP), osteocalcin (OC) and procollagen peptides. All of these can be measured easily by immunoassay techniques. Of these markers, OC has been extensively studied. However, OC undergoes in vitro degradation, thus, assay results are variable. BAP, on the other hand, is much more stable and shows less within-person biological variation. Bone resorption markers include tartrate-resistant acid phosphatase (TRAP) and collagen breakdown products, such as pyridinium cross-links, galactosyl hydroxylysine and cross-linked telopeptides, such as CTx and NTx. Of these, deoxypyridinium (DPD) has been extensively studied. DPD shows diurnal variation and the within-individual biological variation is large. Of the newer assays, NTx appear to show large differences at menopause.

CONCLUSIONS

Thus, serum BAP and DPD or NTx are the current choice of bone markers.

Evaluation of bone metabolic markers in breast cancer with bone metastasis

PURPOSE

In the present study, four bone metabolic markers were examined to clarify them meaning and clinical value in the detection of bone metastasis (BM) from breast cancer.

METHODS

we examined serum carboxyterminal telopeptide of type I collagen (ICTP), tartrate resistant acid phosphatase (TRACP), total alkaline phosphatase (ALP) and urinary type I collagen cross-linked N-telopeptides (NTx) as potential markers. These bone markers were evaluated simultaneously in 156 breast cancer patients; 114 patients without metastasis (group A), 23 patients with BM (group B) and 19 patients with metastasis at sites other than bone (group C).

RESULTS

The mean values of ICTP and TRACP in group B were significantly greater than those in group A. Group B consisted of the patients with varying degrees of BM and variation in their treatments. The patients in group B were divided into BM (+) and BM (++) according to hot spots in bone scan. ICTP and TRACP were elevated in BM (++) patients compared to BM (+) patients (p<0.05). The values of ICTP and TRACP of the twelve patients without treatment in group B were significantly higher than those in group A. In the treated patients of group B, the mean values of ICTP and TRACP were lower in responders and cases of stable disease than those with progression. NTx and ALP were inferior to ICTP and TRACP for clinical evaluation of BM.

CONCLUSIONS

We confirmed that ICTP and TRACP might be useful markers for screening and monitoring BM in breast cancer.

Heparin column analysis of serum type 5 tartrate-resistant acid phosphatase isoforms

The objective of the present study was to develop a specific method for the separation of tartrate-resistant acid phosphatase (TRAP) derived exclusively from osteoclasts. Heparin column-bound TRAP in human serum was separated into three peaks of TRAP activity when eluted with a linear gradient of sodium chloride. The last peak corresponded to TRAP 5b which was first named according to its electrophoretic mobility [Clin. Chem. 24 (1978) 309] and was considered to be derived from osteoclasts [J. Bone Miner. Res. 13 (1998) 683]. The second peak was found to be TRAP 5a. The height of the last peak varied from age to age.

Naphthol-ASBI phosphate as a preferred substrate for tartrate-resistant acid phosphatase isoform 5b

Tartrate-resistant acid phosphatase (TRAP) isoform 5b is a potential serum marker for osteoclastic activity. Biochemical assays for serum TRAP activity with para-nitrophenylphosphate (pNPP) have low specificity for bone because of hydrolysis by unrelated nontype 5 TRAPs of blood cells and by related isoform 5a. Our purpose was to increase the specificity of TRAP assay for osteoclastic activity by using naphthol-ASBI phosphate (N-ASBI-P) as a substrate for serum type 5 TRAP activity and heparin as an inhibitor of isoform 5a. TRAP activity in individual and pooled sera of normal subjects and patients with endstage renal disease (ESRD) and rheumatologic diseases was quantitated using pNPP and N-ASBI-P as substrate at pH 5.5 and 6.1. For some experiments, heparin (23U/ml) was added as a specific inhibitor of isoform 5a activity. Isoforms 5a and 5b were separated from serum pools by cation exchange chromatography and identified by nondenaturing polyacrylamide gel electrophoresis (PAGE). N-ASBI-P was selectively hydrolyzed by TRAP isoform 5b. TRAP assays with pNPP and N-ASBI-P correlated only in ESRD sera, which contained primarily isoform 5b. The two assays did not correlate in normal or rheumatic sera with significant amounts of 5a. Heparin inhibited isoform 5a activity approximately 50% but had little effect on isoform 5b activity. Biochemical assay of serum TRAP activity can be made specific for isoform 5b by using N-ASBI-P and heparin. This method can be adapted to simple microplate biochemical or immunochemical assays. This simplified method for assessment of osteoclastic TRAP 5b activity warrants a detailed investigation in diseases of bone metabolism.

Biochemical markers to survey bone turnover

Molecular markers of bone turnover have gained increasing relevance in the evaluation of patients with metabolic bone diseases. Their clinical applications include the assessment of future osteoporotic fracture risk, complementation of bone density measurements, diagnosis of certain metabolic osteopathies, therapeutic decision making, and monitoring of therapeutic efficacy and patient compliance. One should be aware, however, that the results from large epidemiologic or clinical trials are sometimes difficult to translate into the everyday clinical situation. The individual patient often has more than one disease that might affect either bone turnover or the handling of the parameters mentioned (or both). Analytic and biologic variability of bone markers can be significant and also needs to be considered when using these indices. In the scientific setting, conventional and new markers of bone turnover can help to elucidate formerly unknown mechanisms and pathways. Because the development of ever more specific and sensitive markers of bone metabolism is progressing rapidly, we are likely to witness new insights into the pathophysiology of bone diseases in the near future.

Tartrate-resistant Acid Phosphatase Isoform 5b as Serum Marker for Osteoclastic Activity

Background: Tartrate-resistant acid phosphatase (AcP) 5b is a marker of osteoclastic activity and bone resorption. Immunoassays for serum TRAcP may lack sensitivity and specificity because of the presence of non-bone isoform 5a. The purpose of this study was to isolate the serum isoforms, quantify their disease-related expressions, and test an improved immunoassay for TRAcP 5b.

Methods: We separated TRAcP isoforms chromatographically from pooled sera of healthy, rheumatoid arthritis (RA) and endstage renal disease (ESRD) subjects. TRAcP isoforms were identified by electrophoresis and quantified by biochemical and immunochemical assays. Serum TRAcP activity in healthy, RA, and ESRD cohorts was assessed at pH 5.5 and 6.1, and compared with bone alkaline phosphatase (BAP) and N-telopeptides of type I collagen (NTx).

Results: TRAcP isoforms 5a and 5b were present in all sera; 5b was identical to osteoclastic TRAcP. In serum from healthy subjects, 5a accounted for 87% of the enzyme protein but only 55% of the activity. In RA, both isoforms were increased two- to threefold in protein, but their specific activities were subnormal. In ESRD, only 5b was abnormal, being increased fivefold in protein and threefold in activity. In RA sera, TRAcP activity did not correlate with either BAP or NTx. In ESRD sera, TRAcP activity correlated with BAP and NTx only when measured at pH 6.1.

Conclusions: All sera contained both TRAcP isoforms 5a and 5b, but only 5b was present in bone. TRAcP isoform expression was variable in different diseases. Measurement of TRAcP activity at pH 6.1 improves the specificity of immunoassay for isoform 5b.

Electrophoretic study of tartrate-resistant acid phosphatase isoforms in endstage renal disease and rheumatoid arthritis

The objective of this study was to identify the isoform, type-5a or type-5b, responsible for increased tartrate-resistant acid phosphatase (TRAP) activity in endstage renal disease (ESRD) and TRAP protein in rheumatoid arthritis (RA). We studied 24 sera each from healthy, ESRD and RA subjects. Type-5 TRAP activity and protein were quantitated by immunoassays. Isoform expression was determined by computerized imaging of non-denaturing polyacrylamide gels (PAGE) stained for TRAP activity. Other biochemical markers included: intact parathyroid hormone (iPTH), total and bone-specific alkaline phosphatase (TAP, BAP), N-telopeptides of type-I collagen (NTx), and free pyridinoline (Pyd). Isoform 5a was normal in both ESRD and RA. Isoform 5b was elevated in ESRD only. Serum TRAP activity correlated with both isoforms 5a and 5b in RA, but only with 5b in ESRD. TRAP protein assays did not correlate with PAGE assays for 5a or 5b. TRAP activity, but not protein, correlated with BAP and NTx in RA sera. Both TRAP activity and protein correlated with iPTH, TAP and Pyd in ESRD sera. Increased TRAP activity in ESRD was due to increased osteoclastic isoform 5b and related to bone turnover. Increased TRAP protein in RA was suspected, but not proven, to be isoform 5a and not related to bone turnover. Heterogeneity of serum TRAP and preferential expression of isoforms has clinical significance in different diseases including ESRD and RA.

Biological markers in the diagnosis of the different forms of renal osteodystrophy

Renal osteodystrophy continues to be a long-term complication associated with high rates of morbidity in patients with chronic renal failure. Although bone histomorphometry is the most reliable diagnostic method, several new biochemical markers of bone turnover have been proposed in recent years for the evaluation of bone remodelling in uremic patients. This review assesses the value and the limitations of serum markers of bone formation and resorption in the diagnosis of the major types of renal osteodystrophy. In addition, we consider the hypothetical role of serum beta2-microglobulin and of some local mediators involved in the process of bone cell activation and inhibition, such as circulating cytokines and their inhibitors and receptors.

Tartrate-resistant acid phosphatase 5b: a novel serum marker of bone resorption

Human serum contains two forms of tartrate-resistant acid phosphatase (TRAP), 5a and 5b. Of these, 5a contains sialic acid and 5b does not. We show here that antigenic properties and pH optimum of TRAP purified from human osteoclasts are identical to those of serum TRAP 5b and completely different from those of serum TRAP 5a, suggesting that 5b would be derived from osteoclasts and 5a from some other source. We developed a novel immunoassay specific for 5b using a monoclonal antibody O1A as capture antibody. O1A did not bind acid phosphatase derived from platelets and erythrocytes. Western analysis showed that O1A was specific for TRAP in both human bone and serum. We measured bound TRAP activity at pH 6.1, where 5b is highly active and 5a almost completely inactive. The immunoassay detected more than 90% of the initial TRAP 5b activity after 8-h incubation of serum samples at 25 degrees C and after 3 days incubation at 4 degrees C. Serum TRAP 5b activity decreased significantly after 6 months of hormone replacement therapy (HRT) of postmenopausal women compared with the change observed in postmenopausal women receiving placebo (p < 0.0001). Instead, no significant differences were observed between the changes in the placebo and HRT groups in total serum TRAP amount. These results show that serum TRAP 5b is a specific and sensitive marker for monitoring antiresorptive treatment. Instead, total serum TRAP cannot be used for that purpose. These findings may turn out to be a significant improvement in using serum TRAP as a resorption marker.

Development of a Kinetic Assay for Band 5b Tartrate-resistant Acid Phosphatase Activity in Serum

Background: Band 5 tartrate-resistant acid phosphatase (TrACP; EC 3.1.3.2) consists of two isoenzymes, bands 5a and 5b, of which band 5b TrACP is considered to be derived from bone. However, no kinetic method for the specific measurement of band 5b TrACP in serum is available. Our aim was to develop a kinetic assay method for the specific measurement of band 5b TrACP in serum.

Methods: Band 5b TrACP was measured kinetically in serum as tartrate-resistant fluoride-sensitive heparin-resistant ACP with 2,6-dichloro-4-acetylphenyl phosphate as substrate at pH 6.6.

Results: Heparin inhibited band 5a TrACP but had no effect on band 5b TrACP in serum or in bone extract. The presence of EDTA or ascorbic acid had no effect, but dithiothreitol inhibited enzyme activity. The within-run (n = 20) and between-run (n = 20) CVs of band 5b TrACP activity were 3.3–5.8% and 5.0–7.3%, respectively. The mean ± SD values of band 5b TrACP activity in males (n = 25) and females (n = 57) 20–29 years of age by this method were 8.0 ± 2.2 U/L and 6.4 ± 1.8 U/L, respectively. The band 5b TrACP value was significantly higher in females &gt;50 years of age compared with the younger subjects (20–29 years). The highest band 5b TrACP values were among children younger than 15 years.

Conclusions: This kinetic assay is a simple and specific method for the measurement of band 5b TrACP in serum samples and is useful in the evaluation of bone turnover activity.

Clinical Significance of Immunoassays for Type-5 Tartrate-resistant Acid Phosphatase

Background: Tartrate-resistant acid phosphatase (TRAP; EC 3.1.3.2) is a product of osteoclasts and a biochemical marker of bone resorption rate. However, erythrocytes and platelets contribute to total TRAP activity in serum, reducing the specificity of direct biochemical assays in serum. Osteoclast TRAP is also known as type-5 TRAP and is antigenically unique. Immunoassays are sought to improve the specificity and sensitivity of TRAP as a bone marker.

Methods: We developed two colorimetric microplate assays for type-5 TRAP: an enzyme capture immunoassay to measure antibody-bound enzymatic activity, and a two-site immunoassay to measure bound enzyme protein. Both use the same monoclonal antibody (14G6) to capture type-5 TRAP, which permits determination of specific activity of serum TRAP in health and disease.

Results: Both TRAP assays were linear from one-tenth to fivefold the mean value in 18 healthy subjects. In these subjects, the mean (SD) TRAP activity was 3.2 (0.54) U/L for the enzyme capture assay and 37 (13) μg/L for the two-site assay. Mean TRAP activity was not significantly increased in 64 patients with endstage renal disease requiring hemodialysis (HD) or 99 unselected patients with rheumatic diseases. By contrast, TRAP protein was increased in both the HD and rheumatic disease groups. The specific activity of TRAP in the 17 of 64 HD sera that had increased TRAP activity (0.088 U/μg) was similar to that in healthy subjects (0.091 U/μg). By contrast, the specific activity of TRAP in the 31 of 99 rheumatic sera with increased TRAP protein (0.035 U/μg) was significantly decreased.

Conclusions: Wide sample distributions for TRAP activity in HD patients and TRAP protein in rheumatic disease patients suggest the presence of subpopulations of HD patients with increased TRAP activity and of rheumatic patients with increased TRAP protein. Each assay for TRAP activity and protein may have its own biological significance and clinical applications in specific groups of patients.

Reactivity and assay restriction profiles of monoclonal and polyclonal antibodies to acid phosphatases: a preliminary study

The development of secure diagnostic immunoassays requires, among others, rigorous characterisation of potential antibody reagents. The reactivity profiles of seven antibodies (six monoclonal [MAb] and one polyclonal [PAb]) with putative specificity for tartrate-resistant acid phosphatase (TRAP) and/or osteoclasts were evaluated in enzyme-linked immunosorbent assay (ELISA) and/or immunocytochemistry. MAbs 2H1, 4E6 and 5Cl demonstrated assay restriction: exhibiting reactivity only in ELISA. The remaining three MAbs (G211D, G312G and V35B) and the PAb 8023 recognised recombinant TRAP (rTRAP) in ELISA and native acid phosphatases in selected tissues and cell lines. The latter were cytochemically assessed for both tartrate-sensitive acid phosphatase (TSAP) and TRAP. V35B showed reactivity against the monocytic leukaemia cell line U937 and guinea pig kidney tissue (both TSAP+ and TRAP+) and ECV304 (TSAP+) cells. Interestingly, the reactivity of MAb G211D co-localised with TRAP activity in the membrane of osteoclasts but also detected cytoplasmic components in U937 cells and human embryonic lung fibroblasts (TRAP+ and TRAP+). G211D exhibited immunoreactivity against placental trophoblasts (positive for total AP). Intriguingly, MAbs 2H1, 4E6, 5Cl and PAb 8023 cross-reacted with potato acid phosphatase in ELISA, suggesting reactivity to conformationally similar epitopes. Thus, some of these reagents could be used in the development of standardised diagnostic immunoassays or as drug-targeting agents for conditions in which the pathological process involves bone resorption, the MAbs G211D, 2H1, 4E6, 5Cl and PAb 8023 being useful in ELISA but not immunocytochemical detection of TRAP.

Three-dimensional structure of a mammalian purple acid phosphatase at 2.2 A resolution with a mu-(hydr)oxo bridged di-iron center

The crystal structure of purple acid phosphatase from rat bone has been determined by molecular replacement and the structure has been refined to 2.2 A resolution to an R -factor of 21.3 % (R -free 26.5 %). The core of the enzyme consists of two seven-stranded mixed beta-sheets, with each sheet flanked by solvent-exposed alpha-helices on one side. The two sheets pack towards each other forming a beta-sandwich. The di-iron center, located at the bottom of the active-site pocket at one edge of the beta-sandwich, contains a mu-hydroxo or mu-oxo bridge and both metal ions are observed in an almost perfect octahedral coordination geometry. The electron density map indicates that a mu-(hydr)oxo bridge is found in the metal center and that at least one solvent molecule is located in the first coordination sphere of one of the metal ions. The crystallographic study of rat purple acid phosphatase reveals that the mammalian enzymes are very similar in overall structure to the plant enzymes in spite of only 18 % overall sequence identity. In particular, coordination and geometry of the iron cluster is preserved in both enzymes and comparison of the active-sites suggests a common mechanism for the mammalian and plant enzymes. However, significant differences are found in the architecture of the substrate binding pocket.

Circulating biochemical markers of bone remodeling in uremic patients

Chronic renal failure is often associated with bone disorders, including secondary hyperparathyroidism, aluminum-related low-turnover bone disease, osteomalacia, adynamic osteopathy, osteoporosis, and skeletal beta2-microglobulin amyloid deposits. In spite of the enormous progress made during the last few years in the search of noninvasive methods to assess bone metabolism, the distinction between high- and low-turnover bone diseases in these patients still frequently requires invasive and/or costly procedures such as bone biopsy after double tetracycline labeling, scintigraphic-scan studies, computed tomography, and densitometry. This review is focused on the diagnostic value of several new serum markers of bone metabolism, including bone-specific alkaline phosphatase (bAP), procollagen type I carboxy-terminal extension peptide (PICP), procollagen type I cross-linked carboxy-terminal telopeptide (ICTP), pyridinoline (PYD), osteocalcin, and tartrate-resistant acid phosphatase (TRAP) in patients with chronic renal failure. Most of the observations made by several groups converge to the conclusion that serum bAP is the most sensitive and specific marker to evaluate the degree of bone remodeling in uremic patients. Nonetheless, PYD and osteocalcin, in spite of their retention and accumulation in the serum of renal insufficient patients, are also excellent markers of bone turnover. The future generalized use of these markers, individually or in combination with other methods, will undoubtedly improve the diagnosis and the treatment of the complex renal osteodystrophy.

Tartrate-resistant bone acid phosphatase: large-scale production and purification of the recombinant enzyme, characterization, and crystallization

Tartrate-resistant acid phosphatase (TRAP) is an enzyme expressed in bone-resorbing osteoclasts and certain tissue macrophages in human tissues. The functions of TRAP in biological systems are not known. Elucidation of the three-dimensional structure of the active site could yield important information about the physiological substrate(s) of the enzyme. We have produced recombinant rat bone TRAP using a baculovirus expression vector system. The production was scaled up to a 30-l bioreactor, and a method of purification in large scale was developed. The enzyme is composed of one 34 kDa polypeptide chain. Trypsin digestion resulted in a preparation where two subunits of approximately 23 kDa and approximately 16 kDa appeared after disulfide reduction. Trypsin digestion activated the enzyme. We generated monoclonal antibodies against recombinant TRAP. One of the selected antibodies detected the 23 kDa subunit in Western blotting. The reduced and oxidized forms of the enzyme could be separated by Mono-S cation-exchange chromatography. Crystals of TRAP have been obtained with ammonium sulfate/polyethylene glycol as precipitant. They belong to space group P212121 or P21212 with unit cell dimensions a = 57.2 A, b = 69.5 A, and c = 87.2 A and diffract to at least 2.2 A resolution. A packing density value of 2.55 A3/Da is consistent with one subunit in the asymmetric unit.

Two-site immunoassays for osteoclastic tartrate-resistant acid phosphatase based on characterization of six monoclonal antibodies

Tartrate-resistant acid phosphatase (TRAP), an enzyme expressed in bone-resorbing osteoclasts, is secreted into the circulation during bone resorption. We used six monoclonal antibodies (MAbs) to optimize direct two-site fluoroimmunoassays for determining serum TRAP concentrations. Four of the MABs, 1F1, 2H1, 4E6, and 5C1, were raised against recombinant human TRAP, and the other two, O1A and J1B, against human bone TRAP. 2H1, J1B, and O1A appeared to be highly specific for TRAP. 1F1 and 4E6 were poor in recognizing bone TRAP and were not useful in the assay. 5C1, while having a good affinity for the bone enzyme, was not specific. Serum TRAP is relatively stable, because 7 days of storage of serum samples at 4 degreesC and -20 degreesC or five thawing-freezing cycles, did not change the TRAP concentration detected using the two-site assays. All studied assays detected an increase in serum TRAP concentrations of postmenopausal women compared with premenopausal women, the difference being highest with MAB pairs 2H1-5C1 and O1A-J1B. These results suggest that serum TRAP may be a useful bone resorption marker, and the MAB pairs 2H1-5C1 and O1A-J1B may be useful in determining the bone resorption rate.

Tartrate-resistant acid phosphatase forms complexes with alpha2-macroglobulin in serum

Tartrate-resistant acid phosphatase (TRAP) is a standard histochemical marker of differentiated osteoclasts and has been proposed as a serum/plasma marker for osteoclast activity. Enzyme assays have been described that show elevated TRAP enzyme activity in the serum or plasma of patient groups known to have increased bone metabolism. However, the poor stability of the enzyme and potential contribution from nonosteoclastic sources make it problematic to measure in patient samples. Immunoassays developed to measure TRAP in serum and plasma have yielded widely varying TRAP levels in both normal and disease states. It is not clear if this variability is caused by differences in assay calibration, antibody specificity, and/or TRAP instability. In this paper, we report that purified TRAP spiked into serum forms high molecular weight complexes. Complex formation results in greatly decreased TRAP enzyme activity and immunoreactivity. The complexing protein in serum has been identified as alpha2-macroglobulin (alpha2M). Similar complexes are observed in stored patient samples. In vitro studies with purified components show that TRAP binds to alpha2M primarily through noncovalent ionic interactions. Our results demonstrate that one mechanism of TRAP instability in serum is complex formation with alpha2M and suggest further that current TRAP enzyme and immunoassays may not accurately measure the circulating level of TRAP.

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.

Species specificity of monoclonal antibodies to human tartrate-resistant acid phosphatase

Tartrate-resistant acid phosphatase (TRAP) is expressed abundantly by osteoclasts and is required for bone resorption. This enzyme is emerging as an important biomarker in bone pathology, both for histochemical identification of osteoclasts and as a serum marker of osteoclast activity and increased bone turnover. Rat and mouse models are becoming popular systems for studying osteoclast development, bone physiology and morphogenesis, and bone diseases such as osteoporosis. We have developed two unique antibodies to human TRAP purified from hairy cell leukemia spleen. Both antibodies (9C5 and 14G6) are suitable for immunohistochemistry of osteoclasts and macrophages. Only one (14G6) is capable of immunoprecipitating active TRAP from human cell lysates. Antibody 9C5 reacts with a denatured epitope of TRAP while antibody 14G6 probably reacts with a native, conformational determinant. The high degree of homology among TRAPs of various species predicts that these antibodies should be suitable for work in experimental animals as well as humans. Immunohistochemical staining, electrophoretic analyses, immunoprecipitation and immunoblotting assays of human rat and mouse TRAP were carried out to test the validity of these antibodies as cell markers in rodents. Both antibodies were suitable for immunohistochemistry in all species. Antibody 9C5 was suitable for immunoblotting of denatured TRAP of all species tested. Antibody 14G6 reacted with the native TRAP of humans only and failed to immunoprecipitate mouse or rat TRAP activity. Although TRAP is a phylogenetically conserved protein, subtle, species-specific determinants exist. Care should be exercised when anti-TRAP antibodies are used for immunoassay in experimental animals.

Histochemical and molecular analyses of distraction osteogenesis in a mouse model

A tibial lengthening scheme in the mouse was used to study the molecular and cellular events regulating tissue regeneration during distraction osteogenesis. Here, we report on the surgical technique and frame design and describe the histochemical and molecular aspects of distraction during different phases of treatment. A total of 26 mice were used in this study. The treatment protocol was divided into a latency period of 7 days, a phase of active distraction that lasted 10 days with a distraction rate of 0.42 mm/day, and a maturation phase of 9 days. During latency, the distraction site resembled a stabilized fracture callus on both a histochemical and a molecular level. During active distraction, the gap was characterized by a central fibrous interzone bordered by primary matrix fronts, regenerate bone aligned with the distraction force, parallel columns of vascular sinusoids, and a medullary cavity. Alkaline phosphatase activity was detected in the endosteal and periosteal surfaces of the bone ends. Tartrate resistant acid phosphatase staining revealed that osteoclasts remodeled the bone regenerate as it formed. Collagen type I was expressed in the periosteum and the primary matrix front during distraction, whereas collagen type-II transcripts were localized to discrete regions on the periosteal surfaces, immediately adjacent to the osteotomy ends. Collagen type-II transcripts were not detected in the fibrous interzone. During the maturation phase, cells within the fibrous interzone expressed collagen type I and exhibited abundant alkaline phosphatase activity, suggesting that they had begun to terminally differentiate. Collectively, these data demonstrate the utility of a mouse model to study the molecular and cellular bases for the regeneration and remodeling of tissue.

Biochemical markers of bone turnover. Applications for osteoporosis

The recent discovery of specific and sensitive biochemical markers reflecting the overall rate of bone formation and bone resorption has markedly improved the noninvasive assessment of bone turnover in various metabolic bone diseases, especially in osteoporosis. The immunoassay of human osteocalcin, recognizing the intact molecule and its major proteolytic fragment, and assays for bone alkaline phosphatase and the intact form of the N-terminal extension propeptide of type I collagen are currently the most sensitive markers for assessing bone formation. The best indices of bone resorption are the new immunoassays for pyridinoline cross-links and type I collagen-related peptides in urine and, recently, in serum. Several studies indicate that screening for these bone markers may be useful for improving the prognostic assessment of postmenopausal women in combination with bone mass measurement, that is, their risk for osteoporosis and, ultimately, for fractures and to monitor rapidly the efficacy of antiresorptive drugs.

Characterization of serum tartrate-resistant acid phosphatase and development of a direct two-site immunoassay

Osteoclasts secrete tartrate-resistant acid phosphatase (TRAP) to the circulation, where the amount of TRAP is expected to correlate with the bone resorption rate. We have developed two monoclonal antibodies, O1A and J1B, using purified human bone TRAP as antigen. The antibodies recognized different epitopes, allowing us to develop a two-site fluoroimmunoassay. The immunoreactivity in fresh serum specimens was less than 10% of the concentrations measured from the same specimens after 24 h of storage at 4 degrees C, or after addition of 5 mM EDTA or EGTA to them. When fresh serum was gel filtrated using Sephacryl S-200 column, all of the enzyme eluted in the void volume as a complex with a molecular weight of more than 250 kDa. If the serum was treated with EDTA before the gel filtration, the complex was destroyed and the enzyme eluted in fractions corresponding to a molecular weight of 30 kDa, the size of monomeric purified human bone TRAP. The immunoassay was used to measure TRAP concentrations from serum samples that had been stored at 4 degrees C for 24 h. According to the assay, premenopausal women had 13.1 +/- 3.1, postmenopausal women 17.6 +/- 4.2, and children 32.6 +/- 12.2 microg TRAP/l of serum. We conclude that TRAP circulates in the serum as part of a complex, which also contains Ca2+, and that TRAP-immunoassay is a potentially useful method for determining bone resorption rates, as long as the complex is destroyed before the assay.

Improved method for measuring tartrate-resistant acid phosphatase activity in serum

We describe an improved method for the kinetic measurement of tartrate-resistant acid phosphatase (TrACP; EC 3.1.3.2) activity in serum. Of the TrACP derived from erythrocytes, platelets, and macrophages (osteoclasts and others), that from the first two sources is also resistant to fluoride, whereas skeletal TrACP is sensitive to fluoride. Thus, osteoclast-derived TrACP can be measured specifically by exploiting its sensitivity to fluoride. We measured the activity of tartrate-resistant and fluoride-sensitive acid phosphatase (TrFsACP) by using 2,6-dichloro-4-acetylphenyl phosphate as substrate at pH 6.2. The activity of TrFsACP in serum was increased by adding hexadimethrine bromide (Polybrene) to the reaction mixture. This method was not influenced by hemolysis with hemoglobin concentrations as great as 0.9 g/L. The mean ± SD values of TrFsACP activity by this method were 20.4 ± 2.8 and 16.4 ± 2.3 U/L for young (ages 20–29 years) men (n = 34) and women (n = 50), respectively. The highest mean TrFsACP activity was found among children younger than 15 years, followed by that in elderly subjects (older than 60).

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.

Osteoporosis

Osteoporosis, one of the most prevalent diseases associated with aging, is characterized by a low bone mass and microarchitectural deterioration of bone tissue leading to increased susceptibility of fracture. The management of osteoporotic patients relies especially on the static measurement of bone mass as bone mineral and dynamic changes of bone turn over. New developments of bone mineral density measurement techniques and biochemical markers of bone turnover are reviewed. Their clinical uses for the management of patients with osteoporosis are discussed.

Recombinant human and mouse purple acid phosphatases: expression and characterization

The mammalian purple acid phosphatases (also called tartrate-resistant acid phosphatases) are expressed primarily in actively resorbing osteoclasts and activated macrophages. The enzymes are characterized by the presence of a binuclear iron center at the active site. Recent studies on transgenic mice lacking purple acid phosphatase implicate the osteoclast enzyme in both bone resorption and bone mineralization. To characterize the mammalian enzymes in more detail, particularly with respect to their substrate specificity at the low pH of the osteoclastic resorptive space (2.5-3), we have purified the recombinant human and mouse enzymes from baculovirus-infected insect cells. The properties of the recombinant mouse enzyme are compared with those of the nonrecombinant enzyme isolated from mouse spleen. The kinetics of hydrolysis of the substrates p-nitrophenyl phosphate, phosphotyrosine, and pyrophosphate and a phosphotyrosyl peptide by the recombinant human and mouse enzymes and the nonrecombinant mouse and pig enzymes were analyzed. For all the enzymes the ratio k(cat)/Km was typically approximately 10(6) M(-1) s(-1) and was higher at pH 2.5 than at 4.9. The increase was attributable to a large decrease in Km at the lower pH value. The results indicate that the enzyme exhibits high catalytic efficiency toward substrates such as pyrophosphate and acidic phosphotyrosine-containing peptides, particularly at low pH values typical of the bone resorptive space. The implications of the results for the physiological function of the enzyme are discussed.

Bone markers

The recent development of specific and sensitive biochemical markers reflecting the overall rate of bone formation and bone resorption, has markedly improved the non-invasive assessment of bone turnover in various metabolic bone diseases, especially osteoporosis. The immunoassay of human osteocalcin recognizing the intact molecule and its major proteolytic fragment, along with that of bone alkaline phosphatase, are currently the most sensitive markers to assess bone formation. For bone resorption, the total urinary excretion of pyridinoline crosslinks measured by high pressure liquid chromatography has shown its superiority over all other markers for the clinical assessment of osteoporosis. The recent development of immunoassays recognizing either the free pyridinoline crosslinks or pyridinoline crosslinked-type I collagen peptides in urine and serum should allow a broad use of this sensitive resorption marker. Recent studies, some of them still in progress, define the clinical use of these markers: first, to improve the prognostic assessment of post-menopausal women in combination with bone mass measurement, i.e. their risk of developing osteoporosis and, ultimately, fractures and, second, to monitor the efficacy of anti-resorption drugs.

Characterization of monoclonal antibodies specific to human tartrate-resistant acid phosphatase

A major product of osteoclasts, tartrate-resistant acid phosphatase (TRAP) is an essential but insufficient enzyme for bone resorption. TRAP is an excellent cell marker for osteoclasts and macrophages and is being investigated as a serum marker for osteoclast activity in diseases of bone destruction. For decades, TRAP has also been used as a marker for hairy cell leukemia. Immunoassays for TRAP are sought to increase the sensitivity and specificity of the TRAP test for bone and hairy cells. Our laboratory recently developed a monoclonal antibody to TRAP (9C5) useful for immunohistochemical identification of TRAP-positive cells in paraffin sections. Herein, we characterize 9C5 in greater detail and report production of another anti-TRAP monoclonal antibody antibody (14G6) reactive with native, active enzyme antigen. Enzyme immunoassay, immunoprecipitation, western blot, and immunohistochemical analyses revealed the contrasting properties of 9C5 and 14G6. Antibody 9C5 reacts with a heat-denatured epitope and is suitable for denaturing western blot analysis and for immunohistochemistry. Antibody 14G6 reacts with a conformational determinant destroyed by heat and is suitable for immunoprecipitation of active TRAP, although 20% to 30% of activity is inhibited in the immune complexes. Having characterized several properties of these anti-TRAP antibodies, 9C5 and 14G6 may be useful for development of TRAP-specific immunoassays in bone pathology and hematology. They will certainly be of use for the study of biosynthesis, regulation, expression, and function of TRAP.

Tartrate-resistant acid phosphate activity as osteoclastic marker: sensitivity of cytochemical assessment and serum assay in comparison with standardized osteoclast histomorphometry

Tartrate-resistant acid phosphatase (TRAP) activity is regarded as an important cytochemical marker of osteoclasts; its concentration in serum is utilized as a biochemical marker of osteoclast function and degree of bone resorption. This study was carried out to assess the sensitivity of TRAP activity both as a cytochemical marker in histological sections and as a biochemical marker in serum in comparison with the standardized histomorphometric variables of osteoclasts. To this end we investigated 24 patients (21 women, 3 men; 60 +/- 17 years of age) affected with various metabolic bone diseases. Osteoclast surface (OcS/BS) and osteoclast number (OcN/BS) were evaluated by standardized histomorphometry in iliac crest biopsies. On the basis of TRAP cytochemical activity, TRAP-positive osteoclast surface (TRAP + OcS/BS) and number (TRAP + OcN/BS) were measured. TRAP-positive cells adjacent to bone and showing one nucleus or no nuclei at all in the plane of section were included in the counts as osteoclasts. Serum TRAP activity was determined by spectrophotometric assay. Values of OcS/BS and OcN/BS were much lower than those of TRAP + OcS/BS (-50%) and TRAP + OcN/BS (-60%), respectively. Correlations between OcS/BS and TRAP + OcS/BS, and between OcN/BS and TRAP + OcN/BS, were highly significant. Serum TRAP was significantly correlated with OcS/BS, OcN/BS, and TRAP + OcN/BS. These correlations, however, were rather low. Moreover, serum TRAP did not correlate with TRAP + OcS/BS. From these results, the conclusion can be drawn that while TRAP activity is confirmed as a valid cytochemical marker for identification of osteoclasts, serum TRAP activity is an osteoclastic marker of weak sensitivity. This may be due to known factors, such as synthesis of the enzyme not being unique to osteoclasts, enzyme instability, and the presence of inhibitors in serum. Mononucleated osteoclasts do not significantly influence the serum enzyme levels.