A dual-label immunofluorometric assay for human osteocalcin

Osteocalcin: A bone protein with multiple endocrine functions

Bones are now recognised as endocrine organs with diverse functions. Osteocalcin, a protein primarily produced by osteoblasts, has garnered significant attention. Research into osteocalcin has revealed its impact on glucose metabolism and its unexpected endocrine role, particularly in its undercarboxylated form (ucOC). This form influences organs, affecting insulin sensitivity and even showing correlations with conditions like type 2 diabetes and cardiovascular diseases. However, analytical challenges are impeding advances in clinical research. Various immunoassays like RIA, EIA, ECLIA, IRMA, and ELISA have been developed to analyse osteocalcin. Recent innovations include techniques like OS-ELISA and OS phage Immuno-PCR, enabling fragment analysis. Advancements also encompass porous silicon for detection and ECLIA for rapid measurements. The limitations of immunoassays lead to ucOC measurement discrepancies, prompting the development of mass spectrometry-based techniques. Mass spectrometry increasingly quantifies carboxylated, undercarboxylated, and fragmented forms of osteocalcin. Mass spectrometry improves routine and clinical analysis accuracy. With heightened specificity, it identifies carboxylation status and serum fragmentations, boosting measurement reliability as a reference method. This approach augments analytical precision, advancing disease understanding, enabling personalised medicine, and ultimately benefiting clinical outcomes. In this review, the different techniques for the analysis of osteocalcin will be explored and compared, and their clinical implications will be discussed.

A urine midmolecule osteocalcin assay shows higher discriminatory power than a serum midmolecule osteocalcin assay during short-term alendronate treatment of osteoporotic patients

We isolated and characterized a peptide fragment corresponding to amino acid sequence 14-28 of human osteocalcin in urine from Paget's disease, and developed a polyclonal antibody reactive to this peptide in urine. We used this antibody to measure urinary fragments of osteocalcin and compared to efficacy of the urinary osteocalcin assay with a serum osteocalcin (sOC) assay (ELISA-Osteo, Cis-Bio International) to monitor the short-term changes in bone turnover in response to alendronate treatment. The synthetic peptide-based urinary osteocalcin (uOC) radioimmunoassay (RIA) showed an analytical sensitivity of 6.25 ng/mL, standard curve range of 3.12-400 ng/mL, and mean intra- (n = 20) and interassay (n = 30) coefficient of variation (CV) of <15%. Urine osteocalcin concentrations in postmenopausal osteoporotic patients were approximately 90% higher than in normal premenopausal controls. Series of 24 h urine and matched serum samples were collected at baseline, 30 days, and 90 days after treatment of postmenopausal osteoporotic patients with daily dose of 10 mg alendronate. We measured urinary osteocalcin (uOc) levels and urinary N-telopeptide (uNTx, Ostex) in urine samples and serum N-telopeptide (sNTx), C-telopeptide (sCTx, Osteometer), serum osteocalcin (sOC) as well as bone-specific alkaline phosphatase (sALP) (Alkphose-B, Metra Biosystems) in serum samples. The percent change data obtained between baseline and 30 days (n = 18) posttreatment suggested a rapid decline in uOC concentration (-27%, p < 0.01) in response to alendronate treatment, as compared with a marginal and nonsignificant decrease in sOC (-7.2%, p = 0.417) or sALP (-3.4%, p = 0.689), two specific markers of bone formation. As expected, due to the coupling of bone formation and bone resorption, the concentration of all markers showed a 30%-45% decline compared with baseline values after 90 days (n = 16) of treatment. Correlation of markers after a 30 day treatment with alendronate revealed a higher correlation (r = 0.61, p < 0.01) between uOC and uNTx, as compared with sOC (r = 0.03, p = 0.447) or sALP (r = -0.14, p = 0.295) with uNTx. Similarly, correlation coefficients with r values between 0.48 and 0.55 (p < 0.05) were observed between uOC, sNTx, and sCTx, whereas no significant correlation was observed between sOC and sNTx or sCTx. These results provide indirect evidence that fragments measured by the urine assay probably originated from bone resorption, and suggest that the uOC assay could be used to assess short-term changes in bone metabolism with regard to osteocalcin.

Time-resolved fluorometry in end-point and real-time PCR quantification of nucleic acids

Two time-resolved fluorescence-based methods for nucleic acid quantification are described and their results are compared. Both methods use an exogenous internal standard to eliminate errors arising from different steps of the assay. The first method is a competitive end-point assay, where the standard competes for the same primers with the actual target sequence, prostate-specific antigen (PSA) cDNA. The standard and target are quantified in a dual-label plate hybridization with lanthanide-labelled probes after a fixed number of PCR cycles. The second method is based on real-time monitoring of PCR and on the use of a novel homogeneous signal generation principle that relies on the use of a 5'-->3' exonucleolytic DNA polymerase and a probe labelled with an environment sensitive, stable and fluorescent lanthanide chelate. In this assay, a non-competitive, exogenous internal standard is used. Both assays have a wide linear range (50-5 x 10(6) and 10-5 x 10(7) input PSA cDNA molecules for the end-point and real-time assays, respectively) and there is a strong correlation between the results obtained with the two assays (r = 1.0). Being somewhat faster to perform, the real-time format is better suited for assays that require high throughput.

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.