Multiple Osteocalcin Fragments in Human Urine and Serum as Detected by a Midmolecule Osteocalcin Radioimmunoassay*

Abstract

Reliable markers of bone formation are essential to the investigation of metabolic bone disorders. In this regard, evidence indicates that circulating levels of human osteocalcin (OC) correlate with the skeletal isoenzyme of alkaline phosphatase and can be used as an index of bone formation. A disadvantage of using serum OC as a marker of formation is its diurnal variation. To address this problem we carried out our studies to determine the usefulness of urine in the assessment of bone turnover. Using a midmolecule specific human OC RIA, we were able to detect OC in urine of normal adults (42 mugeq/g creatinine), normal children (849 mu/geq/g creatinine), and Paget's disease patients (613 mugeq/g creatinine). Immunoreactive fragments of OC in human urine and human serum were separated by high pressure liquid chromatography. Multiple fragments were found in normal adult urine that were not detected in normal adult serum. Uremic and Paget's disease sera contain several immunoreactive forms of OC, other than the intact molecule, not found in normal adult serum. Additionally, both Paget's disease sera and urine contained a specific peak of immunoreactive material, eluting at 25% acetonitrile, that was not found in any other serum or urine tested. Urinary OC (uOC) correlated with both skeletal alkaline phosphatase (r = 0.91) and serum OC (r = 0.83), indices of skeletal formation. While uOC has a diurnal variation similar to that of serum OC, determinations of 24-h uOC give integrated values of daily bone turnover rates. Z-Score analysis indicates that uOC (z = 14.04) is better able to distinguish between normal children with high bone turnover and normal adults than either skeletal alkaline phosphatase (z = 8.87) or serum OC (z = 9.01).

Release of Intact and Fragmented Osteocalcin Molecules from Bone Matrix during Bone Resorption in Vitro

“…However, later studies on patients with different metabolic bone diseases have suggested that not all of osteocalcin fragments are derived from the metabolism of osteocalcin in the circulation or peripheral organs but also from osteocalcin embedded in bone (21)(22)(23). Thus, part of osteocalcin found in the blood may also originate from the resorption process, when osteocalcin embedded in the bone matrix is released during bone degradation (Fig.…”

Urinary Osteocalcin Is a Useful Marker for Monitoring the Effect of Alendronate Therapy

“…Circulating osteocalcin (OC), a bone-specific protein produced by osteoblasts, is widely used as an index of bone formation (3,4 ). Fragments of OC are also found in urine (5)(6)(7), and the measurement of urinary OC (U-OC) is another method for monitoring bone metabolism (8 -11 ). We recently described 3 immunoassays for measuring U-OC (11 ).…”

“…Increased serum PDGF concentrations during and after hemodialysis, attributable to PLT-membrane contact at artificial surfaces, may be directly involved in the increased frequency of atherosclerosis in hemodialysis patients (2 ). The role of circulating GFs in many other nonmalignant clinical conditions has also been investigated (3)(4)(5)(6)(7)(8).…”

Studies on Gene Expression in Calvaria and Serum Levels of Insulin-Like Growth Factor-I and Bone Gla Protein in the Methimazole-Induced Congenital Hypothyroid Rat .

“…The BGP molecules, after cleavage of the propeptide, are partly released into the peripheral circulation and can be detected in serum or plasma by RIA [45][46][47]. But previous reports have demonstrated that the majority of their fragments, which derive from the processing during bone resorption, are also released into the circulation and that former RIAs for BGP cross-react with the fragments [48][49][50]. Consequently, osteoblastic bone formation could be evaluated by using a specific assay system to detect intact molecular BGP.…”