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Clinical Chemistry 43: 1663-1665, 1997;
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(Clinical Chemistry. 1997;43:1663-1665.)
© 1997 American Association for Clinical Chemistry, Inc.

Technical Briefs

Simple Method to Evaluate Specificity of Osteocalcin Immunoassays

Jean-Claude Souberbiellea, Delphine Marque, Philippe Bonnet, Patricia Herviaux and Charles Sachs

Service d'Explorations Fonctionnelles, Hôpital Necker-Enfants Malades, 149 rue de Sèvres, 75015, Paris, France
a author for correspondence: fax 33/1 40 61 55 87

Osteocalcin (Oc) is a 49-amino-acid noncollagenous protein synthesized by osteoblasts. Because of the presence of three {gamma}-carboxyglutamic acid (GLA) residues, newly synthesized Oc will be primarily bound to hydroxyapatite present in bone matrix, only a small fraction being released into the circulation (1). Although serum Oc concentration is considered a valid marker of bone turnover in most situations (2), it reflects specifically bone formation when resorption and formation are uncoupled as observed in corticoid-induced osteoporosis (3). Several reports have stressed recently that absolute Oc concentrations obtained with various in-house (4) or commercial (5) assays cannot be compared directly. An important cause for the discrepancies between Oc assays is the variabily of the anti-Oc antibody specificity for the circulating Oc fragments described by Garnero et al. (6). The intact Oc molecule (1–49 Oc) is highly susceptible to tryptic proteolysis because of the presence of 2 Arg-Arg sites in positions 19–20 and 43–44 (7). When serum samples are not rapidly frozen, 1–49 Oc will be degraded quickly, the largest degradation product being a big N-terminal mid-fragment (1–43 Oc). This fragment is accumulated in patients with chronic renal failure (CRF) (6). This problem of degradation is a major drawback in terms of specificity with an Oc assay that recognizes only 1–49 Oc. On the contrary, an Oc assay recognizing both 1–49 Oc and 1–43 Oc with equal affinity will be less sensitive to the effect of proteolysis (8) but of very limited value in CRF patients, as even those with adynamic bone disease will appear to have high serum Oc concentrations (9). Antibody specificity is thus an important variable to be taken into account when interpreting serum Oc concentrations obtained . . . [Full Text of this Article]


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