Two-site assays of bone gla protein (osteocalcin) demonstrate immunochemical heterogeneity of the intact molecule

We developed a panel of monoclonal antibodies to human bone gla protein (BGP; osteocalcin) peptides that span the linear sequence of the molecule, specifically BGP 1-12 (N-terminal), BGP 15-30 (midregion), and BGP 38-49 (C-terminal). These antibodies were evaluated in various combinations of two-site formats in studies of serum BGP concentrations. For clinical studies, we selected from a panel of antibodies the two most sensitive antibody pairs for the intact molecule (N-C); we also used a polyclonal RIA based on BGP-C. For the two-site format, we used two N-terminal antibodies, 029 and 052, adsorbed to polystyrene beads, and radioiodinated a C-terminal antibody, 663. The standard for each of the assays was purified human BGP. The following BGP serum concentrations (microgram/L, mean +/- SE) were measured with the various assays: by the 029-663 assay, results for normal subjects were 7 +/- 3, for patients with renal failure 25 +/- 8, and for patients with Paget disease 12 +/-4; by the 052-663 assay, the respective results were 22 +/- 4, 44 +/- 12, and 31 +/- 7; by the polyclonal assay, the results were 3 +/- 0.2, 13 +/- 2, and 5 +/- 1. The two intact (N-C) assays were significantly (P < 0.01) correlated (r = 0.94), but their serum values differed by more than twofold in terms of the same BGP standard. The polyclonal assay significantly correlated with each of the intact assays (r = 0.83, 0.77), but it, too, gave different serum values for BGP. These studies demonstrate the immunochemical heterogeneity of circulating BGP, heterogeneity that is manifest even in immunoassays specific for the same region of the molecule.