Measurement of serum bone gla-protein (BGP) in humans with an ovine BGP-based radioimmunoassay

Abstract

Most RIAs of serum bone gla-protein (BGP; also called osteocalcin) used for clinical investigation are based on bovine BGP for standard, tracer, and immunogen because of the homology between bovine and human BGP. However, ovine BGP differs from human BGP by only five amino acids, being identical from residues 11 to 49, as compared with homology at residues 20-49 between bovine and human BGP. In screening various anti-ovine BGP polyclonal anti-sera we selected one (R310) that exhibits apparently complete cross-reactivity with human BGP, as assessed by dilutions of 13 human sera from normal subjects and from patients with bone disease. This RIA gave a 42% binding at a 10,000-fold final dilution, with intra- and interassay variations less than 7% and 11%, respectively. Gel-filtration chromatography of human serum showed a single immunoreactive peak. Synthetic fragments of human BGP 1-10, 7-19, 25-37, and 37-49 were not recognized by R310, suggesting that either a mid-molecule region or a conformational epitope was its target. Using this RIA, we determined that serum BGP increased with age in women (P less than 0.02), by a mean of 90% from ages 30 to 70 years. Serum BGP was also increased in patients with primary hyperparathyroidism, renal osteodystrophy, and Paget's disease. In contrast with the "normal" concentrations of BGP detected with an anti-bovine BGP antiserum (R102), serum BGP was increased in patients with postmenopausal osteoporosis as measured with the R310 ovine assay, suggesting a greater sensitivity for the latter assay.