Radioimmunoassay for osteonectin. Concentrations in bone, nonmineralized tissues, and blood

We describe a double antibody radioimmunoassay for the bone protein osteonectin using a polyclonal antibody raised against bovine fetal bone osteonectin. This assay, which cross-reacts with human osteonectin, was used to explore the tissue distribution of the protein. Osteonectin was detected in extracts of nonmineralized tissues from bovine fetuses, with concentrations ranging from 10 ng/100 micrograms of proteins in the lens to about 80 ng/100 micrograms in collagen-rich tissues (e.g., tendon), which represents respectively from 0.01% to 0.5% of the concentrations found in bone extracts. Osteonectin is thus a widespread component of extracellular matrix, although its concentration is minute in tissues other than bone and dentin. Adult human serum contains 200 ng/ml of osteonectin and values obtained in plasma prepared without platelet activation are about one-third of those in matched sera. This platelet-independent fraction might be used as a marker for bone metabolism.