Intertissular variations in osteonectin: a monoclonal antibody directed to bone osteonectin shows reduced affinity...

Osteonectin, a major noncollagenous protein of bone, is also synthesized and secreted by various non-mineralized tissues and by platelets. To establish whether there are structural specificities of osteonectin according to its tissular origin, we raised 12 monoclonal antibodies against bovine bone osteonectin and screened them for their ability to recognize bone and platelet osteonectin. When hybridoma culture media were radioimmunoassayed all MAbs showed the same titer for [125I]human platelet osteonectin and for [125I]bovine bone osteonectin, except MAb 2, which poorly bound platelet osteonectin. Immunoprecipitation and immunoblotting experiments were performed on human bone protein extracts and on material secreted by human platelets upon thrombin stimulation; in these experiments MAb 2 recognized human bone osteonectin and only faintly human platelet osteonectin. A "sandwich" immunoradiometric assay was devised in which osteonectin bound to a solid phase by a first MAb was recognized by a 125I-labeled second MAb. In this assay MAb 2, used as a tracer, showed a 100-fold lower affinity for purified human platelet osteonectin than for purified human bone osteonectin. These results suggest the existence of structural variations in osteonectin obtained from bone and platelets. Whether these variations result from differences in sequence, post-translational processing, or postsecretional fate remains to be established.