The association of a newly discovered protein, called chondrocalcin, with cartilage calcification

A newly identified calcium binding protein called chondrocalcin with two subunits of molecular weight approximately 35 000 has been studied in bovine, rat and human cartilage matrix using a monospecific polyclonal antibody. Although it is present in small amounts in non-calcifying cartilage, it occurs in local high concentrations wherever cartilage calcification is observed, namely in the calcifying part of the growth plate and in calcified articular cartilage. Immunoelectron microscopy revealed that it is present in exactly the same discrete sites where mineral is first detected. Thus it may act as a nucleating agent for apatite formation. It is deposited in the same sites where unusual local high concentrations of proteoglycan and link protein are detected by immunoelectron microscopy. Chondrocalcin may bind either directly or indirectly to these molecules. Its occurrence within hypertrophic chondrocytes immediately prior to its extracellular appearance suggests that it is synthesised and released by these cells. Its absence from osteoid during intramembranous calcification indicates a selective involvement in endochondral calcification.