Special remarks: Addition of PMSF (protease inhibitor) to urine samples is recommended to avoid cleavage of OPN. Store samples at -80°C.

Osteopontin (OPN) is a secretory glycoprotein first isolated from the bone. At present, it is known as a highly acidic calcium-binding phosphorylated protein with sugar chain bonds, that is secreted from diverse cells, including osteoblasts, renal tubular cells, macrophages, activated T lymphocytes and vascular smooth muscle cells. Its molecular weight can vary depending on the sugar chain formation and phosphorylation, and has been reported to range from 44 to 66 kDa. One major characteristic of OPN is that its molecule contains an Arg-Gly-Asp (RGD) amino acid sequence. This motif is also seen in fibronectin, vitronectin and other extracellular proteins. It is known that OPN binds through this motif to members of the integrin family (e.g., avb3) of cell surface receptors. Another unique characteristic of OPN is that it can assume various molecular forms in vivo by undergoing RNA splicing, saccharification, phosphorylation, sulfation, degradation by protease, etc. OPN is considered to coexist with thrombin locally in tissues such as injured tissue, inflamed tissue, vascularized tissue, and tumor tissue. Coexistence with thrombin increases the likelihood of proteolysis, and this process may have an important physiological role. Some investigators have reported evidence of enhancement of the cell adhesion of OPN by thrombin treatment. This suggests that cleavage of OPN with thrombin causes the hidden sequence of the adhering sites to be exposed. It has also been reported that the fragments of OPN produced following cleavage by thrombin react with alpha9beta1 integrin. Furthermore, in view of the finding that OPN has many cell-binding sites and can react with various receptors, interactions between OPN and these cells or receptors may play some unknown roles. This kit can specifically measure the N-terminal OPN fragment cleaved by thrombin. OPN molecules without thrombin cleavage, on the other hand, are hardly detected by the kit.