Bone Proteinases

This chapter surveys the current knowledge of the proteinases expressed in bone. Although previously the osteoclast was considered to be the main producer of proteinases in bone, it has become increasingly clear that osteoblasts play a significant role in the production of many of these proteinases. Based on the structure and the catalytic mechanism of the active site involving particular amino acid residues and zinc, proteinases can be classified into four groups: metalloproteinases e.g., collagenase-3; serine proteinases, e.g., plasminogen activator; cysteine proteinases, e.g., cathepsin K; and aspartic proteinases, e.g., cathepsin D. The osteoblast has the ability to produce proteinases of all four classes, but far more is known about their production of collagenase and plasminogen activators, at least in vitro. The absolute role of any of these osteoblastic enzymes in vivo is still not fully known. Further work with knockouts of the respective enzymes is likely the only way to determine their required functions. These roles may not be restricted to assisting in the resorption process but may include functions to regulate bone development. Additionally, the osteoclast produces MMP-9 and cathepsin K, which appear to have similar roles in the two diverse processes.