Function
Once activated, osteoclasts move to areas of microfracture in the bone by chemotaxis. Osteoclasts lie in small cavities called Howships lacunae, formed from the digestion of the underlying bone. The sealing zone is the attachment of the osteoclasts plasma membrane to the underlying bone. Sealing zones are bounded by belts of specialized adhesion structures called podosomes. Attachment to the bone matrix is facilitated by integrin receptors, such as αvβ3, via the specific amino acid motif Arg-Gly-Asp in bone matrix proteins, such as osteopontin. The osteoclast releases hydrogen ions through the action of carbonic anhydrase (H2O + CO2 → HCO3− + H+) through the ruffled border into the resorptive cavity, acidifying and aiding dissolution of the mineralized bone matrix into Ca2+, H3PO4, H2CO3, water and other substances. Dysfunction of the carbonic anhydrase has been documented to cause some forms of osteopetrosis. Hydrogen ions are pumped against a high concentration gradient by proton pumps, specifically a unique vacuolar-ATPase. This enzyme has been targeted in the prevention of osteoporosis. In addition, several hydrolytic enzymes, such as members of the cathepsin and matrix metalloprotease (MMP) groups, are released to digest the organic components of the matrix. These enzymes are released into the compartment by lysosomes. Of these hydrolytic enzymes, cathepsin K is of most importance. Cathepsin K and other cathepsins
Cathepsin K is a collagenolytic papain-like cysteine protease that is mainly expressed in osteoclasts, and is secreted into the resorptive pit. Cathepsin K is the major protease involved in the degradation of type I collagen and other noncollagenous proteins.
On September 29, 2011, the FDA issued new guidance with regard to the DESI category. This effectively disallowed any new DESI formulations to enter the market. The FDA also has stated that DESI drugs do not have any therapeutic equivalent drugs listed in the “Orange Book”. In their description of therapeutic equivalence, the FDA specifically cites Donnatal®. Therefore, any so-called generic drugs on the market with similar formulation to DESI drugs, are considered illegal drugs as the FDA has not reviewed their composition or therapeutic equivalence. Donnatal, although containing phenobarbital, is exempt from the Controlled Substances Act due to the belladonna alkaloids present in the formulation. References
External links
"Donnatal Extended Release Tablet Information". Archived from the original on 2008-12-27.