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Here is what I understand:

Osteoclasts demineralize/decalcify bone by releasing both protons and proteolytic enzymes (including MMP's).

Bone is composed of two major components: Osteoid (organic) and Hydroxyapatite (mineral). Osteoid is synthesized and released by osteoblasts and is composed of fibers and ground substance. The predominant fiber-type is Type I collagen. The ground substance is mostly made up of chondroiton sulfate and osteocalcin. Hydroxyapatite is made up of calcium and phosphate, which is the mineral component of bone.

The proteolytic enzymes released by osteoclasts digest the osteoid, releasing hydroxyproline. Hydroxyproline (from collagen) can be detected in the urine and is a marker of osteoclast activity. On the other hand, the acid is used to demineralize the bone, releasing CaPO4 (Note: Mineralization is favored by a basic environment while demineralization is favored by an acidic environment)

Hope that helps, Great question!!
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