Calcium metabolism Calcium metabolism Calcium metabolism refers to the movements and regulation of calcium ions (Ca) "in" and "out" of various body compartments, such as the gastrointestinal tract, the blood plasma, the extracellular and intracellular fluids, and bone tissue. An important aspect of calcium metabolism is plasma calcium homeostasis, the regulation of calcium ions in the blood plasma within narrow limits. In this process, bone tissue acts as a calcium storage center for deposits and withdrawals as needed by the blood, via continual bone remodeling. Derangement of this mechanism leads to hypercalcemia or hypocalcemia, both of which can have consequences for health.
Calcium such as osteopontin and bone sialoprotein use both the first and the second. Direct activation of enzymes by binding calcium is common; some other enzymes are activated by noncovalent association with direct calcium-binding enzymes. Calcium also binds to the phospholipid layer of the cell membrane, anchoring proteins associated with the cell surface. As an example of the wide range of solubility of calcium compounds, monocalcium phosphate is very soluble in water, 85% of extracellular calcium is as dicalcium phosphate with a solubility of 2.0 mM and the hydroxyapatite of bones in an organic matrix is tricalcium phosphate at 100 µM.