Fig. 6.
Major mineral transport mechanisms in bone. For Ca2+, transport is largely trans-cellular [82]. Phosphate from sodium-dependent uptake at the basolateral surface [76] is converted to organic phosphate [119] and exported from the apical surface by unknown transport mechanisms. This is followed by release of phosphate by an ecto-enzyme (extracellular membrane attached) alkaline phosphatase [110]. The osteoblast also massively expresses the pyrophosphate transport regulator ANKH [121], possibly capturing PPi produced during collagen synthesis: one pyrophosphate is produced per amino acid added. In the absence of alkaline phosphatase, accumulation of organic phosphates including pyridoxal phosphate [110] and of pyrophosphate [112] occurs, both related to attenuation of mineralization.