Specific activity of skeletal alkaline phosphatase in human osteoblast‐line cells regulated by phosphate, phosphate esters, and phosphate analogs and release of alkaline phosphatase activity inversely regulated by calcium

We assessed the significance of Ca and phosphate (P_i) as determinants of (1) the amount of skeletal alkaline phosphatase (ALP) activity in SaOS‐2 (human osteosarcoma) cells and normal human bone cells, and (2) the release of ALP activity from the cells into the culture medium. After 24 h in serum‐free BGJ_b medium containing 0.25–2 mM P_i, the specific activity of ALP in SaOS‐2 cells was proportional to P_i concentration (r = 0.99, p < 0.001). The P_i‐dependent increase in ALP activity was time dependent (evident within 6 h) and could not be attributed to decreased ALP release, since P_i also increased the amount of ALP activity released (r = 0.99, p < 0.001). Parallel studies with Ca (0.25–2.0 mM) showed that the amount of ALP activity released from SaOS‐2 cells was inversely proportional to the concentration of Ca (r = −0.85, p < 0.01). This effect was rapid (i.e., observed within 1 h) and could not be attributed to a decrease in the amount of ALP activity in the cells. Phase distribution studies showed that the effect of low Ca to increase ALP release reflected increases in the release of both hydrophilic ALP (i.e., anchorless ALP, released by phosphatidylinositol‐glycanase activity) and hydrophobic ALP (i.e., phosphatidylinositol‐glycan–anchored ALP, released by membrane vesicle formation). The range of Ca‐dependent changes in ALP‐specific activity was much smaller than the range of P_i‐dependent changes. The observed correlation between skeletal ALP‐specific activity and P_i was not unique to osteosarcoma cells or to P_i. Similar effects were seen in normal human bone cells in response to P_i (r = 0.99, p < 0.001) and in SaOS‐2 cells in response to a variety of P_i esters and analogs (e.g., β‐glycero‐P_i and molybdate). Further studies indicated that the effects of phosphoryl compounds on ALP‐specific activity could not be correlated with effects on ALP reaction kinetics, cell proliferation, or acid phosphatase activity and that the β‐glycero‐P_i‐dependent increase in ALP activity was blocked by cycloheximide but not actinomycin D. Together these data suggest that the function of skeletal ALP may be regulated by P_i and that Ca may be involved in ALP release.