Effects of zinc on human skeletal alkaline phosphatase activity in vitro

Inorganic phosphate (Pi) can regulate the level of skeletal alkaline phosph atase (ALP) activity in human osteoblast-like cells by stabilizing the enzy me (without affecting transcription, ALP release from the cell surface, or the amount of ALP protein). These observations suggest that Pi determines t he level of ALP activity by modulating a process of irreversible inactivati on. The current studies were intended to examine the hypothesis that this i nactivation of ALP activity is caused by the dissociation of an active cent er Zn and that Pi inhibits that dissociation. Initial studies showed that Z n, Like Pi, could increase ALP specific activity in human osteosarcoma SaOS -2 cells in a time- and dose-dependent manner (e.g., a 50% increase at 0.2 mu mol/liter Zn, P < 0.005). This effect was specific for Zn (i.e., no simi lar effect was seen with Ca, Fe, Co, Mg, Mn, or Cu), but not for SaOS-2 cel ls. Zn also increased ALP specific activity in (human osteosarcoma) MG-63 c ells and in cells derived from normal human vertebrae (P < 0.001 for each). The effect of Zn to increase ALP activity was not associated with parallel increases in total protein synthesis, collagen production, or tartrate-res istant acid phosphatase activity (no change in any of these indices), net I GF-2 synthesis (a Zn-dependent decrease, P < 0.005), or PTH-dependent synth esis of cAMP (a biphasic increase, P < 0.02). Kinetic studies of Pi and Zn as co-effectors of ALP activity showed that Zn was a mixed-type effector wi th respect to Pi, whereas Pi was competitive with respect to Zn. Mechanisti c studies showed that (1) Zn reversed the effect of Pi withdrawal to decrea se ALP activity, but not by reactivating inactive ALP protein (the process required protein synthesis, without increases in ALP mRNA or the level of A LP immunoreactive protein); (2) Zn increased the half-life of ALP activity in intact cells and after a partial purification; and (3) Pi inhibited the process of ALP inactivation by EDTA (which chelates active center Zn). All these findings are consistent with the general hypothesis that Pi increases the half-life of skeletal ALP by preventing the dissociation of active cen ter Zn and with a mechanistic model of skeletal ALP activity in which activ e center Zn participates in Pi-ester binding and/or hydrolysis.