Phosphate regulates the stability of skeletal alkaline phosphatase activity in human osteosarcoma (SaOS-2) cells without equivalent effects on the level of skeletal alkaline phosphatase immunoreactive protein.

Inorganic phosphate (P(i)) can regulate the level of skeletal alkaline phosphatase (ALP) activity in human osteoblast-like cells, but not by means of changes in transcription or release from the cell surface. The current studies were intended to determine whether (1) P(i) affected the inactivation of ALP activity in human osteosarcoma (SaOS-2) cells; and (2) P(i)-dependent changes in ALP-specific activity were associated with equal, concomitant changes in the level of ALP immunoreactive protein. The results of these studies revealed that P(i) increased the stability of skeletal ALP activity without equivalent effects on the level of ALP immunoreactive protein. An increase in P(i) (from 0 to 1.8 mmol/liter) caused a time-dependent increase in the amount of skeletal ALP activity in the SaOS-2 cells, without a parallel increase in the amount of skeletal ALP immunoreactive protein, and a decrease in P(i) (from 1.8 to 0 mmol/liter) caused a time-dependent decrease in the amount of ALP activity, without a significant decrease in the total cellular content of ALP immunoreactive protein. Together, these observations suggest that P(i) may alter the level of skeletal ALP activity in SaOS-2 cells by inhibiting a process of irreversible inactivation that does not effect equal, concomitant changes in the level of skeletal ALP immunoreactive protein.