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.