Effect of skeletal alkaline phosphatase inhibitors on bone cell proliferation in vitro

Chicken skeletal alkaline phosphatase is subject to competitive inhibitions by vanadate (Ki = 0.38 mM in carbonate, Ki = 0.08 mM in Tris, both at pH 7.4) and phenylphosphonate (Ki = 15 mM in carbonate, Ki = 1.3 mM in Tris, both at pH 7.4), and uncompetitive inhibition by levamisole (Ki = 0.08 mM in carbonate, Ki = 0.10 mM in Tris, both at pH 7.4). The competitive inhibitors were more effective in Tris buffer because nonreactive ternary complexes were formed between alkaline phosphatase, the inhibitor and Tris. The effects of vanadate, phenylphosphonate and levamisole on the proliferation of embryonic chick calvarial cells in vitro were biphasic. Low doses of each agent stimulated 3H-thymidine incorporation into TCA-insoluble material; higher doses were inhibitory. Neither effect could be attributed to inhibition of alkaline phosphatase activity (e.g. 20 microM vanadate should inhibit alkaline phosphatase by 3% but stimulated cell proliferation by 187%; 50 microM vanadate should inhibit alkaline phosphatase by 7% but inhibited 3H-thymidine incorporation by 90%). None of the alkaline phosphatase inhibitors tested affected the cellular concentration of the enzyme during the 24-hour incubation. These studies indicate that alkaline phosphatase inhibitors can have nonspecific effects on bone cells in culture, and that for cells in the osteoblast cell line, an inhibition of alkaline phosphatase activity is not consistently related to a decrease in cell proliferation.