Regulation of alkaline phosphatase: implications for calcium pyrophosphate dihydrate crystal dissolution and other alkaline phosphatase functions.

OBJECTIVE

Alkaline phosphatase (ALP), an enzyme with pyrophosphatase (PPiase) activity can dissolve calcium pyrophosphate dihydrate (CPPD) crystals. We studied the effects of enzyme inhibitors such as bisphosphonates, orthovanadate, calcium, cadmium, and ascorbic acid on PPiase activity of ALP as well as on phosphate ester hydrolysis (Pase) activity and compared these effects to those on CPPD crystal dissolution.

METHOD

An in vitro model system for crystal enzyme interaction was used to assess CPPD crystal dissolution.

RESULTS

Bisphosphonates inhibited ALP Pase activity more than ALP PPiase activity at the same concentrations. Calcium inhibited ALP PPiase activity, but not ALP Pase activity. Orthovanadate and cadmium inhibited ALP PPiase activity more than ALP Pase at the same concentrations. The inhibition rates of ALP PPiase at the same concentrations were orthovanadate > cadmium > calcium. Although ALP Pase activity was not inhibited, at high concentrations, ascorbic acid slightly inhibited ALP PPiase activity. Bisphosphonates at high concentrations inhibited ALP CPPD crystal dissolution. The strong inhibitory effects of bisphosphonates on ALP CPPD crystal dissolution compared to those on ALP PPiase activity suggest that bisphosphonates inhibit crystal dissolution by their affinity for the CPPD crystal surface. Calcium, orthovanadate, and cadmium inhibited ALP CPPD dissolution. The inhibition rates of ALP CPPD dissolution at the same concentrations were cadmium > calcium > orthovanadate. Ascorbic acid at high concentrations enhanced ALP CPPD dissolution.

CONCLUSION

These effects of different inhibitors on ALP PPiase and CPPD dissolution suggest that ALP CPPD crystal dissolution depends on binding of ALP CPPD crystals as well as the PPiase activity of the bound ALP. Because of its ubiquitous and broad phosphatase activity including PPiase activity, ALP may have a critical role in cell energy metabolism.