Alkaline phosphatase dissolves calcium pyrophosphate dihydrate crystals

Area of Science:

• Biochemistry
• Biomineralization
• Enzymology

Background:

• Calcium pyrophosphate dihydrate (CPPD) crystal deposition is linked to various arthropathies.
• The dissolution mechanisms of CPPD crystals are not fully understood.
• Yeast pyrophosphatase has previously demonstrated CPPD crystal dissolution capabilities.

Purpose of the Study:

• To investigate the efficacy of alkaline phosphatase (ALP) in dissolving CPPD crystals in vitro.
• To elucidate the specific enzymatic activity of ALP responsible for CPPD dissolution.
• To explore the influence of pH, magnesium, calcium, and matrix molecules on ALP-mediated CPPD dissolution.

Main Methods:

• In vitro incubation of CPPD crystals with purified alkaline phosphatase (ALP).
• Assays to measure pyrophosphatase (PPiase) and phosphoesterase activities of ALP.
• Assessment of CPPD dissolution rates under varying conditions (pH, presence of Mg2+, Ca2+, proteoglycans, chondroitin sulfate).

Main Results:

• Alkaline phosphatase (ALP) effectively dissolved calcium pyrophosphate dihydrate (CPPD) crystals in vitro.
• CPPD dissolution by ALP exhibited a pH optimum of 7.4, coinciding with its pyrophosphatase (PPiase) activity optimum.
• Both CPPD dissolution and PPiase activity of ALP were magnesium-dependent and inhibited by calcium, unlike its phosphoesterase activity.

Conclusions:

• The pyrophosphatase (PPiase) activity of alkaline phosphatase (ALP), not its phosphoesterase activity, is responsible for CPPD crystal dissolution.
• ALP demonstrates higher efficacy in dissolving CPPD crystals compared to soluble pyrophosphate, relative to yeast pyrophosphatase.
• Chondrocyte ALP may play a significant role in the physiological dissolution of CPPD crystals within cartilage.