Inhibition of alkaline phosphatase by cysteine: implications for calcium pyrophosphate dihydrate crystal deposition disease.

OBJECTIVE

Calcium pyrophosphate dihydrate (CPPD) crystal deposition disease, a common arthritis affecting the elderly, is characterized by the deposition of CPPD crystals in articular joints. The mechanism underlying disease expression is unknown, but factors contributing to the pathogenesis may involve changes in enzymatic activities involving pyrophosphate and phosphate metabolism. Tissue nonspecific alkaline phosphatase (TNAP) is one of the major enzymes regulating pyrophosphate concentrations in articular joints. We hypothesized that inhibition of TNAP activity at pH = 7.4 by endogenous molecules can lead to CPPD disease pathogenesis.

METHODS

We investigated the inhibitory effects of the amino acid cysteine on TNAP's phosphatase, inorganic pyrophosphatase, and CPPD crystal dissolution activities. Kinetic parameters V(max), K(M), concentration for 50% inhibition (I(50)), inhibitor constant (K(I)), and specific activities calculated from Initial Velocity, Eadie-Hofstee, Simple, Dixon, and Secondary plots were used to assess enzyme inhibition.

RESULTS

Cysteine inhibited TNAP's phosphatase activity uncompetitively and its inorganic pyrophosphatase activity mix-competitively. CPPD crystal dissolution activity was also inhibited. I(50) values demonstrated that high cysteine concentration is required to inhibit 50% of enzyme activity. K(I) values suggested that inorganic pyrophosphatase activity is inhibited more than the phosphatase activity. Ca(++) and Mg(++) ion concentrations may regulate this inhibition.

CONCLUSION

The control of endogenous inhibitors, such as cysteine, that interfere with TNAP's ability to regulate CPPD crystal formation and dissolution in joints could be a potential therapeutic option for CPPD crystal deposition disease.