Influence of calcium, phosphate and complexing substances on inorganic pyrophosphatase in mineralizing hamster molars.

The influence of calcium and phosphorus on PP_i-ase activity in mineralizing hamster molars was studied in the presence and absence of several complexing substances. These complexing substances inhibited the PP_i-ase activity. Only Zn²⁺ completely reversed this inhibition. The Ca²⁺ stimulation of the PP_i-ase was substrate-dependent with an optimal activity at a Ca:PP_i ratio of 1. At higher concentrations, Ca²⁺ was strongly inhibitory, possibly by forming neutral salts with PP_i. The influence of complexing substances on the enzyme was modified by Ca²⁺, which changes the mol Ca²⁺:PP_i ratio or competes slightly with Zn²⁺ for the complexing agent. There was a small inhibition of the PP_i-ase activity by P_i which was not influenced by the substrate used (PP_i^4?, Mg-PP_i^2?or Ca-PP_i^2?. P_i can reverse the inhibiting effects of complexing substances to a small extent, except for stronger inhibitions obtained for 0.1 mM EHDP and 2mM Cl₂ MDP. These small in-vitro effects of Ca²⁺ and P_i on the PP_i-ase activity could not explain the great changes in PP_i-ase activity in mineralizing molars during normal development and at P_i depletion. This supports a stimulating role for the PP_i-ase in mineralization. It was concluded that, during mineralization, different Ca²⁺ and P_i concentrations can modify the inhibitory effects of the complexing substances on the PP_i-ase only to a very small extent.