ATPases depend on divalent metal ions, including Ca²⁺, Mg²⁺, and Zn²⁺, and we found that depletion of these ions blocked the hydrolysis of ATP and the formation of adenosine in human blood (...)
We added the metal chelator ethylenediaminetetraacetic acid (EDTA) to human whole blood as a model to examine the roles of divalent metal ions in the conversion of plasma ATP and the regulation of PMN functions. EDTA added to whole blood resulted in a dose-dependent accumulation of extracellular ATP, which was paralleled by a decrease in plasma adenosine levels (Fig. 9A). These results demonstrate that divalent metal ions are needed to convert extracellular ATP to adenosine, which can be internalized by cells.
EDTA concentrations as low as 0.5 mM had a significant impact on ATP hydrolysis in human blood. Interestingly, equivalent EDTA concentrations had little to no effect on the corresponding plasma Ca2+ and Mg2+ levels (Fig. 9B).
Removal of divalent ions with EDTA not only blocked ATP breakdown but also increased the activation of ROS production (Fig. 9C), CD63 expression (Fig. 9D), and CD11b and CD66b expression (data not shown) in response to PMN stimulation. EDTA concentrations of up to 0.5 mM enhanced those PMN responses, while higher EDTA concentrations caused suppression, likely due to complete chelation of free Ca2+ ions (see Fig.9B) that are indispensable for cell activation. Addition of Zn2+ to mouse plasma that had been depleted of divalent ions with EDTA dose dependently recovered ATPase activity (Fig. 9E). These findings support the concept that Zn2+ has a central role as a co-enzyme that helps control ATP levels in mouse plasma.
Taken together, these findings suggest that metal ions such as Ca2+, Mg2+, and particularly Zn2+ play important roles in the regulation of extracellular ATP and adenosine levels in mouse and human blood.