Inhibition of an ecto-ATP-diphosphohydrolase by azide

Area of Science:

• Biochemistry
• Enzymology
• Cell Biology

Background:

• Cell surface ATPases (E-ATPases) regulate extracellular nucleotide levels.
• E-ATPases are classified into ecto-ATPases and ecto-ATP-diphosphohydrolases (ecto-ATPDases).
• Ecto-ATPDases, like CD39-related proteins, hydrolyze nucleoside diphosphates and triphosphates, and are azide-sensitive.

Purpose of the Study:

• To conduct the first systematic kinetic study of purified chicken oviduct ecto-ATPDase.
• To investigate the enzyme's kinetics with ATP and ADP substrates.
• To analyze the inhibition patterns of azide, fluoride, vanadate, and pyrophosphate.

Main Methods:

• Purification of chicken oviduct ecto-ATPDase.
• Enzymatic assays measuring ATP and ADP hydrolysis rates.
• Kinetic analysis of substrate affinity (Km) and catalytic efficiency (Vmax).
• Assessment of inhibition by various compounds (azide, fluoride, vanadate, pyrophosphate) under different conditions (pH, divalent ions).

Main Results:

• Chicken oviduct ecto-ATPDase exhibited lower Km values and higher catalytic efficiency for ATP compared to ADP.
• Azide inhibition was complex and dependent on nucleotide substrate, divalent ion, and pH.
• Nearly complete azide inhibition occurred with MgADP at pH 6-6.4, while inhibition was reduced with ATP, Ca2+, and higher pH.
• Azide decreased both Vmax and Km for ADP, but only affected Vmax for ATP, suggesting differential binding modes.
• Other inhibitors (fluoride, vanadate, pyrophosphate) also showed substrate and pH-dependent responses.

Conclusions:

• The chicken oviduct ecto-ATPDase demonstrates clear discrimination between ATP and ADP substrates.
• Azide inhibition mechanisms differ for ATP and ADP hydrolysis, indicating distinct enzyme-ligand interactions.
• The enzyme's response to inhibitors suggests a specific binding affinity for the enzyme-ADP complex over the enzyme-ATP complex.