Ecto-ADPase activity in the rat renal brush-border membranes

Brush-border membrane vesicles purified from rat kidney cortex exhibit an ectoenzyme activity responsible for the hydrolysis of both ATP and ADP, as well as of other nucleoside tri- and diphosphates. In the presence of Ca2+ ions, ADP hydrolysis follows the simple Michaelis-Menten kinetics assuming a single catalytic site. The real substrate for ADPase is a divalent cation conjugated ADP. The pH optimum for the hydrolysis is between 7.2 and 8.6. ADP and ATP hydrolysis show similar heat denaturation curves, and are both resistant to limited proteolysis and to inhibitors of other known ATPases. The enzyme activity is inhibited by: diethyl py- rocarbonate, dithiothreitol, high concentrations of both IV-ethylmale- imide and azide. The diethyl pyrocarbonate inhibition could be reversed by hydroxylamine, indicating the involvement of histidine and/or tyrosine residues in the reaction. It is proposed that both ADPase and ATPase activities reside within the same enzyme protein