engleski

Ecto-ATPases of the kidney

Evidence is presented that brush-border membrane vesicles purified from homogenates of rat kidney cortex posses an ecto-enzyme activity that is responsible for the hydrolysis of both ATP and ADP, as well as for the hydrolysis of other nucleoside tri- and diphosphates. The enzyme could not be classified as ADP diphosphohydrolase (EC 3.6.1.5) or as ATPase (EC 3.6.1.3) for it is sharing some characteristics with both enzymes. The ADP and ATP hydrolyzing activities both have broad pH optimum, between 7.2 and 8.6. In the presence of Ca2+ ions ADP hydrolysis is following the simple Michaelis-Menten kinetics assuming a single catalytic site. However, in the absence of the divalent cations the activity was reduced to a insignificant levels. Results suggest that the true substrate for ADPase is a divalent cation conjugated ADP. It was found that Ca2+ was more stimulating than Mg2+. Denaturation experiments, that involved treatment of vesicles with trypsin and papain , as well as heat denaturation curves, indicated that only one enzyme was involved in the hydrolysis of nucleoside di- and triphosphates. ADP and ATP hydrolysis was found to be resistant to limited proteolysis, while detergents induced strong inhibition. The inhibitors of other known ATPases i.e. ouabain, levamisole, oligomycin, vanadate and concanavalin A, respectively, did not affect the rate of hydrolysis. However, hydrolysis of both ADP and ATP was inhibited by: diethyl pyrocarbonate, dithiothreitol, high concentrations of both N-ethylmaleimide and azide, indicating that histidine and sulphydrile groups may be essential for the catalytic reaction. The inhibition produced by diethyl pyrocarbonate was reversed by subsequent treatment with hydroxylamine supporting importance of histidine and/or tyrosine residues for the ecto-ADPase and ecto-ATPase activities. Based on the results presented it is proposed that both ADPase and ATPase activities reside within the same protein which is capable to hydrolyze extracellular ATP to AMP yielding eventually adenosine in the presence of 5'nucleotidase.

kidney; ecto-ATPases; brush-border membranes

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