title: Inhibition of smooth muscle 5'-nucleotidase by imidazole and its reversal by magnesium.
abstract: Imidazole, commonly used as an effective pH-buffering reagent in aqueous media maintained at pH 7-8, was found to depress the 5'-nucleotidase (5'-ribonucleotide phosphohydrolase, EC 3.1.3.5) activity of microsomal membrane fraction isolated from rat vas deferens smooth muscle in a dose-dependent manner in the absence of added Mg2+. Such an inhibitory effect of imidazole on the smooth muscle 5'-nucleotidase was not dependent upon the purity or integrity of the membrane fractions used and could be fully reversed by the inclusion of 5-10 mM Mg2+ in the assay medium. Of the five different pH-buffering reagents tested, imidazole was specific in exerting inhibitory effect on the 5'-nucleotidase in the absence of Mg2+ and this inhibition could not be accounted for by the impurities present in the imidazole. Differential effects of chelating reagents and other divalent metal ions on the 5'-nucleotidase activity were also observed in imidazole and Tris buffer solutions. The 5'-nucleotidase activity was not affected if the membranes were preincubated and washed with a large volume of 50 mM imidazole and subsequently assayed in 50 mM Tris in the absence of Mg2+. Similar findings were obtained with EDTA treated membrane. These results suggest that imidazole does not act by removal of the activating metal ion but rather interacts directly with 5'-nucleotidase and alters the metal-enzyme interactions.
mesh_terms: D015720:5'-Nucleotidase; D000818:Animals; D002021:Buffers / Q000494:pharmacology; D002413:Cations, Divalent / Q000494:pharmacology; D002462:Cell Membrane / Q000201:enzymology; D002614:Chelating Agents / Q000494:pharmacology; D003593:Cytoplasm / Q000201:enzymology; D007093:Imidazoles / Q000037:antagonists & inhibitors / Q000494:pharmacology*; D066298:In Vitro Techniques; D008274:Magnesium / Q000494:pharmacology*; D008297:Male; D008861:Microsomes / Q000201:enzymology; D009130:Muscle, Smooth / Q000201:enzymology*; D009708:Nucleotidases / Q000037:antagonists & inhibitors*; D051381:Rats; D014649:Vas Deferens / Q000201:enzymology
chemical_list: D002021:Buffers; D002413:Cations, Divalent; D002614:Chelating Agents; D007093:Imidazoles; C029899:imidazole; D009708:Nucleotidases; D015720:5'-Nucleotidase; D008274:Magnesium