Abstract
An ATPase activity stimulated by divalent ions (Mg 2+, Ca 2+, Mn 2+, Zn 2+) has been observed in intact hamster fibroblasts cultured in vitro (BHK line). Such activity has been determined by the incubation (30 min at 37°C) of washed cell suspensions (about 1 mg of proteins) in a medium containing 100 mM NaCl, 20 mM KCl, 15 mM Tris—HCl (pH 7.4), 10 mM NaHCO 3, 5 mM glucose and equimolar concentrations of ATP and divalent cation. Mg 2+-ATPase activity is insensitive to ouabain and lacks specificity towards nucleoside triphosphate substrates. AMP and ADP are not hydrolyzed under these conditions. Apparent K m of 0.76 mM and V max of 1.46 μmol Pi · mg proteins −1 · h −1 have been calculated for Mg-ATP complex. This ATPase is an ectoenzyme, therefore its activity could be used as a suitable index of the action of chemicals like chromium compounds known for their cytotoxic effects on membrane functions. Salts of trivalent (CrCl 3) and hexavalent (K 2Cr 2O 7) chromium at concentrations ranging from 1 mM to 5 mM inhibit Mg 2+-ATPase. The inhibition by K 2Cr 2O 7 is observed after pretreatment of the cells with this compound followed by its absence from the assay medium “per se” for Mg 2+-ATPase, and it is referred to the alterations of membrane bound enzyme structures by the oxidizing hexavalent chromium. The inhibition by CrCl 3 is mainly evident when this compound is present in the incubation medium, and is referred to the interaction of trivalent chromium with Mg 2+-ATP as it is partially reversed by increasing Mg 2+-ATP concentration.
Published Version
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