Inhibition of Na+/K(+)-ATPase and Mg(2+)-ATPase by metal ions and prevention and recovery of inhibited activities by chelators.
Kinetics and inhibition of Na(+)/K(+)-ATPase and Mg(2+)-ATPase activity from rat synaptic plasma membrane (SPM), by separate and simultaneous exposure to transition (Cu(2+), Zn(2+), Fe(2+) and Co(2+)) and heavy metals (Hg(2+) and Pb(2+)) ions were studied. All investigated metals produced a larger maximum inhibition of Na(+)/K(+)-ATPase than Mg(2+)-ATPase activity. The free concentrations of the key species (inhibitor, MgATP(2-), MeATP(2-)) in the medium assay were calculated and discussed. Simultaneous exposure to the combinations Cu(2+)/Fe(2+) or Hg(2+)/Pb(2+) caused additive inhibition, while Cu(2+)/Zn(2+) or Fe(2+)/Zn(2+) inhibited Na(+)/K(+)-ATPase activity synergistically (i.e., greater than the sum metal-induced inhibition assayed separately). Simultaneous exposure to Cu(2+)/Fe(2+) or Cu(2+)/Zn(2+) inhibited Mg(2+)-ATPase activity synergistically, while Hg(2+)/Pb(2+) or Fe(2+)/Zn(2+) induced antagonistic inhibition of this enzyme. Kinetic analysis showed that all investigated metals inhibited Na(+)/K(+)-ATPase activity by reducing the maximum velocities (V(max)) rather than the apparent affinity (Km) for substrate MgATP(2-), implying the noncompetitive nature of the inhibition. The incomplete inhibition of Mg(2+)-ATPase activity by Zn(2+), Fe(2+) and Co(2+) as well as kinetic analysis indicated two distinct Mg(2+)-ATPase subtypes activated in the presence of low and high MgATP(2-) concentration. EDTA, L-cysteine and gluthathione (GSH) prevented metal ion-induced inhibition of Na(+)/K(+)-ATPase with various potencies. Furthermore, these ligands also reversed Na(+)/K(+)-ATPase activity inhibited by transition metals in a concentration-dependent manner, but a recovery effect by any ligand on Hg(2+)-induced inhibition was not obtained.
Krstić
Danijela
D
Department of Chemistry, School of Medicine, University of Belgrade, Visegradska, Belgrade, Serbia & Montenegro.
Krinulović
Katarina
K
Vasić
Vesna
V
D000818:Animals; D002614:Chelating Agents / Q000494:pharmacology*; D004492:Edetic Acid / Q000494:pharmacology*; D004791:Enzyme Inhibitors / Q000494:pharmacology*; D020128:Inhibitory Concentration 50; D007477:Ions / Q000737:chemistry; D007700:Kinetics; D008274:Magnesium / Q000378:metabolism*; D008297:Male; D019216:Metals, Heavy / Q000037:antagonists & inhibitors* / Q000737:chemistry / Q000494:pharmacology*; D051381:Rats; D017208:Rats, Wistar; D000254:Sodium-Potassium-Exchanging ATPase / Q000037:antagonists & inhibitors* / Q000378:metabolism; D013379:Substrate Specificity / Q000187:drug effects
D002614:Chelating Agents; D004791:Enzyme Inhibitors; D007477:Ions; D019216:Metals, Heavy; D004492:Edetic Acid; D000254:Sodium-Potassium-Exchanging ATPase; D008274:Magnesium