title: Ligand-dependent reactivity of (Na+ + K+)-ATPase with showdomycin.
abstract: Showdomycin inhibited pig brain (Na+ + K+)-ATPase with pseudo first-order kinetics. The rate of inhibition by showdomycin was examined in the presence of 16 combinations of four ligands, i.e., Na+, K+, Mg2+ and ATP, and was found to depend on the ligands added. Combinations of ligands were divided into five groups in terms of the magnitude of the rate constant; in the order of decreasing rate constants these were: (1) Na+ + Mg2+ + ATP, (2) Mg2+, Mg2+ + K+, K+ and none, (3) Na+ + Mg2+, Na+, K+ + Na+ and Na+ + K+ + Mg2+, (4) Mg2+ + K+ + ATP, K+ + ATP and Mg2+ + ATP, (5) K+ + Na + + ATP, Na+ + ATP, Na+ + K+ + Mg2+ + ATP and ATP. The highest rate was obtained in the presence of Na+, Mg2+ and ATP. The apparent concentrations of Na+, Mg2+ and ATP for half-maximum stimulation of inhibition (KS0.5) were 3 mM, 0.13 mM and 4 MicroM, respectively. The rate was unchanged upon further increase in Na+ concentration from 140 to 1000 mM. The rates of inhibition could be explained on the basis of the enzyme forms present, including E1, E2, ES, E1-P and E2-P, i. e., E2 has higher reactivity with showdomycin than E1, while E2-P has almost the same reactivity as E1-P. We conclude that the reaction of (Na+ + K+)- ATPase proceeds via at least four kinds of enzyme form (E1, E2, E1 . nucleotide and EP), which all have different conformations.
mesh_terms: D000255:Adenosine Triphosphate / Q000494:pharmacology; D000818:Animals; D000903:Antibiotics, Antineoplastic / Q000494:pharmacology*; D001921:Brain / Q000201:enzymology*; D005033:Ethylmaleimide / Q000494:pharmacology; D007700:Kinetics; D008024:Ligands; D008274:Magnesium / Q000494:pharmacology; D011188:Potassium / Q000494:pharmacology; D011485:Protein Binding; D012787:Showdomycin / Q000494:pharmacology*; D012964:Sodium / Q000494:pharmacology; D000254:Sodium-Potassium-Exchanging ATPase / Q000037:antagonists & inhibitors*; D013552:Swine
chemical_list: D000903:Antibiotics, Antineoplastic; D008024:Ligands; D000255:Adenosine Triphosphate; D012964:Sodium; D012787:Showdomycin; D000254:Sodium-Potassium-Exchanging ATPase; D008274:Magnesium; D005033:Ethylmaleimide; D011188:Potassium