Reversible inhibition of (Na+,K+ )-dependent ATPase by Mg2+ ion, adenosine triphosphate, and K+

Abstract

ATP hydrolysis catalyzed by the plasma membrane (Na+,K+)ATPase isolated from several sources [lamb kidney, rabbit sarcoplasmic reticulum, bovine heart mitochondrial and Electrophorus electricus electroplax] was inhibited by Mg2+, provided that K+ and ATP were also present. Phosphorylation of the ATPase by ATP and by Pi was also inhibited, as was p-nitrophenyl phosphatase activity. EDTA and catecholamines protected and reversed the inhibition of ATP hydrolysis by Mg2+, K+ and ATP. EDTA was protected by chelation of Mg2+ but catecholamines acted by some other mechanism. The specificities of various nucleotides as inhibitors (in conjunction with Mg2+ and K+) and as substrates for the (Na+,K+)ATPase were strikingly different. ATP, ADP, .beta.,.gamma.-CH2-ATP and .alpha.,.beta.-CH2-ADP were active as inhibitors; ITP, CTP, UTP, GTP and AMP were not. ATP and CTP were substrates and .beta.,.gamma.-NH-ATP was a competitive inhibitor of ATP hydrolysis, but not an inhibitor in conjunction with Mg2+ and K+. The Ca2+-ATPase from sarcoplasmic reticulum and F1, the Mg2+-ATPase from the inner mitochondrial membrane, were also inhibited by Mg2+. Catecholamines reversed inhibition of the Ca2+-ATPase, but not that of F1.