Pb2+ and imidazole-activated phosphorylation by ATP of (Na+ + K+)-ATPase.
In order to study whether Pb2+ and imidazole increase the ATP phosphorylation level of (Na+ + K+)-ATPase by the same mechanism, the effects of both compounds on phosphorylation and dephosphorylation reactions of the enzyme have been studied. Imidazole in the presence of Mg2+ increases steady-state phosphorylation of (Na+ + K+)-ATPase by decreasing, in a competitive way, the K+-sensitivity of the formed phospho-enzyme (E-P . Mg). If Pb2+ is present during phosphorylation, the rate of phosphorylation increases and a K+- and ADP-insensitive phosphointermediate (E-P . Pb) is formed. Pb2+ has no effect on the K+-sensitivity of E-P . Mg and EDTA is unable to affect the K+-insensitivity of E-P . Pb. These effects indicate that Pb2+ acts before or during phosphorylation with the enzyme. Binding of Na+ to E-P . Pb does not restore K+-sensitivity either. However, increasing Na+ during phosphorylation in the presence of Pb2+ leads to formation of the K+-sensitive intermediate (E-P . Mg), indicating that E-P . Pb is formed via a side path of the Albers-Post scheme. ATP and ADP decrease the dephosphorylation rate of both E-P . Mg and E-P . Pb. Above optimal concentration, Pb2+ also decreases the steady-state phosphorylation level both in the absence and in the presence of Na+. This inhibitory effect of Pb2+ is antagonized by Mg2+.
Swarts
H G
HG
Department of Biochemistry, University of Nijmegen, The Netherlands.
Zwartjes
H A
HA
Schuurmans Stekhoven
F M
FM
de Pont
J J
JJ
D000255:Adenosine Triphosphate / Q000378:metabolism*; D000818:Animals; D004789:Enzyme Activation / Q000187:drug effects; D007093:Imidazoles / Q000494:pharmacology*; D007679:Kidney Medulla / Q000201:enzymology; D007700:Kinetics; D007854:Lead / Q000494:pharmacology*; D008274:Magnesium / Q000494:pharmacology; D010766:Phosphorylation; D011188:Potassium / Q000494:pharmacology; D011817:Rabbits; D012964:Sodium / Q000494:pharmacology; D000254:Sodium-Potassium-Exchanging ATPase / Q000378:metabolism*
D007093:Imidazoles; D007854:Lead; C029899:imidazole; D000255:Adenosine Triphosphate; D012964:Sodium; D000254:Sodium-Potassium-Exchanging ATPase; D008274:Magnesium; D011188:Potassium