Pb 2+ and imidazole-activated phosphorylation by ATP of [formula omitted

Abstract

In order to study whether Pb 2+ 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 Mg 2+ increases steady-state phosphorylation of ( Na + + K +)- ATPase by decreasing, in a competitive way, the K +-sensitivity of the formed phosoho-enzyme ( E-P · Mg ). If Pb 2+ is present during phosphorylation, the rate of phosphorylation increases and a K +- and ADP-insensitive phosphointermediate ( E-P · Pb ) is formed. Pb 2+ 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 Pb 2+ 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 Pb 2+ 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, Pb + also decreases the steady-state phosphorylation level both in the absence and in the presence of Na +. This inhibitory effect of Pb 2+ is antagonized by Mg 2+.