Divalent cation interactions with Na,K-ATPase cytoplasmic cation sites:: Implications for the para-nitrophenyl phosphatase reaction mechanism

The interactions of divalent cations with the adenosine triphosphatase (ATPase) and para-nitrophenyl phosphatase (pNPPase) activity of the purified dog kidney Na pump and the fluorescence of fluorescein isothiocyanate (FITC)-labeled pump were determined. Sr2+ and Ba2+ did not compete with K+ for ATPase (an extracellular K+ effect). Sr2+ and Ba2+ did compete with Na+ for ATPase (an intracellular Na+ effect) and with K+ for pNPPase (an intracellular K+ effect). These results suggest that Ba2+ or Sr2+ can bind to the intracellular transport site, yet neither Ba2+ nor Sr2+ was able to activate pNPPase activity; we confirmed that Ca2+ and Mn2+ did activate. As another measure of cation binding, we observed that Ca2+ and Mn2+, but not Ba2+, decreased the fluorescence of the FITC-labeled pump; we confirmed that K+ substantially decreased the fluorescence. Interestingly, Ba2+ did shift the K+ dose-response curve. Ethane diamine inhibited Mn2+ stimulation of pNPPase (as well as K+ and Mg2+ stimulation) but did not shift the 50% inhibitory concentration (IC50) for the Mn2+-induced fluorescence change of FITC, though it did shift the IC50 for the K+-induced change. These results suggest that the Mn2+-induced fluorescence change is not due to Mn2+ binding at the transport site. The drawbacks of models in which Mn2+ stimulates pNPPase by binding solely to the catalytic site vs. those in which Mn2+ stimulates by binding to both the catalytic and transport sites are presented. Our results provide new insights into the pNPPase kinetic mechanism as well as how divalent cations interact with the Na pump.