The Na+/K+ pump is involved in the regulation of cell osmolarity, electrical activity, and the energized transport of ions and solutes across the plasma membrane. Our laboratory utilizes high-resolution, time-resolved methods to characterize the dynamic behavior of the enzymatic, electrical and conformational reactions catalyzed by the Na+/K+ pump. Our objective is to understand how these events are coordinated in order to accomplish active Na+ and K+ transport by this pump. Detergent-solubilized membrane fragments containing eel electric organ Na+/K+-ATPase have been used to identify the conformational interactions between catalytic (a) subunits during ATP-dependent cycling. Native membranes exhibit similar steady state levels of the principal Na+/K+-ATPase phosphoenzyme intermediates, E1P and E2P, despite the rapid conversion of E1P to E2P and slow hydrolysis of E2P (which favor E2P accumulation). Exposure to detergent, which produces a monomers, depletes E1P by increasing its turnover rate. This implies that the steady state formation of E1P is stabilized by intersubunit conformational interactions which are relieved by detergents. These quaternary interactions may have a critical role in energy conservation during the transport cycle. Computer analysis of the enzymatic reactions and transient state pump currents produced by Na+/K+-ATPase-containing membrane fragments from electric organ and pig kidney have demonstrated that electrogenic Na+ transport is delayed with respect to K+-activated E2P hydrolysis. This implies that K+ is bound to the pump and is translocated before Na+ is released. This behavior conflicts with the traditional pump model (Albers-Post scheme) in which K+ binding takes place after Na+ is released (ping-pong mechanism).
