Kinetic Analysis and Voltage Dependency of Alpha-3 Na+/K+-Pump

The Na+/K+-pump generates Na+ and K+ gradients across the plasma membrane, driving both various Na+-dependent co-transporting systems and the action potentials in excitable cells. There are three types of Na+/K+-pump in mammalian tissues. A general one, designated as al Na+/K+-ATPase, is ubiquitously distributed in all tissues. The second one, α2 Na+/K+-ATPase, exists in nerve, muscle, heart and adipocytes. The third type is α3 Na+/K+-ATPase, existing in central nervous tissues and the Purkinje fibers of mature animals. Despite the distinct tissue-specificity, no differences in enzyme characteristics among those three types have been found except in ouabain sensitivity. In order to get more information, we examined the kinetic characteristics of the al and the α3 enzymes. As the α3 enzyme is localized in the CNS with the α1 and α2 enzymes and the separation of α3 from α1 and α2 is very difficult, the precise nature of α3 has remained unclear. So, we established an α3 cDNA-transfected cell line, expressing 5–7 fold excess of α3 enzyme activity in relation to the endogenous α1 enzyme activity. We previously reported that α3 Na+/K+-ATPase expressed in the transformant showed a very high ouabain-sensitivity (Ki=1×10-7M), like that of rat brain Na+/K+-ATPase (4). Here we describe a difference in rate-limiting step between the al and α3 enzymes and suggest that α3 Na+/K+-ATPase may be regulated by the membrane potential.