Na؉ /K ؉ -transport through mammalian cell membranes by Na ؉ /K ؉ -ATPase (EC 3.6.1.37) needs the interaction of ATP sites with different binding affinities during catalysis: one with catalytic (high affinity site) and one with regulatory properties (low affinity site). To find affinity labels for the latter one, the effects of 2-Odansylated ATP analogs on Na -ATPase binds ATP with high affinity to the sodium exporting E 1 -form (E 1 ATP binding site) and is consequently phosphorylated. After the release of sodium at the outer cell side and the following dephosphorylation, the enzyme needs a second binding of ATP. Therefore, ATP binds with low affinity to the E 2 -form (E 2 ATP binding site) of Na ϩ /K ϩ -ATPase and enhances the rate-limiting step of deocclusion of potassium during import (2, 3). In contrast to expectations deriving from this single ATP site model with its subsequent formation of the ATP sites, use of substitution-inert MgATP complex analogs has led to the postulate of a coexistence (in time and at different places) of both ATP sites (4). The Repke-Schön-Stein model (5) attempts to explain such a situation by shifting the energy excess of the sodium-transporting subunit to the potassium-transporting subunit. Each subunit follows a whole Albers-Post cycle but 180°out of phase. The bicyclic model of Plesner, on the other hand, gets its power from two ATP binding sites whose partial activities (Na ϩ -ATPase, K ϩ -phosphatase) are lower than the overall reaction (Na ϩ /K ϩ -ATPase). A single subunit does not have to pass all of the intermediates of the Albers-Post circle, but the sum fulfills all steps required for a whole turnover (6). However, there is still a lot of discussion about the intermediates shared by the partial reactions and the overall reaction (7,8).Substitution-inert MgATP complex analogs like CrATP 1 or CoATP are helpful tools to dissect the overall Na ϩ /K ϩ -ATPase activity by specific modifications of either the E 1 ATP site or the E 2 ATP site (9, 10). The activities of the E 1 ATP binding site (for example ATP/ADP exchange and "frontdoor phosphorylation") are unaffected by the inactivation of the E 2 ATP binding site by Co(NH 3 ) 4 PO 4 (11). Similarly, CrAMP-PCP, which inactivates the E 1 ATP site but is unable to phosphorylate it, does not affect activities of the E 2 ATP site, namely 86 Rb ϩ occlusion, K ϩ -activated phosphatase activity, and "backdoor phosphorylation" (12, 13). Although substitution-inert metal ATP complexes are on the one hand helpful tools to get information on basic prop-