Sodium Ion Discharge from Pig Kidney Na+, K+-ATPase Na+-Dependency of the E1P⇌E2P Equilibrium in the Absence of KCl1

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Abstract
The two phosphoenzymes (E1P and E2P) of Na+ K+were measured as ADP-sensitive and K+-sensitive fractions. The sum of these fractions was nearly 1 in the range of 50 to 1,200 mM NaCl. The effects of Na+ on the levels of E1P and E2P, on the rate constant of E2P→E1P transition (k2 on the rate constant of E2P dephosphorylation (k3) on the rate constant of E1P→E2P transition (k1 and on the apparent equilibrium constant between E1P and E2P (Kapp were examined. k1 was found to decrease with increasing Na+concentration, whereas k increased. Kapp was found to be directly proportional to the third power of Na+ concentration. k3 increased with increasing Na+ concentration and saturated at about 1 M NaCL. These results are consistent with a simple model in which ATP hydrolysis occurs through effectively only two phosphoenzyine intermediates in the absence of K+ and three sodium ions are discharged cooperatively from the enzyme during the E1P→E2P conversion.