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Physiological Laboratory, University of Cambridge, Cambridge CB2 3EG

1. The occlusion of rubidium ions by Na, K-ATPase has been investigated by suspending enzyme prepared from pig kidney outer medulla in media containing low concentrations of ⁸⁶Rb, forcing the suspensions rapidly through small columns of cation-exchange resin, and measuring the amounts of radioactivity emerging from the columns.

2. When the suspension media contained 2 mM-ATP or ADP, or 15 mM-NaCl, the amounts of radioactivity emerging from the columns were greatly (and similarly) reduced, presumably because both nucleotides and sodium ions stabilized the enzyme in the E₁ form. (See p. 19 for definition of E₁ and E₂). The extra radioactivity carried through the columns when nucleotides and sodium were absent was taken as a measure of the amount of rubidium occluded within the enzyme (in the E₂ form) when it emerged from the resin.

3. By varying the flow rate, and therefore the time spent by the enzyme on the resin, and relating this to the amount of radioactivity emerging from the columns, we have been able to estimate the rate constant for the conformational change (E₂ E₁) that allows the occluded rubidium ions to escape. At 20 °C, and in the absence of nucleotides, it is about 0·1 S^-1.

4. The rate constant for rubidium release was the same in a sodium-containing as in a potassium-containing medium. The opposite effects of sodium and potassium ions on the poise of the equilibrium between the E₁ and the E₂ forms of the enzyme must, therefore, be due solely to opposite effects of these ions on the rate of conversion of E₁ to E₂.

5. The rate constant for rubidium release was greatly increased by ATP and by ADP. Both nucleotides appeared to act at low-affinity sites and without phosphorylating the enzyme.

6. Orthovanadate, in the presence of magnesium ions, stabilized the enzyme in the occluded-rubidium (E₂Rb) form.

7. Ouabain, in the presence of magnesium ions, prevented the occlusion of rubidium ions.

8. We have measured the amount of rubidium occluded by the enzyme as a function of rubidium concentration, and estimate that at saturating rubidium concentrations about three rubidium ions can be occluded per phosphorylation site (or per ouabain-binding site).

9. We have found that the occluded-rubidium form of the enzyme can also be formed by allowing rubidium ions to catalyse the hydrolysis of phosphoenzyme generated by the addition of ATP to enzyme suspended in a high-sodium medium.

10. The properties of the occluded-rubidium form of the enzyme, and of the two routes that can lead to its formation, suggest that an analagous occluded-potassium form plays a central role in the transport of potassium ions through the sodium—potassium pump. This hypothesis is supported by a detailed consideration of the probable magnitudes of the rate constants of the individual reactions making up the two routes.

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