Abstract
( rmNa + + K +)- ATPase from kidney outer medulla was incorporated into artificial dioleoylphosphatidylcholine vesicles. In the reconstituted system the pump can be activated by adding ATP to the external medium. ATP-driven potassium extrusion by the Na +,K +-pump was studied using a voltage-sensitive dye in the presence of valinomycin. ADP strongly reduced the turnover rate of the pump with a concentration for half-maximal inhibition of c D, 1 2 = 0.1 mM . c D, 1 2 was found to be virtually independent of ATP concentration, indicating that the inhibition is non-competitive with respect to ATP. The non-competitive inhibition by ADP can be explained on the basis of the Post-Albers reaction cycle of the Na +,K +-pump, assuming that the main action of ADP is the reversal of the phosphorylation step. A similar ‘product inhibition’ was observed with inorganic phosphate, but at much higher concentrations ( c p, 1 2 = 14 mM ).
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