Effects of the ATP, ADP and inorganic phosphate on the transport rate of the Na+,K+-pump.

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Abstract

(Na+ + 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 cD,1/2 = 0.1 mM. cD,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 (cP,1/2 = 14 mM).

Keyword(s)

Animals; Kinetics; Liposomes; Mathematics; Models, Biological; Phosphatidylcholines; Rabbits; Spectrometry, Fluorescence; enzymology: Kidney Medulla; metabolism: Na(+)-K(+)-Exchanging ATPase; pharmacology: Adenosine Diphosphate; pharmacology: Adenosine Triphosphate; pharmacology: Phosphates

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