Sidedness of the ATP-Na+-K+ interactions with the Na+ pump in squid axons.
Using dialysed squid axons we have been able to control internal and external ionic compositions under conditions in which most of the Na+ efflux goes through the Na+ pump. We found that (i) internal K+ had a strong inhibitory effect on Na+ efflux; this effect was antagonized by ATP, with low affinity, and by internal Na+, (ii) a reduction in ATP levels from 3 mM to 50 microM greatly increased the apparent affinity for external K+, but reduced its effectiveness compared with other monovalent cations, as an activator of Na+ efflux, and (iii) the relative effectiveness of different K+ congeners as external activator of the Na+ efflux, though affected by the ATP concentration, was not affected by the Na+/K+ ratio inside the cells. These results are consistent with the idea that the same conformation of the (Na+ + K+)-ATPase can be reached by interaction with external K+ after phosphorylation and with internal K+ before rephosphorylation. They also stress a nonphosphorylating regulatory role of ATP.
D000255:Adenosine Triphosphate / Q000378:metabolism; D000818:Animals; D001369:Axons / Q000201:enzymology*; D001693:Biological Transport, Active; D049832:Decapodiformes; D010766:Phosphorylation; D011188:Potassium / Q000494:pharmacology; D011487:Protein Conformation; D012964:Sodium / Q000378:metabolism*; D000254:Sodium-Potassium-Exchanging ATPase / Q000378:metabolism*
D000255:Adenosine Triphosphate; D012964:Sodium; D000254:Sodium-Potassium-Exchanging ATPase; D011188:Potassium