Effects of lipophilic ions, tetraphenylphosphonium (TPP) and tetraphenylboron (TPB), on interactions of Na and K with Na,K-ATPase were studied with membrane-bound enzyme from bovine brain, pig kidney, and shark rectal gland.

Na and K interactions with the inward-facing binding sites, monitored by eosin fluorescence and phosphorylation, were not influenced by lipophilic ions.

Phosphoenzyme interactions with extracellular cations were evaluated through K-, ADP-, and Na-dependent dephosphorylation. TPP decreased: 1) the rate of transition of ADP-insensitive to ADP-sensitive phosphoenzyme, 2) the K affinity and the rate coefficient for dephosphorylation of the K-sensitive phosphoenzyme, 3) the Na affinity and the rate coefficient for Na-dependent dephosphorylation. Pre-steady state phosphorylation experiments indicate that the subsequent occlusion of extracellular cations was prevented by TPP. TPB had opposite effects.

Effects of lipophilic ions on the transition between phosphoenzymes were significantly diminished when Na was replaced by N-methyl-D-glucamine or Tris, but were unaffected by the replacement of Cl by other anions.

Lipophilic ions affected Na-ATPase, Na,K-ATPase, and p-nitrophenylphosphatase activities in accordance with their effects on the partial reactions.

Effects of lipophilic ions appear to be due to their charge indicating that Na and K access to their extracellular binding sites is modified by the intramembrane electric field.

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