Palytoxin-induced Effects on Partial Reactions of the Na,K-ATPase

The interaction of palytoxin with the Na,K-ATPase was studied by the electrochromic styryl dye RH421, which monitors the amount of ions in the membrane domain of the pump. The toxin affected the pump function in the state P-E₂, independently of the type of phosphorylation (ATP or inorganic phosphate). The palytoxin-induced modification of the protein consisted of two steps: toxin binding and a subsequent conformational change into a transmembrane ion channel. At 20°C, the rate-limiting reaction had a forward rate constant of 10⁵ M⁻¹s⁻¹ and a backward rate constant of about 10⁻³ s⁻¹. In the palytoxin-modified state, the binding affinity for Na⁺ and H⁺ was increased and reached values between those obtained in the E₁ and P-E₂ conformation under physiological conditions. Even under saturating palytoxin concentrations, the ATPase activity was not completely inhibited. In the Na/K mode, ∼50% of the enzyme remained active in the average, and in the Na-only mode 25%. The experimental findings indicate that an additional exit from the inhibited state exists. An obvious reaction pathway is a slow dephosphorylation of the palytoxin-inhibited state with a time constant of ∼100 s. Analysis of the effect of blockers of the extracellular and cytoplasmic access channels, TPA⁺ and Br₂-Titu³⁺, respectively, showed that both access channels are part of the ion pathway in the palytoxin-modified protein. All experiments can be explained by an extension of the Post-Albers cycle, in which three additional states were added that branch off in the P-E₂ state and lead to states in which the open-channel conformation is introduced and returns into the pump cycle in the occluded E₂ state. The previously suggested molecular model for the channel state of the Na,K-ATPase as a conformation in which both gates between binding sites and aqueous phases are simultaneously in their open state is supported by this study.