Figure 2. Isotopic measurements of the substitution of bound ⁸⁶Rb⁺ with K⁺ congeners.

Substitution of ⁸⁶Rb⁺ in shark rectal gland Na⁺,K⁺-ATPase in E2·MgF₄²⁻·2Rb⁺ with Rb⁺ (a) or Tl⁺ (b) at 20 ^oC. The lines represent single exponential fits to the measurements in the absence of ouabain (circles) up to 30 s with time constants of 14±1 s (a) and 17±2 s (b), assuming that only half of the bound Rb⁺ is substituted. Broken line and squares in a show Rb⁺ substitution with the preformed ATPase–ouabain complex measured after 2 h of incubation. In the normalization of the data, zero is taken at 10%, corresponding to the dilution of ⁸⁶Rb⁺ in the dissociation buffer. Note that substitution ceases within 30 s when approximately half of the initially bound ⁸⁶Rb⁺ has been substituted.