Eosin fluorescence changes during Rb+ occlusion in the Na+/K(+)-ATPase

Area of Science:

• Biochemistry
• Enzyme Kinetics
• Membrane Transport Proteins

Background:

• Na+/K+-ATPase is a crucial ion pump in cell membranes.
• Understanding its conformational changes and ion binding is vital for cellular function.
• Rubidium (Rb+) serves as a potassium (K+) congener to study Na+/K+-ATPase mechanisms.

Purpose of the Study:

• To compare the effects of Rb+ on the time course and equilibrium of eosin fluorescence and Rb+ occlusion in Na+/K+-ATPase.
• To elucidate the relationship between fluorescence changes and Rb+ occlusion during enzyme conformational transitions.

Main Methods:

• Utilized partially purified Na+/K+-ATPase from pig kidney.
• Measured eosin fluorescence and Rb+ occlusion under identical conditions without ATP.
• Employed rapid-mixing techniques to analyze time courses and equilibrium states.

Main Results:

• Increased Rb+ concentration led to decreased fluorescence and increased Rb+ occlusion, following rectangular hyperbolas with similar half-maximal values.
• Equilibrium attainment involved a rapid initial phase beyond apparatus resolution, followed by at least two exponential time-dependent processes.
• Fluorescence changes exhibited a faster initial exponential component compared to Rb+ occlusion.

Conclusions:

• Fluorescence changes are coupled to Rb+ occlusion but represent a process that completes before Rb+ occlusion reaches equilibrium.
• This suggests distinct but coordinated conformational events within the Na+/K+-ATPase during ion binding.