Small differences in tryptophan fluorescence spectra of 'sodium' and 'potassium' forms of (Na+, K+)-dependent...

A detailed comparative analysis of tryptophan fluorescence spectra of 'sodium' and 'potassium' forms of (Na+, K+)-activated ATPase was carried out. The 'potassium' form spectrum is shifted relative to that of the 'sodium' form by approximately 0.5-1 nm towards shorter wavelengths. The maximal amplitude of the difference spectrum for these forms makes up about 2% of maximal fluorescence intensity of any of the forms. The shape of the difference spectrum does not depend on the solution temperature or ionic strength. The spectral differences between the forms are reversible upon addition of a functionally opposite cation (K+ for 'sodium' form and vice versa) into the medium. The results suggest that if the differences in fluorescence spectra of the 'sodium' and 'potassium' forms of (Na+, K+)-ATPase resulted from the differences in the protein structure, they may be caused by an alteration in local environment of no more than one or two tryptophan residues.