Conformational changes of purified (Na+ + K+)-ATPase detected by a sulfhydryl fluorescence probe.

The fluorescent sulfhydryl reagent S-mercuric-N-dansyl cysteine (Dn-Cys-Hg+) has been used to label a purified preparation of the (Na+ + K+)-ATPase obtained from the electric organ of Electrophorous electricus. The labelled (Na+ +K+)-ATPase (ATP phosphohydrolase, EC 3.6.1.3), although reversibly inhibited, was capable of undergoing conformational changes associated with the active enzyme that could be monitored fluorometrically. The presence of ligands (Na+ + Mg2+ + ATP or Mg2+ + Pi) which are known to convert the enzyme from the E-1 state to the E-2-P state brought about large (97--100%) increases in fluorescence of the dimethylaminonaphthalene sulfonyl (Dn) label. An E-2 state could be achieved by the addition of Mg2+ which caused only a 32.3% increase in fluorescence over the E-1 state. Neither AMP nor TTP with or without Mg2+ or Na+ or Pi added without Mg2+ had any effect on the Dn fluorescence. If the enzyme was denatured, no fluorescence changes were observed. Small changes in the polarization of fluorescence of the Dn moiety were observed under all the conditions used. These small polarization changes and the large increases in the fluorescence intensity suggest that the enzyme can change conformational states in the presence of appropriate ligands and these conformational changes may take place in a relatively limited region of the protein's structure.