The active site structure of Na+- and K+-stimulated ATPase. Location of a specific fluorescein isothiocyanate reactive site.

Fluorescein 5'-isothiocyanate (FITC) has been shown to specifically inactivate the Na+- and K+-stimulated adenosine triphosphatase ((Na,K)-ATPase) at low concentrations (Karlish, S. J. D. (1979) Na+,K+ATPase Structure and Kinetics 115-128). The site of modification of purified dog kidney (Na,K)-ATPase by FITC has been investigated by enzymatic cleavage and fluorescence resonance energy transfer. The binding of FITC, which occurs at a stoichiometry of approximately one site per ATP binding site, causes an ATP-protectable inactivation of ATPase activity suggesting that it is reacting at the ATP hydrolysis site. The FITC reaction site apparently is located near the center of the COOH-terminal 77,000-dalton peptide fragment obtained by chymotryptic cleavage of the alpha subunit. Addition of ouabain to the native enzyme in the presence of chymotrypsin enhances cleavage at this site and releases the fluorescein moiety from the membrane. It is further shown that the distance from the FITC reaction site to the ouabain binding site, as judged by fluorescence resonance energy transfer from anthroyl ouabain to FITC, is approximately 74 A. These results demonstrate that ouabain inhibits the (Na,K)-ATPase by causing a protein conformational change which extends an unusually large distance across the membrane.