Microenvironment of the high affinity ATP-binding site of Na+/K+-ATPase isslightly acidic

Fluorescein-5'-isothiocyanate (FITC) was used to study the high-affinity AT P-binding site of Na+/K+-ATPase. The molar ratio of specifically bound FITC per cy-subunit of Na+/K+-ATPase was found to be 0.5 as followed from pretr eatment experiments with another specific E(1)ATP-inhibitor Cr(H2O)(4)AdoPP [CH2]P. This indicated an existence of one high affinity ATP-binding site ( E(1)ATP-binding site) in the native (alpha beta)(2)-diprotomer of Na+/K+-AT Pase. Fluorescence dual-excitation ratio of specifically bound FITC reveale d that at external pH 7.5, the pH value inside the E(1)ATP-binding site is 6.95 +/- 0.18. In addition, FITC fluorescence quenching by anti-fluorescein and by iodide choline indicated the limited access of water into the small pocket of the E(1)ATP-binding site. (C) 1999 Academic Press.