Inhibition of ion pump ATPase activity by 3'-O-(4-benzoyl)benzoyl-ATP (BzATP): assessment of BzATP as an active site-directed probe

Nucleotide binding to IAF-labelled Na+/K(+)-ATPase measured by steady state fluorescence quenching by TNP-ADP

Nucleotide binding to 5-iodoacetamidofluorescein (IAF) labelled Na+/K(+)-ATPase was measured by steady state fluorescence quenching of the fluorescein label via energy transfer to trinitrophenyl (TNP) labelled nucleotide. TNP-nucleotides are valuable probes of nucleotide binding to ATPases. Interpretation of these and other experiments in our laboratory using TNP-nucleotides with the Na+/K(+)-ATPase rely on having a good model for the interaction of TNP-nucleotide with the enzyme. Sets of fluorescence quenching curves obtained by titrating the enzyme with TNP-ADP in the presence of various concentrations of ADP could not be adequately modelled using a simple model with a single nucleotide binding site. Therefore, we compare various models which allow for additional TNP-nucleotide binding to the enzyme. In the two-site model, the additional binding is to a second specific site for which TNP-nucleotide and unlabelled nucleotide compete. In two other models, the additional binding (in one case saturable, and in the other case non-saturable) of TNP-nucleotide is not blocked by or affected by unlabelled nucleotide, and is, therefore, referred to as non-specific binding of the TNP-nucleotide. The goal of this work is to determine which of the distinctly different physical pictures associated with these models most accurately describes the interaction of TNP-nucleotide with the enzyme. We find that the interaction of TNP-ADP with IAF-labelled Na+/K(+)-ATPase is best described by a model in which there are two classes of binding: TNP-ADP and ADP compete for a specific binding site with dissociation binding constants of 0.13 microM for TNP-ADP and 2.0 microM for ADP; and non-saturable non-specific binding of TNP-ADP.

Binding of 2'(3')-O-(2,4-6-trinitrophenyl) ADP to soluble alpha beta protomers of Na, K-ATPase modified with fluorescein isothiocyanate. Evidence for two distinct nucleotide sites

The overall reaction of well-defined solubilized protomers of Na,K-ATPase (one alpha plus one beta subunit) retains the dual ATP dependence observed with the membrane-bound enzyme, with distinctive ATP effects in the submicromolar and submillimolar ranges (Ward, D. G., and Cavieres, J. D. (1993) Proc. Natl. Acad. Sci. U. S. A. 90, 5332-5336). We have now found that the K+/-phosphatase activity of the alpha beta protomers is still inhibited by 2'(3')-O-(2,4,6-trinitrophenyl)adenosine 5'-diphosphate (TNP-ADP). What is most significant is that the TNP-ADP effect can be observed clearly with protomeric enzyme whose high affinity ATP site has been blocked covalently with fluorescein isothiocyanate. We conclude that nucleotides can bind at two discrete sites in each protomeric unit of Na,K-ATPase.

Simultaneous binding of phosphate and TNP-ADP to FITC-modified NA+,K(+)-ATPase

Double-reciprocal plots of the rate of ATP hydrolysis by Na+,K(+)-ATPase versus ATP concentration are not linear, and may reflect either two distinct binding sites for ATP or a single ATP binding site whose affinity for the nucleotide alternates between high-affinity and low-affinity states. In order to determine whether multiple nucleotides or nucleotide analogs can bind simultaneously to Na,+,K(+)-ATPase, the effects of nucleotides on the hydrolysis of p-nitrophenyl phosphate and on the dephosphorylation rate of Na+,K(+)-ATPase modified by fluorescein 5'-isothiocyanate (FITC) were measured. FITC blocks the high-affinity binding site for ATP on the Na+K(+)-ATPase and inhibits ATP hydrolysis at ATP concentrations as high as 8.3 mM. The hydrolysis of p-nitrophenyl phosphate and phosphoenzyme formation from inorganic phosphate and Mg2+ were not affected by FITC modification. The p-nitrophenylphosphatase activity of unmodified Na+,K(+)-ATPase was stimulated by low concentrations of ATP (10-100 microM) and other nucleotides, and was inhibited at higher nucleotide concentrations. In contrast, there was no effect on p-nitrophenyl phosphate hydrolysis by FITC-modified Na,K(+)-ATPase at ATP concentrations less than 100 microM. The hydrolysis of p-nitrophenyl phosphate by FITC-modified Na+,K(+)-ATPase was inhibited at ATP concentrations greater than 100 microM. These observations demonstrate that the effects of ATP acting at high-affinity sites are absent in FITC-modified Na+,K(+)-ATPase but the effects of ATP acting at low-affinity sites are still observed. In unmodified Na+,K(+)-ATPase, the rate of dephosphorylation of the phosphoenzyme formed from inorganic phosphate and Mg2+ was inhibited by ATP.(ABSTRACT TRUNCATED AT 250 WORDS)

Permeabilization of transformed mouse fibroblasts by 3'-O-(4-benzoyl)benzoyl adenosine 5'-triphosphate and the desensitization of the process

A photoreactive analogue of ATP, 3'-O-(4-benzoyl)benzoyl adenosine 5'-triphosphate (BzATP) altered the plasma membrane permeability of transformed 3T6 mouse fibroblasts to normally impermeant molecules as previously reported for ATP, but at lower concentrations. BzATP-induced permeabilization was modulated by pH, temperature, and the ionic composition of the medium similar to the permeabilizing effects of ATP. Conditions known to enhance ATP-induced permeabilization, such as treatment with the mitochondrial uncoupler carbonyl cyanide-m-chlorophenylhydrazone (CCCP) or the Ca2+-calmodulin antagonist trifluoperazine also enhanced BzATP-induced permeabilization. Conditions inhibitory to ATP-induced permeabilization, including chloride replacement or treatment with furosemide or dithiothreitol (DTT), inhibited permeabilization induced by BzATP. The ionic strength of the medium modulated the responsiveness of the cells to ATP and BzATP; a decrease in the ionic strength below isotonicity increased the sensitivity of the cells to the nucleotides, whereas an increase in ionic strength above isotonicity inhibited permeabilization. Prolonged exposure to ATP under non-permeabilizing conditions caused the cells to become insensitive to ATP and BzATP. The densensitization phenomenon provides support for the theory that the permeabilization process is mediated by a receptor for ATP.