Spectroscopic Properties of 2‘-(or-3‘)-O-(2,4,6-Trinitrophenyl) Adenosine 5‘-Triphosphate Revealed by Time-Resolved Fluorescence Spectroscopy

A review of TNP-ATP in protein binding studies: benefits and pitfalls

We review 50 years of use of 2',3'-O-trinitrophenyl (TNP)-ATP, a fluorescently tagged ATP analog. It has been extensively used to detect binding interactions of ATP to proteins and to measure parameters of those interactions such as the dissociation constant, K_d, or inhibitor dissociation constant, K_i. TNP-ATP has also found use in other applications, for example, as a fluorescence marker in microscopy, as a FRET pair, or as an antagonist (e.g., of P2X receptors). However, its use in protein binding studies has limitations because the TNP moiety often enhances binding affinity, and the fluorescence changes that occur with binding can be masked or mimicked in unexpected ways. The goal of this review is to provide a clear perspective of the pros and cons of using TNP-ATP to allow for better experimental design and less ambiguous data in future experiments using TNP-ATP and other TNP nucleotides.

Analysis of Trinitrophenylated Adenosine and Inosine by Capillary Electrophoresis and γ-Cyclodextrin-Enhanced Fluorescence Detection

Monitoring molecules such as adenosine (Ado) and inosine (Ino) in the central nervous system has enabled the field of neuroscience to correlate molecular concentrations dynamics to neurological function, behavior, and disease. In vivo sampling techniques are commonly used to monitor these dynamics; however, many techniques are limited by the sensitivity and sample volume requirements of currently available detection methods. Here, we present a novel capillary electrophoresis-laser-induced fluorescence detection (CE-LIF) method that analyzes Ado and Ino by derivatization with 2,4,6-trinitrobenzenesulfonic acid to form fluorescent trinitrophenylated complexes of Ado (TNP-Ado) and Ino (TNP-Ino). These complexes exhibit ∼25-fold fluorescence enhancement upon the formation of inclusion complexes with γ-cyclodextrin (γ-CD). Association constants were determined as 4600 M(-1) for Ado and 1000 M(-1) for Ino by CE-LIF. The structure of the TNP-Ado:γ-CD complex was determined by 2D nuclear magnetic resonance (NMR) spectroscopy. Optimal trinitrophenylation reaction conditions and CE-LIF parameters were determined and resulted in the limit of detection of 1.6 μM for Ado and 4 μM for Ino. Ado and Ino were simultaneously quantified in homogenized rat forebrain samples to illustrate application of the technique. Simulated biological samples, desalted by ultrafiltration in the presence γ-CD, were concentrated on-capillary by large-volume sample stacking (LVSS) to achieve detection limits of 32 and 38 nM for TNP-Ado and TNP-Ino, respectively.

Characterization of the N-terminal domain of NrtC, the ATP-binding subunit of ABC-type nitrate transporter of the cyanobacterium Phormidium laminosum

The N-terminal domain of NrtC, the ATP-binding subunit of nitrate/nitrite ABC-transporter in the cyanobacterium Phormidium laminosum, has been expressed in Escherichia coli as a histidine-tagged fusion protein (His(6)NrtC1). Binding of ATP to the pure His(6)NrtC1 was characterized using the nucleotide analogue TNP-ATP [2'(3')-O-(2,4,6-trinitrophenyl) adenosine 5'-triphosphate]. Fluorescence assays showed that His(6)NrtC1 specifically binds Mg(2+) TNP-ATP with high affinity, binding being dependent on protein concentration. The presence of ATP prevents the covalent modification of His(6)NrtC1 by fluorescein 5'-isothiocyanate (FITC), suggesting that this probe reacts at the nucleotide-binding site of NrtC. The active form of the truncated NrtC is a dimer that shows high affinity for TNP-ATP (K(d)=0.76+/-0.1 microM). Evidence for the presence of two nucleotide-binding sites per dimer protein is given. Our results indicate that nucleotide binding is strongly dependent on the dimerization of NrtC and that the N-terminal domain of the protein contains the binding site for ATP. No ATPase activity catalyzed in vitro by the truncated subunit was detected.