Transmembrane 19F NMR chemical shift difference of fluorinated solutes in liposomes, erythrocytes and erythrocyte ghosts

Interaction of 5-fluorouracil loaded nanoparticles with 1,2-dimyristoyl-sn-glycero-3-phosphocholine liposomes used as a cellular membrane model

Nuclear magnetic resonance (NMR) spectroscopy and steady-state fluorescence anisotropy were used to study the behavior and interaction of 5-fluorouracil, both in a free form (5FU) and included in the polymer matrix of poly(butylcyanoacrylate) nanoparticles (5FUPBCN) with a phospholipid bilayer of large unilammellar vesicles composed of 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC), as a model system of biomembranes. The results confirm an interaction and penetration of 5FU into the phospholipid bilayer of DMPC liposomes. Different mechanisms of drug transfer from the aqueous environment into the model membrane environment, for the free drug and that incorporated into polymer nanoparticles, are suggested: (i) concentration-dependent reversible diffusion of the free 5FU and (ii) sustained 5FU release from nanoparticles adsorbed on the liposome surface resulting in continuous delivery of the drug into the phospholipid bilayers of the DMPC liposomes.

Protein (19)F NMR in Escherichia coli

Although overexpression and (15)N enrichment facilitate the observation of resonances from disordered proteins in Escherichia coli, (15)N enrichment alone is insufficient for detecting most globular proteins. Here, we explain this dichotomy and overcome the problem while extending the capability of in-cell NMR by using (19)F-labeled proteins. Resonances from small (approximately 10 kDa) globular proteins containing the amino acid analogue 3-fluoro-tyrosine can be observed in cells, but for larger proteins the (19)F resonances are broadened beyond detection. Incorporating the amino acid analogue trifluoromethyl-L-phenylalanine allows larger proteins (up to 100 kDa) to be observed in cells. We also show that site-specific structural and dynamic information about both globular and disordered proteins can be obtained inside cells by using (19)F NMR.

Detection of the non-steroidal anti-inflammatory drug niflumic acid in humans: a combined 19F-MRS in vivo and in vitro study

This study describes for the first time results of a (19)F-MRS study on humans exposed to the fluorinated non-steroidal anti-inflammatory drug niflumic acid. The accumulation and elimination of this commercially available selective prostaglandin synthase inhibitor is studied after an oral bolus in the human liver, in blood plasma and in urine samples. The in vivo spectra of the liver display two resonances with a similar increase in signal intensity during the investigation period of 240 min. One resonance refers to the parent compound niflumic acid (P), whereas the second resonance corresponds to a metabolite (M1) formed by the biotransformation by liver enzymes. The spectroscopic comparison with model compounds suggests 4'-hydroxyniflumic acid as the metabolite. During the entire experiment the concentration ratios of these resonances (P/M1) ranged between 0.7 and 0.9, indicating a high metabolite concentration most probably due to an efficient first pass metabolism. Both resonances (P, M1) were observed in the in vitro study of the blood plasma samples after plasma protein denaturation. However, in comparison to the liver spectra, the amount of the metabolite M1 is very small with a P/M1-ratio of 36.6 after 90 min and 16.1 after the end of measurement. This finding suggests an efficient biliary excretion of the metabolite M1, which bypasses the blood circulation system. Both resonances are also identified in the native urine samples. The signal intensity of the parent compound dominates the spectra of all urine samples, whereas the signal intensity of M1 increases slowly reaching a similar value to the parent compound P at the end of the measurement. This observation demonstrates an effective renal elimination of niflumic acid and suggests the existence of an enterohepatic circuit with a re-entry mechanism for the biliary excreted metabolite M1. In the urine spectra, an additional metabolite M2 is found. This resonance exhibits a low but constant signal intensity. The chemical origin of this metabolite is unclear.

19F NMR study of the uptake of 2'-fluoro-5-methyl-beta-L-arabinofuranosyluracil in erythrocytes: evidence of transport by facilitated and nonfacilitated pathways

The 19F NMR resonances of intra- and extracellular 2'-fluoro-5-methyl-beta-L-arabinofuranosyluracil (L-FMAU) in suspensions of human erythrocytes are well resolved. This phenomenon allows its transport behavior to be monitored in a 19F NMR time-course experiment. The rate of L-FMAU uptake at 25 degrees in a suspension containing L-FMAU at an initial extracellular concentration of 4 mM was 7.6 +/- 1.0 x 10(-7) pmol cell(-1) sec(-1) (N = 5). Concentration-dependent uptake studies of L-FMAU indicate the existence of both saturable and nonsaturable transport mechanisms, with a Km for the saturable uptake of approximately 1 mM. Although the transport of L-FMAU at 25 degrees was inhibited significantly (54-65%) by nitrobenzylthioinosine (NBTI) and dipyridamole, consistent with the participation of the nucleoside transporter, these inhibitors did not achieve complete blockage of L-FMAU uptake. The participation of the nucleobase transporter in L-FMAU uptake was ruled out by the absence of competition with uracil uptake, and by the lack of inhibition by papaverine. In addition, the NBTI-insensitive uptake of L-FMAU was not affected by pretreatment of the cells with the sulfhydryl reagent, p-chloromercuriphenylsulfonic acid (pCMBS). However, the NBTI- and dipyridamole-insensitive transport of L-FMAU was found to increase upon treatment of the erythrocytes with butanol, an agent that affects membrane fluidity. The partition coefficient of L-FMAU in octanol/phosphate-buffered saline determined by absorption spectrophotometry was 0.31. These data indicate that under the conditions of the studies, L-FMAU uptake by erythrocytes proceeds by both the nucleoside transporter and nonfacilitated membrane diffusion.

Anomeric dependence of fluorodeoxyglucose transport in human erythrocytes

The transport of several n-fluoro-n-deoxy-D-glucose derivatives across the human erythrocyte membrane has been studied under equilibrium exchange conditions using one- and two-dimensional nuclear magnetic resonance (NMR) techniques. This approach is based on the intracellular 19F shift, which was found to depend on the anomeric form and on the F/OH substitution position. Since the transport behavior of both glucose anomers can be followed simultaneously, this approach is particularly sensitive to differences in anomeric permeability. For 2-, 3-, 4-, and 6-fluorodeoxyglucose analogs, the alpha anomers permeate more rapidly, and the P alpha/P beta ratio is dependent on the position of fluorination, with values of 1.1, 1.3, 2.5, and 1.6, respectively, obtained at 37 degrees C. These results have been analyzed in terms of a simple alternating conformation model for the glucose transporter. Although mutarotase activity has been reported for red cells, mutarotation behavior for all anomers was found to be completely negligible on the transport and spin-lattice relaxation time scales. Metabolic transformation of the fluorinated glucose analogs, primarily to fluorinated gluconate and sorbitol analogs, is very slow and does not significantly interfere with the transport measurements. A mean ratio of 2.6 was found for the extracellular/intracellular fluorine spin-lattice relaxation rates.

Imidazol-1-ylalkanoic acids as extrinsic 1H NMR probes for the determination of intracellular pH, extracellular pH and cell volume

The synthesis and biological evaluation of a novel series of extrinsic probes for intracellular pH (pHi), extracellular pH (pHo) and cell volume determination by 1H NMR is described. Imidazol-1-ylacetate, malonate, 3-glutarate and 2-succinate esters were synthesized by reaction of imidazole with alpha-bromo esters or with alpha, beta-unsaturated esters. The corresponding acids were prepared by hydrolysis. Rat erythrocytes were readily permeable to methyl imidazol-1-ylacetate, moderately permeable to diethyl 2-imidazol-1-ylsuccinate and impermeable to diethyl 3-imidazol-1-yl-glutarate esters. Imidazol-1-ylacetic acid was the only acid derivative which penetrated the erythrocyte interior when added directly to the incubation medium. Transport of the permeable compounds to the erythrocyte interior was non-saturable up to 200 mM added compound. Addition of methyl imidazol-1-ylacetate or diethyl 2-imidazol-1-ylsuccinate esters to erythrocyte suspensions, resulted in hydrolysis to imidazol-1-ylacetic acid and 2-imidazol-1-ylsuccinic acid mono-ethyl ester in the intracellular and extracellular spaces, respectively. pHi and pHo were determined from the different chemical shifts of the H-2 proton of the acid derivatives in the intracellular (H-2i) and extracellular (H-2o) compartments. In addition, the relative intracellular and extracellular volumes could be calculated from the areas of the intracellular and extracellular H-2 resonances.