Interaction of benzoic acid derivatives with the transport system of glucose in human erythrocytes

Resolution of the direct interaction with and inhibition of the human GLUT1 hexose transporter by resveratrol from its effect on glucose accumulation

Resveratrol acts as a chemopreventive agent for cancer and as a potential antiobesity and antidiabetic compound, by leading to reduced body fat and improved glucose homeostasis. The exact mechanisms involved in improving hyperglycemic state are not known, but most of the glucose uptake into mammalian cells is facilitated by the GLUT hexose transporters. Resveratrol is structurally similar to isoflavones such as genistein, which inhibit the glucose uptake facilitated by the GLUT1 hexose transporter. Here we examined the direct effects of resveratrol on glucose uptake and accumulation in HL-60 and U-937 leukemic cell lines, which express mainly GLUT1, under conditions that discriminate transport from the intracellular substrate phosphorylation/accumulation. Resveratrol blocks GLUT1-mediated hexose uptake and thereby affects substrate accumulation on these cells. Consequently, we characterized the mechanism involved in inhibition of glucose uptake in human red cells. Resveratrol inhibits glucose exit in human red cells, and the displacement of previously bound cytochalasin B revealed the direct interaction of resveratrol with GLUT1. Resveratrol behaves as a competitive blocker of glucose uptake under zero-trans exit and exchange kinetic assays, but it becomes a mixed noncompetitive blocker when zero-trans entry transport was assayed, suggesting that the binding site for resveratrol lies on the endofacial face of the transporter. We propose that resveratrol interacts directly with the human GLUT1 hexose transporter by binding to an endofacial site and that this interaction inhibits the transport of hexoses across the plasma membrane. This inhibition is distinct from the effect of resveratrol on the intracellular phosphorylation/accumulation of glucose.

Noncompetitive blocking of human GLUT1 hexose transporter by methylxanthines reveals an exofacial regulatory binding site

Glucose transporter (GLUT)1 has become an attractive target to block glucose uptake in malignant cells since most cancer cells overexpress GLUT1 and are sensitive to glucose deprivation. Methylxanthines are natural compounds that inhibit glucose uptake; however, the mechanism of inhibition remains unknown. Here, we used a combination of binding and glucose transport kinetic assays to analyze in detail the effects of caffeine, pentoxifylline, and theophylline on hexose transport in human erythrocytes. The displacement of previously bound cytochalasin B revealed a direct interaction between the methylxanthines and GLUT1. Methylxanthines behave as noncompetitive blockers (inhibition constant values of 2–3 mM) in exchange and zero- trans efflux assays, whereas mixed inhibition with a notable uncompetitive component is observed in zero- trans influx assays (inhibition constant values of 5–12 mM). These results indicate that methylxanthines do not bind to either exofacial or endofacial d-glucose-binding sites but instead interact at a different site accessible by the external face of the transporter. Additionally, infinite- cis exit assays (Sen-Widdas assays) showed that only pentoxifylline disturbed d-glucose for binding to the exofacial substrate site. Interestingly, coinhibition assays showed that methylxanthines bind to a common site on the transporter. We concluded that there is a methylxanthine regulatory site on the external surface of the transporter, which is close but distinguishable from the d-glucose external site. Therefore, the methylxanthine moiety may become an attractive framework for the design of novel specific noncompetitive facilitative GLUT inhibitors.

Endofacial competitive inhibition of the glucose transporter 1 activity by gossypol

Gossypol is a natural disesquiterpene that blocks the activity of the mammalian facilitative hexose transporter GLUT1. In human HL-60 cells, which express GLUT1, Chinese hamster ovary cells overexpressing GLUT1, and human erythrocytes, gossypol inhibited hexose transport in a concentration-dependent fashion, indicating that blocking of GLUT1 activity is independent of cellular context. With the exception of red blood cells, the inhibition of cellular transport was instantaneous. Gossypol effect was specific for the GLUT1 transporter since it did not alter the uptake of nicotinamide by human erythrocytes. Gossypol affects the glucose-displaceable binding of cytochalasin B to GLUT1 in human erythrocyte ghost in a mixed noncompetitive way, with a Kivalue of 20 μM. Likewise, GLUT1 fluorescence was quenched ∼80% by gossypol, while Stern-Volmer plots for quenching by iodide displayed increased slopes by gossypol addition. These effects on protein fluorescence were saturable and unaffected by the presence of d-glucose. Gossypol did not alter the affinity of d-glucose for the external substrate site on GLUT1. Kinetic analysis of transport revealed that gossypol behaves as a noncompetitive inhibitor of zero- trans (substrate outside but not inside) transport, but it acts as a competitive inhibitor of equilibrium-exchange (substrate inside and outside) transport, which is consistent with interaction at the endofacial surface, but not at the exofacial surface of the transporter. Thus, gossypol behaves as a quasi-competitive inhibitor of GLUT1 transport activity by binding to a site accessible through the internal face of the transporter, but it does not, in fact, compete with cytochalasin B binding. Our observations suggest that some effects of gossypol on cellular physiology may be related to its ability to disrupt the normal hexose flux through GLUT1, a transporter expressed in almost every kind of mammalian cell and responsible for the basal uptake of glucose.

Method for measuring the logarithm of the octanol-water partition coefficient by using short octadecyl-poly(vinyl alcohol) high-performance liquid chromatography columns

A simple, quick, versatile and inexpensive HPLC method to estimate the logarithm of the octanol-water partition coefficient (log Pow) employing a methanol-water gradient and a short octadecyl-poly(vinyl alcohol) (ODP) column is described. This method is different from published HPLC-based log Pow methods because it uses retention times from a rapid methanol-water gradient to directly generate log Pow estimates, rather than from a series of isocratic mixtures extrapolated to 100% water. These HPLC log Pow values have good precision and correlate well with traditional shake-flask log Pow values. If necessary, the log Pow determination (including replications) can easily be carried out using only a milligram of sample. By suppressing ionization of acids and bases by the use of a buffer in the aqueous phase, the method can measure the log Pow of neutral organic molecules at any pH between 2 and 13. The method can be used with impure material and is rapid, 7 min per run and 4 min equilibration; it lends itself to and has been utilized for high-throughput hydrophobicity determinations (we have now carried out thousands of HPLC log Pow measurements by this method).

Multifunctional transporter models: lessons from the transport of water, sugars, and ring compounds by GLUTs

Facilitative glucose transporters (GLUTs) have recently been shown to be multifunctional, transporting substrates other than sugars, such as water and ring compounds as large as nitrobenzene-diazol-aminoglucose. Other membrane proteins, including transporters and cystic fibrosis transmembrane conductance regulator, have also revealed a finite permeability to water. We compare the alpha-helical and beta-barrel models for the structure of GLUTs, discuss recent evidence, and argue that a beta-barrel fold explains it better. We show a model for GLUTs consisting of a relatively rigid beta-barrel translocation unit ("channel") of diameter ample enough to allow permeation of the above substrates (approximately 20 A) but gated shut by mobile loops at both ends. Such gates would open only after aromatic interactions would lead to binding of the ring substrates for GLUTs; water would, however, traverse crevices in the closed gates. Using the insights gained from GLUTs, we propose that other transporters may share with GLUTs the motif of a beta-barrel channel and would be permeable to water due to the presence of such channels together with similarly behaving gates.

Preservatives in the vehicle of naloxone: pharmacological effects

The pharmacological activity of the preservatives methyl and propyl hydroxybenzoate, until recently components of the vehicle of naloxone (Narcan), was investigated in vitro. This vehicle produced reversible, concentration-dependent relaxation of guinea pig trachea, not mediated via adrenergic or cholinergic receptors, prostanoid activity or phosphodiesterase inhibition. Sensitivity of the tissue to calcium-induced contraction was decreased. In single isolated rat hepatocytes, surface receptor stimulation elicits repetitive transient rises in intracellular free calcium measured with the photoprotein aequorin. The vehicle reversibly inhibited these transients. These observations suggest that the effect of hydroxybenzoates may be mediated via a perturbation of intracellular calcium-related processes.

Asymmetric transport of a fluorescent glucose analogue by human erythrocytes

A fluorescent glucose analogue, 6-deoxy-N-(7-nitrobenz-2-oxa-1,3-diazol-4-yl)-aminoglucose (NBDG), was synthesized and its interactions with the hexose transport system of the human red blood cell were investigated. NBDG entry is inhibited by increasing concentrations of D-glucose (Ki = 2 mM). However, NBDG exit is unaffected by D-glucose in red blood cells. Cytochalasin B was found to inhibit both NBDG entry and exit. NBDG accumulates in the red blood cell above the theoretical equilibrium concentration. Accumulation of NBDG is temperature-sensitive and is due to the binding of NBDG to some intracellular substance. The binding of NBDG to purified hemoglobin suggests that accumulation of NBDG by erythrocytes is due to the intracellular binding of NBDG to hemoglobin. NBDG does not accumulate in pink erythrocyte ghosts, while its rate of uptake is still inhibited by D-glucose and cytochalasin B. Although there was no apparent D-glucose inhibition of NBDG exit by intact red blood cells, D-glucose was able to inhibit NBDG exit by pink erythrocyte ghosts. The differing properties of NBDG influx and efflux support the interpretation that the hexose transport system of the human red blood cell appears asymmetric although it may be intrinsically symmetric.

Effect of lidocaine on in vitro absorption of glucose in jejunal and ileal segments of the rat

The influence of lidocaine on the intestinal glucose absorption was investigated on Tyrode-perfused isolated intestinal segments of the rat. In ileal segments the transmural transfer of glucose decreased with increasing lidocaine concentration (5 X 10(-9) -2 X 10(-4) mol/l), whereas the water movement was unaffected. No inhibitory effect on glucose and water absorption was observed in the proximal jejunum.

The competitive inhibition of glucose transport in human erythrocytes by compounds of different structures

By kinetic analysis we found that the transport protein for glucose in human erythrocyte membranes has different binding sites for competitive inhibitors. They all change the transport protein with the effect that it loses its affinity to glucose. Some of the competitive inhibitors alter the conformation of the transport protein, so that other ones cannot be bound. There are inhibitors, however, which do not affect the affinity of other competitive inhibitors. A schematic model of our assumption about the mechanism of the competitive inhibition of glucose transport is presented.