Transport studies in red blood cells by measuring light scattering

The inhibitory effects of flavonoids and antiestrogens on the Glut1 glucose transporter in human erythrocytes

Flavonoids and isoflavonoids are potent inhibitors of glucose efflux in human erythrocytes. Net changes of sugars inside the cells were measured by right angle light scattering. The inhibitory potency of hydroxylated flavonoids depends on the pH of the medium. The apparent affinity is maximal at low pH where the molecule is in the undissociated form. The following K(i)-values at pH 6.5 in microM have been obtained: phloretin 0.37+/-0.03, myricetin 0.76+/-0.42, quercetin 0.93+/-0.28, kaempferol 1.33+/-0.17, isoliquiritigenin 1.96, genistein 3.92+/-0.62, naringenin 8.88+/-1.88, 7-hydroxyflavone 17.58+/-3.15 and daidzein 18.62+/-2.85. Flavonoids carrying hydroxyl groups are weak acids and are deprotonated at high pH-values. From spectral changes pK-values between 6.80 (naringenin) and 7.73 (myricetin) have been calculated. No such pK-value could be obtained from quercetin which was rather unstable at alkaline pH. Flavone itself without a hydroxyl group does not demonstrate any absorbance changes at different pH-values and no significant change in inhibition of glucose transport with pH (K(i)-value around 35 microM). In this respect it is similar to the antiestrogens diethylstilbestrol, tamoxifen and cyclofenil with K(i)-values for glucose efflux inhibition of 2.61+/-0.30, 6.75+/-2.03 and 3.97+/-0.54 microM. Except for phloretin, the flavonoids investigated have planar structures. The inhibitory activity in glucose efflux of planar flavonoids increases exponentially with the number of hydroxyl groups in the molecule.

Different activation energies in glucose uptake in Saccharomyces cerevisiae DFY1 suggest two transport systems

The analysis of initial glucose uptake in Saccharomyces cerevisiae at 25 degrees, 20 degrees, 15 degrees and 10 degrees C by computer-assisted nonlinear regression analysis predicts two transport systems. The first demonstrates Michaelis-Menten kinetics and the second shows first order behaviour. The activation energies of these two systems were calculated by the Arrhenius equation at four different growth phases, namely early exponential (EE), middle exponential (ME2), late exponential (LE) and early stationary (ES) with 2% glucose in the batch medium. The activation energies calculated from the V(m) values in EE, ME, LE and ES growth phases were 15.8 +/- 1.7, 13.5 +/- 1.0, 15.1 +/- 0.8 and 13.5 +/- 0.7 kcal/mol. These values are in agreement with activation energies calculated for the first mechanism, facilitated diffusion, which is the mechanism deduced from countertransport experiments. The activation energies derived for the second transport system from the first order rate constants in cells grown to EE, ME2, LE and ES were 8.0 +/- 2.1, 8.1 +/- 1.3, 9.6 +/- 3.0 and 7.5 +/- 2.6 kcal/mol. These values are still significantly higher than for free diffusion of glucose in water and lower as predicted for passage of glucose through the lipid phase. Therefore, we assume in addition to carrier-mediated facilitated diffusion the entrance of glucose into the cell through a pore.

Expression of GLUT-2 cDNA in human B lymphocytes: analysis of glucose transport using flow cytometry

The molecular characterization of transport proteins is often limited by transient functional expression or the need for a simple method to select functional cDNA clones. We used a mammalian expression system to obtain long-term expression of GLUT-2, an isoform of glucose permease. Rat GLUT-2 cDNA was ligated into an EBV vector (pLPP) and transfected into B lymphocytes which lack GLUT-2. Northern and Western analyses confirmed expression of GLUT-2 protein in membranes of transfected cells. Two functional assays using flow cytometry were developed to distinguish GLUT-2 transfectants from control/pLPP transfectants. Uptake of NBD-glucosamine, a fluorescent analogue of glucose, was increased in GLUT-2 transfectants. In addition, when exposed to hypertonic glucose medium, GLUT-2 transfectants and control/pLPP transfectants exhibited a difference in forward-angle light scatter (FALS), an index of cell volume, indicating a difference in glucose permeability. Independent measurements of glucose uptake (isotopic) and cell volume (video microscopy) confirmed the flow cytometry observations. This expression system used in combination with flow cytometry is useful for studying the functional properties of glucose and other solute transporters.

Phloretin keto-enol tautomerism and inhibition of glucose transport in human erythrocytes (including effects of phloretin on anion transport)

Under various pH conditions phloretin demonstrates keto-enol tautomerism with a pK value of 7.26 +/- 0.06. As Wilbrandt has shown ((1950) Arch. Exp. Pathol. Pharmacol. 212, 9-29) phloretin added to erythrocytes inhibits glucose efflux, but not glucose influx. At pH 6.5 a Ki value of 0.36 and at pH 9 of 22.7 microM was measured; only the ketonic form of phloretin contributes to the inhibition of glucose efflux. This was also the case for inhibition of galactose efflux and anion exchange. The geometry optimization of a large number of conformations of the ketonic and enolic forms of phloretin demonstrates different shapes of the molecules. Only the ketonic form shows several overlapping structures with beta-D-glucopyranose. Considering surplus binding of phloretin under glucose efflux conditions as being equivalent to the number of glucose transporters, a number of about 200,000 molecules was determined. By two independent methods 210,000 and 171,000 molecules per cell were calculated. This result is in close agreement with the number of glucose transporter sites of the erythrocyte.

Hemoglobin polymerization in sickle cells studied by circular polarized light scattering

We have studied intracellular polymerization of hemoglobin S in suspensions of small populations of sickle cells using circular polarized light scattering. We argue that the preferential scattering of right circular polarized light (as expressed by measurements of the S14 Mueller scattering matrix element) directly reflects the amount of polymer inside cells. This technique has made it possible to investigate the effect of oxygen tension, cell density and osmotic stress on intracellular hemoglobin polymerization. Using S14 to determine hemoglobin polymer, we show that the polymer increases with deoxyhemoglobin concentration, that cells containing higher hemoglobin concentrations show significantly more polymer than cells containing less hemoglobin, and that polymerization occurs in sickle-trait cells in hypertonic solutions as the oxygen tension in the suspension is reduced. We also present kinetic measurements of polymerization, including that induced by osmotic shock. Finally, we demonstrate that the total light scattered (S11 Mueller scattering matrix element) that is routinely measured simultaneously with S14 can be used to estimate the percent of reduced (deoxy) Hb in the sample. These experiments demonstrate the potential of this technique to monitor hemoglobin polymerization simultaneously with oxygen dissociation under a wide variety of physiological conditions.

Kinetic analysis and simulation of glucose transport in plasma membrane vesicles of glucose-repressed and derepressed Saccharomyces cerevisiae cells

In this study experimental data on the kinetic parameters investigated by other authors 1-5, 11 together with own data on plasma membrane vesicles, have been subjected to a computer simulation based on the equations describing facilitated diffusion. The simulation led to an ideal fit describing the above data. From this it can be concluded that glucose is transported by facilitated diffusion, and not by active transport as was postulated by Van Steveninck 14,15. The simulation method also demonstrates that the fast sampling technique used by these authors 1-5, 11 underestimated the fluxes. Thus, the parameters given do not contribute to the understand of glucose transport under different metabolic conditions. The K value of plasma membrane vesicles prepared from glucose-repressed cells is around 7 mM. Derepression, particularly by galactose, causes a highly significant increase in affinity as shown by a decrease in the K value to 2 mM. The highest affinity was measured in a triple kinaseless mutant grown on glycerol with a K value of 1 mM. It seems, therefore, that the kinetic parameters derived from initial uptake rates of glucose in intact cells 1-5, 11 using single flux analysis, such as Eadie-Hofstee- or Lineweaver-Burk-plots, are in error.