Cytochalasin B inhibition and temperature dependence of 3-O-methylglucose transport in fat cells

Carrier-mediated fructose uptake significantly contributes to carbohydrate metabolism in human skeletal muscle

To determine whether fructose can be utilized as a metabolic substrate for skeletal muscle in man, we investigated its incorporation into glycogen, its oxidation and lactate production in isolated human skeletal muscle. Rates of fructose oxidation and incorporation into glycogen increased in the presence of increasing fructose concentrations (0.1-1.0 mM). Lactate production increased 3-fold when extracellular fructose was increased from 0.1 to 0.5 mM. Cytochalasin B, a competitive inhibitor of hexose transport mediated by the GLUT1 and GLUT4 facilitative glucose transporters, completely inhibited insulin-stimulated glucose incorporation into glycogen and glucose oxidation (P < 0.01), but did not alter fructose incorporation into glycogen or fructose oxidation. Insulin (1000 mu-units/ml) increased glucose incorporation into glycogen 2.7-fold and glucose oxidation 2.3-fold, whereas no effect on fructose incorporation into glycogen or fructose oxidation was noted. A physiological concentration of glucose (5 mM) decreased the rate of 0.5 mM fructose incorporation into glycogen by 60% (P < 0.001), whereas fructose oxidation was not altered in the presence of 5 mM glucose. Irrespective of fructose concentration, the majority of fructose taken up underwent non-oxidative metabolism. Lactate production accounted for approx. 80% of the fructose metabolism in the basal state and approx. 70% in the insulin (1000 mu-units/ml)-stimulated state. In the presence of 5 mM glucose, physiological concentrations of fructose could account for approximately 10-30% of hexose (glucose + fructose) incorporation into glycogen under non-insulin-stimulated conditions. In conclusion, fructose appears to be transported into human skeletal muscle via a carrier-mediated system that does not involve GLUT4 or GLUT1. Furthermore, under physiological conditions, fructose can significantly contribute to carbohydrate metabolism in human skeletal muscle.

Effects of glycaemia on glucose transport in isolated skeletal muscle from patients with NIDDM: in vitro reversal of muscular insulin resistance

We investigated the influence of altered glucose levels on insulin-stimulated 3-0-methylglucose transport in isolated skeletal muscle obtained from NIDDM patients (n = 13) and non-diabetic subjects (n = 23). Whole body insulin sensitivity was 71% lower in the NIDDM patients compared to the non-diabetic subjects (p < 0.05), whereas, insulin-mediated peripheral glucose utilization in the NIDDM patients under hyperglycaemic conditions was comparable to that of the non-diabetic subjects at euglycaemia. Following a 30-min in vitro exposure to 4 mmol/l glucose, insulin-stimulated 3-0-methylglucose transport (600 pmol/l insulin) was 40% lower in isolated skeletal muscle strips from the NIDDM patients when compared to muscle strips from the non-diabetic subjects. The impaired capacity for insulin-stimulated 3-0-methylglucose transport in the NIDDM skeletal muscle was normalized following prolonged (2h) exposure to 4 mmol/l, but not to 8 mmol/l glucose. Insulin-stimulated 3-0-methylglucose transport in the NIDDM skeletal muscle exposed to 8 mmol/l glucose was similar to that of the non-diabetic muscle exposed to 5 mmol/l glucose, but was decreased by 43% (p < 0.01) when compared to non-diabetic muscle exposed to 8 mmol/l glucose. Despite the impaired insulin-stimulated 3-0-methylglucose transport capacity demonstrated by skeletal muscle from the NIDDM patients, skeletal muscle glycogen content was similar to that of the non-diabetic subjects. Kinetic studies revel a Km for 3-0-methylglucose transport of 9.7 and 8.8 mmol/l glucose for basal and insulin-stimulated conditions, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)

Temperature dependence of glucose transport in erythrocytes from normal and alloxan-diabetic rats

Alloxan diabetes increased 3-O-methylglucose transport rates in rat red blood cells (RBC) at temperatures below 30 degrees C and decreased them above 30 degrees C. Preincubation of RBC from control rats with 20 mM glucose, 3-O-methylglucose, 2-deoxyglucose or xylose greatly elevated transport at 14 degrees C by increasing Vmax. The effect was slight at 40 degrees C. Preincubation with glucose or deoxyglucose alone caused a 50% depression of transport rates at 40 degrees C as a result of a rise in the Km, which is similar to findings in cells from alloxan-diabetic rats. Measurement of intracellular glucose metabolites suggested inhibition of glycolysis in cells from diabetic rats and a positive correlation between the level of intracellular hexose monophosphates and transport inhibition. Membrane fatty-acid and cholesterol composition and membrane lipid-ordering as monitored by electron paramagnetic resonance were not altered by alloxan diabetes. It is concluded that intracellular sugar and sugar metabolism alter the temperature dependence of glucose transport kinetics. Glucose metabolism can feed back to inhibit transport by increasing the transport Km at physiological temperatures only.

Cytochalasin inhibition of hexose transport by platelets

Previously we described a two-transporter model (T1, T2) for galactose uptake by platelets (Horne, M.K. and Hart, J.S. (1986) Biochim. Biophys. Acta 856, 448-456). In the current work we have sought corroborative evidence for this model by studying the effects of cytochalasins on this transport system. Of the various cytochalasins tested, cytochalasin B was the most potent inhibitor (I) of galactose transport, whereas cytochalasin A was less inhibitory and dihydrocytochalasin B and cytochalasin E had no inhibitory effect. The same order of potency was observed for the inhibition of L-glucose diffusion into platelets. The mechanism of cytochalasin B inhibition was investigated in detail. Inhibition of T1 was competitive and required a higher concentration of cytochalasin B (Ki1 approximately 1.7 microM) than inhibition of T2, which was of a mixed type (Ki2 approximately 0.8 microM). The effect of cytochalasin B on T2 could be accounted for by a membrane alteration which enhanced the affinity of the transporter for galactose while simultaneously preventing passage of the TSI complex into the cell. Since a similar effect on membrane permeability would also explain cytochalasin B inhibition of L-glucose diffusion, it is hypothesized that cytochalasin B binds to a membrane structure shared by T2 and the passage for L-glucose. The differences in cytochalasin B sensitivity and mechanism of inhibition manifested by T1 and T2 support our original hypothesis that galactose is indeed transported by kinetically distinct agencies and suggest that these may be physically distinct as well.

Photolabeling of the adipocyte hexose carrier with an aryl azide derivative of maltose

A nitrophenyl azide derivative of maltose, N-(4-azido-2-nitrophenyl)-maltosylamine (NAP-maltosylamine), was synthesized as a potential photoaffinity label for the hexose carrier of the rat adipocyte. This derivative inhibited 3-O-methylglucose uptake with a Ki of 1.3 mM in the dark, while that of maltose was 10.0 mM. Carbon-14-labeled maltose and NAP-maltosylamine entered adipocytes via the hexose carrier, the latter in a concentrative fashion. Photolysis of NAP-[14C]maltosylamine in the presence of an adipocyte low density microsomal membrane fraction labeled several electrophoretic bands. Among these are a 45 kDa band which showed features expected of the hexose carrier: its labeling was decreased 40% by D- but not L-glucose and pretreatment of intact adipocytes with insulin decreased labeling of the 45 kDa band by 10-40%, as predicted by the translocation theory of insulin-stimulated transport activation. These studies show the suitability of using carbon-1-modified sugar photoaffinity labels as probes for the hexose carrier and possibly of its regulation in rat adipocytes.

The effect of hyperthermia on glucose transport in normal and thermal-tolerant Chinese hamster ovary cells

The effect of hyperthermia on glucose transport was studied in CHO cells to test the hypothesis that interference with membrane transport might be related to cell death at elevated temperatures. It was shown that passive diffusion of 2-deoxyglucose increases steadily over the temperature range 4-50 degrees C. Facilitated diffusion increases from 4 degrees C to 35 degrees C then exhibits a broad optimum before decreasing rapidly above 45 degrees C. The temperature dependence of glucose transport in thermally resistant cells was not however different from that of normal cells suggesting that this membrane transport process is not a critical target in cell killing by heat.

Interaction of hydrophobic bis (D-mannose) derivatives with adipocyte and erythrocyte sugar transport systems

The inhibition of sugar uptake by a series of hydrophobic bis(D-mannose) derivatives has been measured in rat adipocytes. When the D-mannose moieties of the bis compounds are separated by a hexane bridge the transport inhibition constant (Ki) is greater than for a decane-bridged molecule. This is probably due to the increased hydrophobicity of the bridge of the decane-bridged compound. The enhancement in affinity due to the second sugar in the bis(D-mannose) derivatives is probably only 2-fold, since half reduction of the bis(D-mannosyloxy)hexane increases Ki approx. 2-3-fold. N'-DNP-1,3-bis(D-mannos-4'-yloxy)propyl-2-amine has very high affinity in insulin-treated cells. The affinity is approx. 1000-fold higher than for D-mannose. This enhancement is probably due to the hydrophobicity of the DNP group. The distance from the sugar to the hydrophobic group is important because an increase in Ki occurs if an aminocaproyl spacer is introduced between the DNP group and 1,3-bis(D-mannos-4'-yloxy)propyl-2-amine. Aminocaproyl and glycyl spacers also increase the Ki for NAP derivatives of 1,3-bis(D-mannos-4'-yloxy)propyl-2-amine. Each of the hydrophobic bis(D-mannose) derivatives has a lower Ki in insulin-treated cells. This may be due to an insulin responsive hydrophobic interaction between the hydrophobic portion of the sugar and a hydrophobic domain in the transport system. The inhibition constants for the hydrophobic bis(D-mannose) compounds have also been measured in human erythrocytes.

Accelerated net efflux of 3-O-[14C]methylglucose in isolated fat cells

A flow-tube apparatus suited for measurement of rapid efflux of sugars from adipocytes is described. Due to heterogeneity of fat cell populations, a conventional analysis of the time-course of net efflux of 3-O-methylglucose based on the integrated rate equation can produce gross errors in estimates of kinetic parameters. The half-saturation constant and maximum transport capacity for 3-O-methylglucose transport were found to be about 3-fold higher for net efflux than for equilibrium exchange flux, both in insulin-stimulated and non-stimulated adipocytes. This suggests asymmetric kinetic parameters for 3-O-methylglucose transport.

Temperature optimum of insulin-stimulated 2-deoxy-D-glucose uptake in rat adipocytes. Correlation of cellular transport with membrane spin-label and fluorescence-label data

The effects of temperature alterations between 22 degrees C and 48 degrees C on basal and insulin-stimulated 2-deoxy-D-[1-14C]glucose uptake were examined in isolated rat adipocytes. A distinct optimum was found near physiological temperature for uptake in the presence of maximally effective insulin concentrations where insulin stimulation and hexose uptake were both conducted at each given assay temperature. Basal uptake was only subtly affected. Control and maximally insulin-stimulated cells incubated at 35 degrees C subsequently exhibited minimal temperature-sensitivity of uptake measured between 30 and 43 degrees C. The data are mostly consistent with the concept that insulin-sensitive glucose transporters are, after stimulation by insulin, functionally similar to basal transporters. Adipocyte plasma membranes were labelled with various spin- and fluorescence-label probes in lipid structural studies. The temperature-dependence of the order parameter S calculated from membranes labelled with 5-nitroxide stearate indicated the presence of a lipid phase change at approx. 33 degrees C. Membranes labelled with the fluorescence label 1,6-diphenylhexa-1,3,5-triene, or the cholesterol-like spin label nitroxide cholestane, reveal sharp transitions at lower temperatures. We suggest that a thermotropic lipid phase separation occurs in the adipocyte membrane that may be correlated with the temperature-dependence of hexose transport and insulin action in the intact cells.

Dependence of fluid-phase pinocytosis in arterial smooth-muscle cells on temperature, cellular ATP concentration and the cytoskeletal system

125I-labelled poly(vinylpyrrolidone) was used as a marker of fluid-phase pinocytosis in cultured pig arterial smooth-muscle cells. The rate of pinocytosis was temperature-dependent. A decrease in cellular ATP concentrations as a result of inhibition of either glycolysis or oxidative phosphorylation was associated with a similar decrease in pinocytosis. A microfibrillar-disruptive agent, cytochalasin B, caused a concentration-dependent stimulation of pinocytosis, whereas the microtubular-disruptive agents colchicine and vinblastine decreased pinocytosis to approximately half of control values at all concentrations used. These results indicate that fluid-phase pinocytosis in smooth-muscle cells is dependent on a continuing supply of energy and the integrity of the microtubules. Furthermore, microfilaments appear to exert a certain degree of constraint on pinocytosis, possibly by restricting invagination of the plasma membrane.

Cytochalasin E: inhibition of intestinal glucose absorption in the mouse

In situ glucose absorption in the mouse was significantly inhibited by cytochalasin E. Cytochalasin E (5 micrograms/ml) also inhibited glucose absorption up to 55.5% in mouse jejunum in vitro. During its inhibition transmural potential difference (PD) was increased from -7.4 to -0.4 mV, together with a decrease in glucose accumulation in the intestinal tissues. Furthermore, it was also found that cytochalasin E induced an alteration in Km value from 2.9 X 10(-3) to 4.0 X 10(-2) M and a constant Vmax value of 55.5 mumol/100 mg wet wt tissue/min. It is postulated that cytochalasin E is a possible competitive inhibitor of glucose at the receptor sites of carriers on the microvillar membrane of the intestinal absorptive cells.

Modulation of substrate transport to the brain

Variations of substrate transport across the cerebral capillary endothelium were examined in response to variations of the substrate demand of the brain tissue, and to variations of substrate concentration in the blood. The substrates examined included glucose and ketone bodies. The transport changes were measured in rats, using an indicator fractionation method modified by the reviewer. Four mechanisms appeared to contribute to the adjustment of substrate transport to variations in substrate demand. The first and least important mechanism was the change of concentration gradient across the endothelium that occurred when the substrate consumption rate changed. The second mechanism was the flow-dependency of the average capillary substrate concentration: the higher the perfusion rate, the higher the average capillary concentration. This mechanism failed to account for the changes of substrate transport observed during marked increases of the metabolic rate. The third and most important mechanism was a change of the capillary diffusion capacity, probably associated with a change of the number of perfused capillaries. The fourth mechanism, not previously described, was an adaptation of transport to permanent changes of substrate concentration in the blood. This mechanism appeared to reflect changes of the concentration (and affinity?) of transport proteins in the plasma membranes of endothelial cells, possibly in association with changes of cellular protein synthesis and gene expression.

Alcohols inhibit adipocyte basal and insulin-stimulated glucose uptake and increase the membrane lipid fluidity

Benzyl alcohol and ethanol, at aqueous concentrations that cause local anesthesia of rat sciatic nerve, affect structural and functional properties of rat adipocytes. The data strongly suggest that structurally-intact membrane lipids are required for the proper cellular uptake of glucose and for the physiologic response of adipocytes to insulin. The structure of adipocyte membrane lipids was examined with the spin label method. Isolated adipocyte 'ghost' membranes were labeled with the 5-nitroxide stearate spin probe I(12,3). Order parameters that are sensitive to the fluidity of the lipid environment of the incorporated probe were calculated from ESR spectra of labeled membranes. Benzyl alcohol and ethanol dramatically increased the fluidity of the adipocyte ghost membrane, as indicated by decreases in the polarity-corrected order parameter S. This concentration-dependent fluidization commenced at approx. 10 mM benzyl alcohol and progressively increased at all higher concentrations tested (up to 107 mM). S decreased approx. 5.7% at 40 mM benzyl alcohol, a change in S comparable in magnitude to that induced by a 6 degrees C increase in the incubation temperature. Benzyl alcohol and ethanol inhibited basal glucose uptake in adipocytes and uptake maximally stimulated by insulin. Temperature-induced increases in membrane fluidity, detected with I(12,3), that closely paralleled the fluidity effects of alcohols were associated only with increases in basal and insulin-stimulated glucose uptake. The contention that the membrane lipid fluidity plays a role in insulin action needs further study.

Endotoxin-induced alterations in glucose transport in isolated adipocytes

2-Deoxyglucose and 3-O-methylglucose were used to assess endotoxin-induced changes in glucose transport in rat adipocytes. 6 h after Escherichia coli endotoxin injection insulin-stimulated 2-deoxyglucose uptake was significantly depressed (V decreased, Km unaltered), phosphorylation of 2-deoxyglucose was seemingly unimpaired; basal 3-methylglucose entry was significantly increased, insulin-stimulated uptake was unaltered. Insulin significantly reduced Km in control and endotoxin-treated cells. Cytochalasin B-insensitive uptake of both 2-deoxy-glucose and 3-methylglucose, a small fraction of total transport, increased significantly in endotoxic cells. Endotoxin reduced spermine- and insulin-stimulated 2-deoxyglucose uptake to a similar extent. Results are consistent with the hypotheses that (1) a site of endotoxin-induced insulin resistance is at the cell membrane level and may reflect a decrease in number of activity of effective carrier units, rather than alterations in affinity, (2) endotoxin does not compromise the hexokinase system, (3) the cell membrane-localized effect of endotoxin on hexose transport is not necessarily mediated by the insulin receptor and (4) the entry of 2-deoxyglucose and 3-methyl-glucose may involve two separate transport systems.

Hydrogen bonding requirements for the insulin-sensitive sugar transport system of rat adipocytes

(1) The t 1/2 for 1.3 mM D-allose uptake and efflux in insulin-stimulated adipocytes is 1.7 +/- 0.1 min. In the absence of insulin mediated uptake of D-allose is virtually eliminated and the uptake rate (t 1/2 = 75.8 +/- 4.99 min) is near that calculated for nonmediated transport. The kinetic parameters for D-allose zero-trans uptake in insulin-treated cells are Koizt = 271.3 +/- 34.2 mM, Voizt = 1.15 +/- 0.12 mM . s-1. (2) A kinetic analysis of the single-gate transporter (carrier) model interacting with two substrates (or substrate plus inhibitor) is presented. The analysis shows that the heteroexchange rates for two substrates interacting with the transporter are not unique and can be calculated from the kinetic parameters for each sugar acting alone with the transporter. This means that the equations for substrate analogue inhibition of the transport of a low affinity substrate such as D-allose can be simplified. It is shown that for the single gate transporter the Ki for a substrate analogue inhibitor should equal the equilibrium exchange Km for this analogue. (3) Analogues substituted at C-1 show a fused pyranose ring is accepted by the transporter. 1-Deoxy-D-glucose is transported but has low affinity for the transporter. High affinity can be restored by replacing a fluorine in the beta-position at C-1. The Ki for D-glucose = 8.62 mM; the Ki for beta-fluoro-D-glucose = 6.87 mM. Replacing the ring oxygen also results in a marked reduction in affinity. The Ki for 5-thio-D-glucose = 42.1 mM. (4) A hydroxyl in the gluco configuration at C-2 is not required as 2-deoxy-D-galactose (Ki = 20.75 mM) has a slightly higher affinity than D-galactose (Ki = 24.49 mM). A hydroxyl in the manno configuration at C-2 interferes with transport as D-talose (Ki = 35.4 mM) has a lower affinity than D-galactose. (5) D-Allose (Km = 271.3 mM) and 3-deoxy-D-glucose (Ki = 40.31 mM) have low affinity but high affinity is restored by substituting a fluorine in the gluco configuration at C-3. The Ki for 3-fluoro-D-glucose = 7.97 mM. (6) Analogues modified at C-4 and C-6 do not show large losses in affinity. However, 6-deoxy-D-glucose (Ki = 11.08 mM) has lower affinity than D-glucose and 6-deoxy-D-galactose (Ki = 33.97 mM) has lower affinity than D-galactose. Fluorine solution at C-6 of D-galactose restores high affinity. The Ki for 6-fluoro-D-galactose = 6.67 mM. Removal of the C-5 hydroxymethyl group results in a large affinity loss. The Ki for D-xylose = 45.5 mM. The Ki for L-arabinose = 49.69 mM. (7) These results indicate that the important hydrogen bonding positions involved in sugar interaction with the insulin-stimulated adipocytes transporter are the ring oxygen, C-1 and C-3. There may be a weaker hydrogen bond to C-6. Sugar hydroxyls in non-gluco configurations may sterically hinder transport.

Hexose transport in Novikoff rat hepatoma cells. A simple carrier with directional symmetry, but variable relative mobilities of loaded and empty carrier

The kinetics of transport of the non-metabolizable hexose, 3-O-methyl-D-glucose, have been measured in Novikoff rat hepatoma cells by both zero-trans entry and equilibrium exchange procedures. Transport conformed to a simple carrier model which operates symmetrically with respect to direction, but with greater mobility of the loaded than of the empty carrier. Although a complete kinetic description of the transporter can, in theory, be obtained by application of integrated equations describing the time course of substrate equilibrium across the membrane beginning from the zero-trans situation, statistical analysis of hypothetical data indicated that directional asymmetry or differential mobilities of loaded and empty carrier cannot be discerned reliably from such data alone. The difference in mobility of loaded and empty carrier, apparent in a comparison of zero-trans entry and exchange data, ranged from 1.5--7-fold in different batches of cells. It is concluded that the magnitude of the difference is not an inherent property of the transporter, but is determined physiologically, and may be involved in regulation of hexose transport.

D-Glucose transport across the apical membrane of the surface epithelium in Nereis diversicolor

Epidermal D-glucose transport was investigated in vivo in the brackish-water polychaete worm Nereis diversicolor. Transfer across the apical membrane is rate-limiting to D-glucose uptake, but the cuticle and/or mucus presents some resistance to D-glucose diffusion between bulk solution and transporting membrane. Maximal D-glucose influx is about 10(-12) mol sec-1 per cm2 of apical plasmalemma. Under natural conditions (approximately 1 microM D-glucose in the medium), backflux from the epidermal transport pool is negligible, but a significant paracellular outflux may occur. D-glucose influx across the apical membrane is Na+-dependent and completely inhibitable by phlorizin and harmaline; phloretin is less effective, and cytochalasin B has no effect. Influx is moderately depressed by KCN and iodoacetate, alpha-methyl-D-glucopyranoside is an effective substitute of D-glucose in transport. Animals acclimated to a low salinity, in which epidermal salt transport takes place, show a marked decrease of D-glucose transport capacity. On transfer of animals from a high to a low salinity, or vice versa, the corresponding change of influx occurs after a time-lag of at least an hour. Permeability of the epidermis to simple diffusion of D-glucose is 8 X 10(-8) cm sec-1 (on basis of gross epidermal area).

Inhibition by dexamethasone of the in vitro transport of 3-O-methylglucose into rat thymocytes

Reduced glucose transport across the plasma membrane and reduced phosphorylation may both be responsible for the early inhibitory effect of physiological concentrations of glucocorticoids on glucose uptake by rat thymocytes. The early inhibitory effects of glucocorticoids (5 . 10(-7) M dexamethasone) on glucose consumption and 14CO2 formation from D-[U-14C]glucose were reproduced. The total uptake curve of 4.8 microM 3-O-[14C]methyl-D-glucose was biexponential with t 1/2 of 1.1 min and 36 min, respectively, the rapid part comprising about 50% of the equilibrated intracellular water space. The latency of the effect of 5 . 10(-7) M dexamethasone on 3-O-[14C]methyl-D-glucose uptake ranged from 15 to 100 min and the inhibition varied from 15 to 55% independently of the lag period. The effect of 3-O-methylglucose concentration on the initial uptake by steroid-responsive cell preparations was tested after 45 min of preincubation with or without 5 . 10(-7) M dexamethaone. In 12 experiments dexamethasone reduced V from 1.36 +/- 0.16 mmol . min-1 . l-1 cell water to 0.81 +/- 0.10 mmol . min-1 . l-1 cell water with insignificant change of Km (6.0 mM versus 5.9 mM). Dexamethasone had similar effect after 90 or 120 min. The variabilities of control cell transport capacity, the lag period and the magnitude of the dexamethasone effect could not be accounted for by changes in pH, effects of cell density, concentrations of albumin, ethanol, nucleosides, pyruvate or correlated to age and sex of the rats. In conclusion the inhibition of glucocorticoids on glucose consumption by thymocytes appears to be an inhibited plasma membrane transport capacity.

Coordinate modulation of D-glucose transport activity and bilayer fluidity in plasma membranes derived from control and insulin-treated adipocytes

The cis-monoenoic fatty acids vaccenate and oleate stimulate D-glucose transport when partitioned into isolated plasma membranes from rat adipocytes. The magnitude of hexose transport stimulation due to these agents is equal to that observed in plasma membranes derived from insulin-treated adipocytes. Addition of cis-unsaturated fatty acids to plasma membranes derived from insulin-treated cells results in no further stimulation of glucose transport over that due to the hormone alone. In contrast, treatment of membranes exhibiting insulin-activated D-glucose transport activity with saturated fatty acids reduces transport activity to control levels. No effect of the saturated fatty acids was observed on D-glucose transport in control membranes. Because cis-unsaturated fatty acids fluidize plasma membranes under the conditions used in these experiments, these data demonstrate a positive correlation between membrane fluidity and adipocyte D-glucose transport system activity. In addition, the results suggest that enhanced bilayer fluidity or increased affinity of the glucose transporter for fluid microenvironments of the membrane may play a key role in transport regulation by insulin.

Resistance of the intact and reconstituted adipocyte hexose transport system to irreversible inhibition by sulfhydryl and amino reagents

Sensitivity of the adipocyte D-glucose transport system in intact plasma membranes or following solubilization and reconstruction into phospholipid vesicles to several protein-modifying reagents was investigated. When intact plasma membranes were incubated with N-ethylmaleimide (20 mM) or fluorodinitrobenzene (4 mM), D-glucose transport activity was virtually abolished. However, washing the membranes free of unreacted reagents restored transport activity, indicating that covalent interaction with the membrane did not mediate the transport inhibition. Reaction of [3H]N-ethylmaleimide with plasma membranes under similar conditions resulted in extensive labeling of all protein fractions resolved on dodecyl sulfate gels. Similarly, addition of N-ethylmaleimide to cholate-solubilized membrane protein had no effect on transport activity in artificial phospholipid vesicles reconstituted under conditions where the membrane protein was free of unreacted N-ethylmaleimide. Transport activity in plasma membranes was also inhibited by both reduced and oxidized dithiothreitol or glutathione (15 mM) in a readily reversible manner. consistent with a noncovalent mode of inhibition. Thus, the insulin-responsive adipocyte D-glucose transport system differs from the red cell hexose transport system in its remarkable insensitivity to modulation by covalent blockade of sulfhydryal or amino groups by the reagents studied.