Investigation by amperometric methods of intracellular reduction of 2,6-dichlorophenolindophenol in normal and transformed hepatocytes in the presence of different inhibitors of cellular metabolism

The reduction of 2,6-dichlorophenolindophenol (DCIP) was measured by amperometric methods in Morris hepatoma 3924A cells, normal isolated rat hepatocytes and in mitochondria isolated from normal rat liver. The influence of aerobic and anaerobic atmospheres and of various inhibitors of cellular metabolism, especially of the respiratory chain (KCN, NaN3, oligomycin), on DCIP-reduction were studied using glucose, succinate, beta-hydroxybutyrate, alpha-keto-glutarate and oxalacetate as substrates. Under the influence of KCN and oligomycin the velocity of DCIP-reduction was increased in both cell types. Azide showed a similar effect on tumour cells and to a lower extent on hepatocytes. Using isolated mitochondria total DCIPred was increased by KCN and azide using various mitochondrial metabolites as substrates and with ADP/Pi present. The effects of KCN, azide and oligomycin could be explained by taking DCIP as an artificial coupling site in mitochondria which is only used when oxygen is absent or when the respiratory chain is blocked by inhibitors of cytochrome oxidase. Evaluation of the reaction kinetics revealed differences between normal and transformed cells in terms of the pseudo-first-order rate constants and the activity of overall oxidoreductases. The results apparently reflect quantitative differences in enzymatic equipment and the metabolic pathways predominating in normal and neoplastic cells.     

Initial reaction kinetics of succinate dehydrogenase in mouse liver studied with a real-time image analyser system.

The initial reaction kinetics of succinate dehydrogenase in situ were investigated in sections of mouse unfixed liver using an ARGUS-100 image analyser system. The sections were incubated on substrate-containing agarose gel films. Images of a section, illuminated with monochromatic light (584 nm), were captured with the image analyser in real time at intervals of 10 s during the incubation. The absorbances of selected hepatocytes in the successive images were determined as a function of time. In every cell, the absorbance increased nonlinearly after the first minute of incubation. The initial velocity of the dehydrogenase was calculated from the linear activities during the first 20 s of incubation. Hanes plots of the initial velocities and succinate concentration yielded the following mean kinetic constants. For periportal hepatocytes, the apparent Km = 1.2 +/- 0.8 mM and Vmax = 29 +/- 2 mumol hydrogen equivalents formed/cm3 hepatocyte cytoplasm per min. For pericentral hepatocytes, Km = 1.4 +/- 1.0 mM and Vmax = 21 +/- 2 mumol hydrogen equivalents/cm3 per min. The Km values are very similar to those determined previously from biochemical assays. These results, and the observed dependence of the initial velocity on the enzyme concentration, suggest that the technique reported here is valid for the histochemical assay of succinate dehydrogenase.     

Kinetics of utilization of organic substrates by Mycoplasma mycoides subsp. mycoides in a salts solution: a flow-microcalorimetric study

The metabolism of various organic substrates by suspensions of Mycoplasma mycoides subsp. mycoides in a salts solution was followed by microcalorimetry. Enthalpy changes associated with metabolism were in good agreement with theoretical values. Substrate utilization showed Michaelis kinetics, allowing saturation constants (Km) and maximum specific rates of substrate utilization (Vmax) to be determined. In cells grown on a complex medium containing glucose, Km values were: glucose, fructose, N-acetylglucosamine, glycerol and pyruvate, less than 5 microM; lactate, 20 microM; glucosamine, 130 microns, and mannose, 1 mM. Values of Vmax for glycerol, pyruvate and lactate were similar and approximately twice those for glucose, mannose, glucosamine and N-acetylglucosamine; Vmax for fructose was one-quarter of that for glucose. In cells grown on complex medium in which glucose was replaced by mannose, glucosamine or N-acetylglucosamine, Vmax and Km for the respective growth sugars and for glucose were not significantly affected. However, in cells grown in the presence of fructose, Vmax for fructose increased to the value observed for glucose. It is suggested that M. mycoides is adapted to, and is constitutive for, the utilization of a single sugar (glucose), and a single amino sugar (N-acetylglucosamine), but that in the presence of fructose a fructose-utilizing pathway is induced.     

Transport of glucose and fructose in rat hepatocytes at 37 degrees C

The kinetic parameters for transport of the nonmetabolizable glucose analogue 3-O-methyl-D-glucose and the relationship between transport and metabolism of D-glucose and D-fructose were determined in isolated rat hepatocytes at 37 degrees C and pH 7.4. 3-O-Methylglucose at a very low concentration (0.1 mM) equilibrated with the intracellular water with a rate constant of 0.41 s-1. Km for equilibrium exchange entry was 5.5 mM and Vmax was 2.2 mM X s-1 and similar results were obtained when using the zero-trans entry protocol. The rate constant for entry of tracer D-glucose was 0.15 s-1 and Km for glucose was about 20 mM. The phosphorylation rate for D-glucose was much slower than the transport rate. The rate constant for D-fructose entry was about 0.04 s-1, the apparent Km was about 100 mM and Vmax about 5 mM X s-1. The concentration dependence of 3-O-methylglucose inhibition of labelled fructose transport revealed biphasic kinetics indicating that fructose was transferred by both the glucose transporter and a fructose transporter. At concentrations lower than 1 mM, fructose metabolism appeared to be limited by the transport step.     

Hexose transport in normal and SV40-transformed human endothelial cells in culture

The mechanism of glucose entry into human vascular endothelial cells was studied in monolayer cultures of normal (primary) and virally (SV40) transformed umbilical vein endothelium. Radioisotopic uptake studies with the glucose analogues 2-deoxy-D-glucose, and 3-O-methyl-D-glucose, and the nonmetabolizable stereoisomer L-glucose, indicated the presence of a saturable, stereospecific hexose carrier mechanism in both cell types. In other experiments with D-glucose and 3-O-methyl-D-glucose, the phenomenon of countertransport was demonstrable. Hexose transport was not affected by KCN, dinitrophenol, or ouabain, but was inhibited by phloretin and phlorizin in a pattern consistent with facilitated diffusion. Kinetic constants were obtained for both 2-deoxy-D-glucose and 3-O-methyl-D-glucose uptake. Similar Km values (range, 3.3-4.7 mM) were noted with normal and transformed cells, whereas the apparent Vmax was 0.56 nmol/microliter cytosol/minute for primary cells and 1.7-2.5 nmol/mu cytosol/minute for transformed cells. Under standard culture conditions, as well as following 18 hours of serum deprivation, insulin at concentrations up to 10(-5) M did not appear to influence hexose uptake in either cell type. Metabolism of 14C(U)-D-glucose to 14CO2 also was not stimulated by insulin. The presence of an insulin-insensitive, facilitated transport system for glucose in vascular endothelium has relevance for glucose metabolism in this tissue, and potentially for the association of certain vascular diseases (e.g., diabetic microangiopathy, atherosclerosis) with altered glucose homeostasis.     

Reassessment of the translocation hypothesis by kinetic studies on hexose transport in isolated rat adipocytes

The effect of insulin and factors which have insulin-like activity on the kinetic parameters of 3-O-methyl-D-glucose (MeGlc) transport in rat adipocytes were assessed. Carrier-mediated uptake of MeGlc was estimated by the difference in the amounts of [14C]MeGlc and L-[3H]glucose taken up in cells under equilibrium exchange conditions at 37 degrees C. The Km and Vmax values in basal cells were 17.4 mM and 0.24 nmol/10(6) cells/s, respectively. Removal of endogenous adenosine by adenosine deaminase resulted in a 26% decrease in the basal rate due to a slight increase in the Km (19.6 mM) and a decrease in the Vmax value (0.20 nmol/10(6) cells/s). The maximum concentration (10 nM) of insulin decreased the Km to approximately one-half of the basal (7.1 mM) concomitant with an 8.5-fold increase in the Vmax value (2.04 nmol/10(6) cells/s). Submaximal concentrations (50 and 150 pM) of insulin, N6-phenylisopropyladenosine (1 microM), mechanical agitation of cells by centrifugal force (160 x g), low temperature (15 degrees C), 12-O-tetradecanoylphorbol-13-acetate (1 microM), and hydrogen peroxide (10 mM) all decreased the basal Km value to a range of 13.5-7.3 mM, concomitant with a 1.7-7.4-fold increase in the Vmax. A possible explanation for the alterations in the kinetic parameters may be that insulin and other factors cause the translocation of the mobile low-Km glucose transporters from an intracellular site to the cell surface, where the stationary high-Km transporters are located. Thus, when the Km and Vmax values of the hypothetical high-Km transporters were assumed to be 20 mM and 0.20 nmol/10(6) cells/s, respectively, and the Km of the low-Km transporters was assumed to be 7 mM, the theoretical Km decreased from 20 to 7.5 mM as the Vmax of the low-Km transporters increased from near 0 to 2.0 nmol/10(6) cells/s. The relation between empirical Km and Vmax values as affected by several agents and conditions followed closely the relation predicted by the above two-transporter model.     

Modulation of basal glucose transporter Km in the adipocyte by insulin and other factors

We have previously described experimental conditions where basal methylglucose transport in adipocytes exhibited an apparent Km of approximately 35 mM. Under those conditions insulin stimulated transport predominantly by decreasing the transport Km (Whitesell, R. R., and Abumrad, N. A. (1985) J. Biol. Chem. 260, 2894-2899). Our findings were in contrast with earlier reports that the Km of basal glucose transport was low (3-5 mM) and similar to that of transport in insulin-treated cells. In this study we have investigated the effect of different experimental conditions on the kinetics of basal glucose transport in adipocytes. When transport was assayed at 37 degrees C, cell agitation for 10 min prior to the transport assay decreased the basal Km from 35 to 12 mM. Deprivation of metabolic substrate produced a further reduction down to 2 mM. Refeeding starved cells with 1 mM glucose returned the Km back up to 12 mM in agitated cells and to 40 mM in stabilized cells. The effects of agitation to lower and of glucose to raise the basal Km were prevented by preincubating cells with dinitrophenol. Cell agitation or substrate lack did not alter the Vmax of basal transport and were without effect on both Km and Vmax in insulin-treated cells. The temperature dependencies of the kinetics of basal and stimulated transport were studied. A decrease in the assay temperature from 37 to 23 degrees C caused both basal Km and Vmax to drop proportionately from 25 to 5 mM, and 13 to 3.6 nmol/(microliter X min), respectively. In insulin-stimulated cells, only the Vmax was decreased (Km went from 3.5 to 3 mM, Vmax from 45 to 17 nmol/(microliter X min]. The results support the concept that experimental conditions can produce large changes in the Km of basal glucose transporters. Furthermore they explain why, under certain assay conditions (with temperatures around 23 degrees C or with deprivation of metabolic substrate), the effect of insulin on transport Km is not observed. Our data also suggest that basal transport characteristics do not persist in insulin-treated cells. We would propose that one of the actions of insulin (in addition to raising Vmax) is to change the characteristics of basal transporters by overriding metabolic factors which keep the Km high. Alternatively, insulin could cause the disappearance of basal transporters as new and different ones are recruited from intracellular stores.     

Kinetics of the oxidation of ferrocyanide by lactoperoxidase compound II

The kinetics of the oxidation of ferrocyanide by lactoperoxidase compound II has been studied over the pH range 5.2-9.9 at 25 degrees C and an ionic strength of 0.11 M. For all pH values, exponential decay curves are obtained for the reaction of compound II in the presence of ferrocyanide which yielded pseudo-first-order rate constants kobs. The spontaneous decay of compound II in the absence of ferrocyanide occurs at an appreciable rate which was measured independently and used in the data analysis. At all pH values two striking effects were observed when the rate of the decay reaction in the presence of ferrocyanide, kobs, was plotted against ferrocyanide concentration: a saturation effect and positive intercepts which are attributable to the spontaneous decay. The plots of kobs versus ferrocyanide concentration were analyzed in terms of the following parameters: a first-order rate constant k3,obs, a Michaelis constant Km,obs and a spontaneous-decay rate constant k4. The parameters k3,obs and Km,obs describe the reaction of compound II with ferrocyanide, independently of the spontaneous decay. The parameter k4 has only a small pH dependence, whereas plots of the logs of k3,obs and Km,obs versus pH have slopes of -1 at high pH. The major part of the pH dependence can be explained by the influence of a single heme-linked acid group in the LPO-compound-II-ferrocyanide complex.     

Phosphorylation reaction of vertebrate smooth muscle myosin: an enzyme kinetic analysis

Phosphorylation of vertebrate smooth muscle myosin or its isolated 20 000-dalton light chains by myosin light-chain kinase (MLCK) was found to follow first-order kinetics not only at low ([M] much less than Km) but also at high ([M] greater than or equal to Km) substrate concentration. This observation can most simply be explained by a product inhibition for which the Michaelis constants (Km) of the enzyme for the substrate (dephosphorylated myosin) and for the product (phosphorylated myosin) are approximately the same. For such a case, integration of the kinetic velocity equation gives an exponential formula similar to that of a true first-order reaction, the only difference being that its rate constant (k) depends additionally on the initial substrate concentration ([M]0). The standard kinetic constants (k, Km, Vmax) have been calculated by using this pseudo-first-order relationship. Independent evidence for the validity of the derived kinetic relationship was obtained from binding studies with myosin and MLCK. These showed that MLCK binds to phosphorylated and dephosphorylated myosin with approximately equal affinity (Ks = 30 X 10(-9) M). The possible applicability of the same kinetic relationship to other enzyme systems is discussed.     

Microcalorimetric study of the anaerobic growth of Escherichia coli: measurements of the affinity of whole cells for various energy substrates.

Microcalorimetry has been used to determine the affinity of whole cells of Escherichia coli for glucose, galactose, fructose, and lactose. Anaerobic growth thermograms were analyzed, and the Km and Vmax values for these energy substrates were measured at pH 7.8. Results obtained with this technique using various organisms growing anaerobically on different sugars are compared. This comparison shows that in practically all cases the cellular rate of catabolic activity is a hyperbolic function of the energy substrate concentrations at low sugar concentrations. In some cases this technique also allows determination of kinetics at high sugar concentrations.     

Infinite-cis kinetics support the carrier model for erythrocyte glucose transport

It has been claimed that the Km for infinite-cis uptake of glucose in human erythrocytes is so low that the carrier model for transport must be rejected. We redetermined this parameter for three experimental conditions and found instead that the Km values were in good agreement with the model. For each of a variety of cis glucose concentrations, cells were preequilibrated with various concentrations of glucose, and the apparent Km was determined as the intracellular concentration reducing the initial rate of net uptake by half. The dependence of the apparent Km values on the cis glucose was as predicted by the carrier model; the infinite-cis Km was determined from both this concentration dependence and the extrapolated value at infinite cis glucose. The resulting values were 15 mM for fresh blood at 0 degrees C, 39 mM for outdated blood at 0 degrees C, and 11 mM for outdated blood at 25 degrees C. Previous measurements of the Km at room temperature yielded values of 2-3 mM. These earlier studies used a time course procedure that indicated rapid changes in rates during the initial 10 s of uptake but did not directly measure such changes. We examined the uptake of 60 mM glucose at 20 degrees C into cells containing 0 and 5 mM glucose; rapid changes in rates were not observed in the first few seconds, and the time courses were more consistent with our higher Km values. Our new values, together with other initial rate measurements in the literature, support the adequacy of the carrier model to account for the kinetics of glucose transport in human erythrocytes.     

The relative importance of passive and P-glycoprotein mediated anthracycline efflux from multidrug-resistant cells.

For the four anthracyclines idarubicin, daunorubicin, epirubicin and doxorubicin the passive and active efflux rates in intact multidrug resistant cells were compared. Although highly similar structurally, these anti-tumor agents differ in lipophilicity and membrane permeability (k). The method we used was based on the continuous measurement of the cellular efflux and determination of the ratio (RVp) of transport rates just before and just after inhibition of the active transport with verapamil (Vp). Hence, RVp - 1 should reflect the active transport rate relative to the passive transport rate. If cells were single, well-stirred compartments, RVp - 1 should equal Vmax/(k.Km), where Vmax is the maximal pumping rate and Km is the Michaelis constant. However, using the plasma membrane permeabilizing agent digitonin, we found an effective intracellular anthracycline store. Particularly, when the efflux was fast, e.g. with idarubicin or in intensively pumping cells, the intracellular transport began to control the cellular efflux. Under these conditions, k underestimated the true plasma membrane permeability (k0) and RVp - 1 underestimated Vmax/(k.Km). Based on the effects of digitonin on the efflux rates in pumping and nonpumping cells, we developed an index (RVp,corrected - 1) which should equal Vmax/(k0. Km). The term Vmax/(k0.Km) varied substantially between the drugs. It appears that differences in lipophilicity between the drugs do not affect passive efflux and pumping equally. This demonstrates that passive permeation plays a substantial and independent role in determining the drug resistance for these anthracyclines. The methods developed here enable dissection of this role from that of drug pumping and intracellular subcompartmentation.     

Regional modulations in tegumental glucose transporter kinetics in the rat tapeworm

Comparisons of glucose transporter kinetics in 8-day (Km = 0.34 mM, Vmax = 14 nmole.min-1.g-1), 10-day (Km = 0.46 mM, Vmax = 18 nmole.min-1.g-1), the first quartile of 17-day (Km = 0.51 mM, Vmax = 21 nmole.min-1.g-1), and the first quartile of 32-day (Km = 0.33 mM, Vmax = 39 nmole.min-1.g-1) rat tapeworms (Hymenolepis diminuta) suggest maximal velocities may vary with age. A gradient in glucose transporter density is suggested in the rat tapeworm by changes in the estimated transporter Vmax in the first through fourth quartiles. Alterations in the physiological efficiency (as indicated by the Vmax/Km ratio) and permeability (indicated by the unsaturated permeability-area product) of the glucose transporter were determined to be significantly greater in the first quartile than in other quartiles of 17-day hymenolepids. A similar trend was apparent in older (32-day) worms. In tapeworms maintained for 30 min in glucose-free medium, maximal velocities were highest in the anterior (first) quartile, and reductions were seen in successive second, third, and fourth quartiles. When worms were maintained in a medium containing 11 mM glucose, maximal velocities were about twofold greater, but the Vmax increased in each successive quartile. The apparent half-saturation constants, which indicate that concentration of external glucose at which half of the glucose transporter proteins are occupied, are reduced approximately 50% in tapeworms maintained in glucose-free medium. These studies demonstrate that regional differences exist in the glucose transporter of the rat tapeworm, analogous to the intestinal glucose gradient. Furthermore, substrate-induced modulations in the transporter may also exhibit independent regional variability.     

Kinetics of sulfate and hydrogen uptake by the thermophilic sulfate-reducing bacteria thermodesulfobacterium sp. Strain JSP and thermodesulfovibrio sp. Strain R1Ha3

Half-saturation constants (Km), maximum uptake rates (Vmax), and threshold concentrations for sulfate and hydrogen were determined for two thermophilic sulfate-reducing bacteria (SRB) in an incubation system without headspace. Km values determined for the thermophilic SRB were similar to the constants described for mesophilic SRB isolated from environments with low sulfate concentrations.     

Succinate transport in Rhizobium leguminosarum.

The transport of succinate was studied in an effective streptomycin-resistant strain of Rhizobium leguminosarum. High levels of succinate transport occurred when cells were grown on succinate, fumarate, or malate, whereas low activity was found when cells were grown on glucose, sucrose, arabinose, or pyruvate as the sole carbon source. Because of the rapid metabolism of succinate after transport into the cells, a succinate dehydrogenase-deficient mutant was isolated in which intracellular succinate accumulated to over 400 times the external concentration. Succinate transport was completely abolished in the presence of metabolic uncouplers but was relatively insensitive to sodium arsenate. Succinate transport was a saturable function of the succinate concentration, and the apparent Km and Vmax values for transport were determined in both the parent and the succinate dehydrogenase mutant. Malate and fumarate competitively inhibited succinate transport, whereas citrate and malonate had no effect. Succinate transport mutants were isolated by transposon (Tn5) mutagenesis. These mutants were unable to transport succinate or malate and were unable to grow on succinate, malate, or fumarate as the sole carbon source. The mutants grew normally on pyruvate, oxaloacetate, citrate, or arabinose, and revertants isolated on succinate minimal medium had regained the ability to grow on malate and fumarate. From these data, we conclude that R. leguminosarum possesses a C4-dicarboxylic acid transport system which is inducible and mediates the active transport of succinate, fumarate, and malate into the cell.     

Simultaneous uptake of galactose and glucose by Azotobacter vinelandii.

Azotobacter vinelandii growing on galactosides induced two distinct permeases for glucose and galactose. The apparent Vmax and Km of the galactose permease were 16 nmol galactose/min per 10(10) cells and 0.5 mM, respectively. The apparent Vmax and Km of the glucose permease were 7.8 nmol glucose/min per 10(10) cells and 0.04 mM, respectively. Excess glucose had no effect on the galactose uptake. However, excess galactose inhibited glucose transport. The galactosides-induced glucose permease also exhibited different uptake kinetics from that induced by glucose.     

Kinetic parameters of maltose hydrolysis and glucose intake in the rat small intestine in a chronic experiment

"True" (corrected for the influence of the pre-epithelial layer) kinetic constants of maltose hydrolysis (Km and Vmax) and Glucose active transport (Kt and Jmax) in the isolated loop of the rat small intestine in chronic experiments were determined using a new mathematical approach. The Km (4.260.25 mM) does not differ from that, obtained in in vitro experiments on the homogenates of mucous membrane taken from the same intestinal loops, and the Vmax (0.72 +/- 0.07 mol/(min.cm)) is 1.7 times lower than that in in vitro experiments. The Kt and Jmax values are 3.18 +/- 0.68 mM and 0.73 +/- 0.07 mol/(min.cm), resp. The estimated values of Km, Kt and Vmax are in accordance with the corresponding published data, whereas the Jmax is several times higher than the value generally believed on the basis of acute experiments in vivo. A high level of glucose absorption in the small intestine of unanesthetized animals is achieved mainly due to a high permeability of the pre-epithelial layer and a high capacity of the active transport as a major mechanism of glucose absorption in the small intestine under normal conditions.     

Microcalorimetric determination of the kinetics of substrate utilisation by non-growing suspensions of Neisseria sicca

The metabolism of various substrates by non-growing suspensions of Neisseria sicca was investigated by a flow-microcalorimetric technique. Substrate utilisation showed Michaelis kinetics allowing determination of saturation constants (Km) and maximum specific rates of substrate utilisation (Vmax). Pyruvate, lactate, a number of tricarboxylic acid cycle intermediates, and amino acids (aspartate, glutamate and proline) were rapidly metabolised [Vmax 5-35 mumol (g dry wt cells)-1 min-1]; Km values were between 4 and 20 microM. Glucose, glycerol, acetate and the other amino acids investigated gave only a slight or no increase in power. The pattern of substrate utilisation is discussed in relation to the role of carbonic anhydrase in N. sicca.     

Uptake of C4 dicarboxylates and pyruvate by Rhodopseudomonas spheroides.

The uptake of C4 dicarboxylates by cells from exponential cultures of Rhodopseudomonas spheroides followed saturation kinetics at concentrations below 100 muM with Km values for succinate, malate, and fumarate of 2.7, 2.3, and 0.8, respectively. Corresponding Vmax values of 50, 52, and 67.5 nmol/min per mg of protein at 20 C were obtained. Each of these compounds interfered competitively with uptake of the others, and a common transport system appears to be involved. Fructose-grown cells took up C4 dicarboxylates only at very low rates, and pyruvate-grown cells took up C4 dicarboxylates at one-third the rates found with succinate-grown cultures. Malonate and maleate inhibited uptake less severely, and aspartate and alpha-ketoglutarate had no effect at 100-fold excess. Divalent metals stimulated uptake. Light or respiration was required for uptake, and entered materials were rapidly converted to other metabolities, notably amino acids. Pyruvate entry appeared to be mediated by several systems, of which only one could be resolved kinetically. This system had a Km of 13 muM and Vmax of 5.6 nmol/min per mg of protein at 20 C. A number of related mono- and dicarboxylates interfered with pyruvate uptake. The pyruvate uptake system was distinguishable from the C4 dicarboxylate system by the absence of divalent cation stimulation and by substrate and inhibitor specificity.     

Replacement of intracellular C-terminal domain of GLUT1 glucose transporter with that of GLUT2 increases Vmax and Km of transport activity.

The intracellular C-terminal domain is diverse in size and amino acid sequence among facilitative glucose transporter isoforms. The characteristics of glucose transport are also divergent, and GLUT2 has far higher Km and Vmax values compared with GLUT1. To investigate the role of the intracellular C-terminal domain in glucose transport, we expressed in Chinese hamster ovary cells the mutated GLUT1 protein whose intracellular C-terminal domain was replaced with that of GLUT2 by means of engineering the chimeric cDNA. Cytochalasin B, for which GLUT2 protein has much lower affinity, bound to this chimeric protein in a fashion similar to GLUT1. In contrast, greater transport activity was observed in this chimeric glucose transporter compared with the wild-type GLUT1 at 10 mM 2-deoxy-D-glucose concentration. The kinetic studies on 2-deoxy-D-glucose uptake revealed a 3.8-fold increase in Km and a 4.3-fold increase in Vmax in this chimeric glucose transporter compared with the wild-type GLUT1. Thus, replacement of the intracellular C-terminal domain confers the GLUT2-like property on the glucose transporter. These results strongly suggest that the diversity of intracellular C-terminal domain contributes to the diversity of glucose transport characteristics among isoforms.