Interaction of ascorbate and alpha-tocopherol in resealed human erythrocyte ghosts. Transmembrane electron transfer and protection from lipid peroxidation

A role for ascorbate-derived electrons in protection against oxidative damage to membrane lipids was investigated in resealed human erythrocyte ghosts. Incubation of resealed ghosts with the membrane-impermeant oxidant ferricyanide doubled the ghost membrane concentration of F2-isoprostanes, a sensitive marker of lipid peroxidation. Incorporation of ascorbate into ghosts during resealing largely prevented F2-isoprostane formation due to extravesicular ferricyanide. This protection was associated with a rapid transmembrane oxidation of intravesicular ascorbate by extravesicular ferricyanide. Transmembrane electron transfer, which was measured indirectly as ascorbate-dependent ferricyanide reduction, correlated with the content of alpha-tocopherol in the ghost membrane in several respects. First, ascorbate resealed within ghosts protected against ferricyanide-induced oxidation of endogenous alpha-tocopherol in the ghost membrane. Second, when exogenous alpha-tocopherol was incorporated into the ghost membrane during the resealing step, subsequent ferricyanide reduction was enhanced. Last, incubation of intact erythrocytes with soybean phospholipid liposomes, followed by resealed ghost preparation, caused a proportional decrease in both the membrane content of alpha-tocopherol and in ferricyanide reduction. Incorporation of exogenous alpha-tocopherol during resealing of ghosts prepared from liposome-treated cells completely restored the ferricyanide-reducing capacity of the ghosts. These results suggest that the transmembrane transfer of ascorbate-derived electrons in erythrocyte ghosts is dependent in part on alpha-tocopherol and that such transfer may help to protect the erythrocyte membrane against oxidant stress originating outside the cell.

Characterization of 8-(N-methylisopropyl)amino-N6-(5'-endohydroxy- endonorbornyl)-9-methyladenine (WRC-0571), a highly potent and selective, non-xanthine antagonist of A1 adenosine receptors

Previous studies in our laboratory identified N6-endonorbornyl-9-methyladenine (N-0861) as a highly selective (100-fold) A1-adenosine receptor antagonist (KB = 500 nM). However, its moderate potency limits the degree of A1-receptor blockade that can be achieved by systemically administered N-0861. Structure activity studies were undertaken to invent a compound that had greater affinity for the A1-adenosine receptors than N-0861. C8-N-methylisopropylamino-N6-5'-endohydroxy-N-0861 (WRC-0571) inhibited [3H]-N6-cyclohexyladenosine (CHA) binding to guinea pig A1-receptors with a Ki value of 1.1 nM. WRC-0571 was 200-fold less potent at inhibiting [3H]-5'-N-ethylcarboxamidoadenosine binding to bovine A2a receptors (Ki = 234 nM). WRC-0571 also inhibited the binding of radioligands to cloned human A1, A2a and A3 adenosine receptors with affinities of 1.7, 105 and 7940 nM, respectively. Thus in human adenosine receptors, WRC-0571 is 62-fold selective for the A1 vs. A2a and 4670-fold selective for the A1 vs. A3 receptors; WRC-0571 is therefore the most A1 vs. A3 selective compound yet described. In guinea pig isolated atria, WRC-0571 antagonized the A1-mediated negative inotropic responses to 5'-N-ethylcarboxamidoadenosine (NECA) with a KB of 3.4 nM. WRC-0571 was more than 2500-fold less potent at antagonizing NECA-induced A2b-mediated relaxation in guinea pig aorta. In anesthetized rats WRC-0571 antagonized adenosine-induced bradycardia at concentrations as low as 1 nmol/kg but failed to antagonize A2-mediated hindquarter vasodilation at concentrations up to 10,000 nmol/kg. WRC-0571 is orally active at concentrations as low as 0.3 mumol/kg. WRC-0571 is therefore a highly potent, highly selective antagonist of A1-adenosine receptors both in vitro and in vivo.

Functional organization of mammalian hexokinase II. Retention of catalytic and regulatory functions in both the NH2- and COOH-terminal halves

The mammalian hexokinase (HK) family includes three closely related 100-kDa isoforms (HKI-III) that are thought to have arisen from a common 50-kDa precursor by gene duplication and tandem ligation. Previous studies of HKI indicated that a glucose 6-phosphate (Glu-6-P)-regulated catalytic site resides in the COOH-terminal half of the molecule and that the NH2-terminal half contains only a Glu-6-P binding site. In contrast, we now show that proteins representing both halves of human and rat HKII have catalytic activity and that each is inhibited by Glu-6-P. The intact enzyme and the NH2- and COOH-terminal halves of the enzyme each increase glucose utilization when expressed in Xenopus oocytes. Mutations corresponding to either Asp-209 or Asp-657 in the intact enzyme completely inactivate the NH2- and COOH-terminal half enzymes, respectively. Mutation of either of these sites results in a 50% reduction of activity in the 100-kDa enzyme. Mutation of both sites results in a complete loss of activity. This suggests that each half of the HKII molecule retains catalytic activity within the 100-kDa protein. These observations indicate that HKI and HKII are functionally distinct and have evolved differently.

Accessibility and reactivity of ascorbate 6-palmitate bound to erythrocyte membranes

Lipophilic derivatives of ascorbic acid may protect lipid bilayers and micelles against lipid peroxidation. In this work the binding, accessibility, and reducing capacity of ascorbate 6-palmitate (A6P) were studied in human erythrocyte membranes. In contrast to less lipophilic carbon-6-modified ascorbate derivatives, A6P bound to erythrocyte membranes in a concentration-dependent manner. This binding was preserved following centrifugation washes, but was largely reversed by extraction with bovine serum albumin. Most of the ascorbyl groups of membrane-bound A6P were readily accessible to oxidation by water-soluble oxidants. Ferricyanide quantitatively oxidized membrane-bound A6P, but the latter spared endogenous tocopherols from destruction. In EPR studies, A6P was much more effective than ascorbate in reducing nitroxide spin labels positioned at either carbon-5 or carbon-16 of membrane-bound stearic acid in both intact cells and in membranes. A6P, thus, appears to intercalate into the erythrocyte membrane with the ascorbyl group located superficially, but with access to the hydrophobic membrane interior, and with the ability to recycle endogenous alpha-tocopherol during oxidant stress.

Ascorbate recycling in human erythrocytes: role of GSH in reducing dehydroascorbate

Human erythrocytes regenerate ascorbate from its oxidized product, dehydroascorbate. The extent to which such ascorbate recycling occurs by a GSH-dependent mechanism was investigated. In the presence of glucose, erythrocytes took up over 90% of extracellular [14C]dehydroascorbate and rapidly converted it to [14C]ascorbate, which was trapped within the cells. Dehydroascorbate uptake and reduction was not associated with generation of a monoascorbyl free radical intermediate. Uptake and reduction of dehydroascorbate by glucose-depleted erythrocytes coordinately decreased GSH and raised GSSG concentrations in erythrocytes. This effect was reversed by D-glucose, but not by L-lactate. Conversely, depletion of cellular GSH decreased the ability of cells to recycle dehydroascorbate to ascorbate, as reflected in the extent to which cells were able to reduce extracellular ferricyanide. Monoascorbyl free radical was formed during the reduction of extracellular ferricyanide, indicating that one electron transfer steps were involved in this process. In GSH-depleted cells, addition of L-lactate as an energy source for glycolysis-dependent NADH regeneration did cause a partial recovery of the ability of cells to reduce ferricyanide. However, in resealed erythrocyte ghosts containing either 4 mM GSH or 400 mu M NADH, only the GSH-containing ghosts supported regeneration of ascorbate from added dehydroascorbate. These results suggest that in human erythrocytes ascorbate regeneration from dehydroascorbate is largely GSH dependent, and that it occurs through either enzymatic or nonenzymatic reactions not involving the monoascorbyl free radical.

Y1 and Y2 receptor selective neuropeptide Y analogues: evidence for a Y1 receptor subclass

Neuropeptide Y (NPY), a 36-residue polypeptide produced abundantly in both nervous and peripheral tissues, appears to play a significant role in the regulation of diverse biological processes, including feeding behavior and cardiovascular and psychotropic functions. The actions of NPY are mediated through effective binding to specific receptors of which two, designated Y1 and Y2, have been well characterized. A shortened cyclic analogue of NPY, des-AA10-17-cyclo-7/21[Cys7,21]NPY, was shown to retain high affinity for both human neuroblastoma SK-N-MC and SK-N-BE2 cell types (expressing Y1 and Y2 receptors, respectively). Increasing the size of the ring (des-AA10-17-cyclo-2/27[Cys2,27]NPY) in the present study produced a high-affinity analogue (Ki = 3.0 vs 0.3 nM for NPY) that bound exclusively to Y2 receptors. Using the feedback from structure-activity relationships, we also describe the optimization of specific substitutions and bridging arrangements leading to the production of other truncated, high-affinity Y1 selective analogues which bind, as does NPY itself, in the low-nanomolar range. Of greatest significance, des-AA10-17-cyclo-7/21[Cys7,21,Pro34]NPY (11) was found to possess agonistic properties with an affinity comparable to that of the native NPY molecule when tested for its ability to inhibit norepinephrine-stimulated cAMP release in SK-N-MC human neuroblastoma cells. Compound 11 also caused an increase in blood pressure in anesthetized rats. However, in two central nervous system models of Y1 receptor function, stimulation of feeding and anxiolytic activity, this analogue was inactive, which suggests the presence of a new subclass of receptors. In summary, the present results demonstrate that residues 10-17 of NPY are not directly involved in either Y1 or Y2 receptor recognition or activation. This suggests that the selectivity of NPY receptors is highly dependent on subtle conformational changes such as the substitution of residue 34 to a proline or the introduction of intramolecular constraints. Additionally, we have produced an analogue of NPY that selectively activates peripheral NPY Y1 receptors.

Ascorbic acid recycling enhances the antioxidant reserve of human erythrocytes

The role of ascorbate transport and metabolism in the response of human erythrocytes to an extracellular oxidant stress was investigated. Rates of entry and exit of [14C]dehydroascorbate from erythrocytes were more than 10-fold greater than those of [14C]ascorbate. Both the reduced and oxidized forms of the vitamin were transported largely by the glucose transporter. Inside erythrocytes, dehydroascorbate was converted to ascorbate, increasing intracellular ascorbate concentrations 2-3-fold over those in the medium. In such ascorbate-loaded cells, the membrane-impermeant oxidant ferricyanide induced a transmembrane oxidation of intracellular ascorbate to dehydroascorbate. The latter escaped the cells on the glucose transporter, which resulted in a halving of the net entry of [14C]dehydroascorbate in the presence of ferricyanide. Treatment of ascorbate-loaded cells with H2O2 and Cu2+ also oxidized ascorbate and induced efflux of [14C]dehydroascorbate. Ferricyanide-dependent intracellular oxidation of ascorbate resulted in a corresponding reduction of extracellular ferricyanide, which served as an integrated measure of ascorbate recycling. Ferricyanide reduction was proportional to the loading concentration of dehydroascorbate and was enhanced when loss of dehydroascorbate from cells was decreased, either by blockade of the glucose transporter or by concentrating the cells. Selective depletion of cellular ascorbate lowered rates of ferricyanide reduction by two-thirds, suggesting that ascorbate rather than NADH is the major donor for the transmembrane ferricyanide oxidoreductase activity. On the basis of the ascorbate-dependent rate of ferricyanide reduction, erythrocytes at a 45% hematocrit can regenerate the ascorbic acid present in whole blood every 3 min. Erythrocyte ascorbate recycling may thus contribute more to the antioxidant reserve of blood than is evident from plasma ascorbate concentrations alone.

Ascorbate is the major electron donor for a transmembrane oxidoreductase of human erythrocytes

Ascorbic acid is an important antioxidant in human blood. Erythrocytes contribute to the antioxidant capacity of blood by regenerating ascorbate and possibly by exporting ascorbate-derived reducing equivalents through a transmembrane oxidoreductase. The role of ascorbate as an electron donor to the latter enzyme was tested in human erythrocytes and ghosts using nitroblue tetrazolium as an electron acceptor. Although nitroblue tetrazolium was not directly reduced by ascorbate, erythrocyte ghosts facilitated reduction of nitroblue tetrazolium in the presence of ascorbate and ascorbate derivatives containing a reducing double bond. The resulting blue monoformazan product was deposited directly in ghost membranes. Ascorbate-induced monoformazan deposition showed several features of an enzyme-mediated process, including hyperbolic dependence on substrate and acceptor concentrations, as well as sensitivity to enzyme proteolysis, detergent solubilization, and sulfhydryl reagents. Incubation of intact erythrocytes with nitroblue tetrazolium caused deposition of the monoformazan in ghost membranes prepared from the cells. This deposition reflected the intracellular ascorbate content and was inhibited by extracellular ferricyanide, a known electron acceptor for the transmembrane oxidoreductase. Although nitroblue tetrazolium did not cross the cell membrane, like the cell-impermeant ferricyanide, it oxidized intracellular [14C]ascorbate to [14C]dehydroascorbate, which then exited the cells. In resealed ghosts, both monoformazan deposition and ferricyanide reduction were proportional to the intravesicular ascorbate concentration. NADH was only about half as effective as a donor for the enzyme as ascorbate in both open and resealed ghosts. These results suggest that not only can ascorbate donate electrons to a transmembrane oxidoreductase, but that it may be the major donor in intact erythrocytes.

Coupled glucose transport and metabolism in cultured neuronal cells: determination of the rate-limiting step

In brain and nerves the phosphorylation of glucose, rather than its transport, is generally considered the major rate-limiting step in metabolism. Since little is known regarding the kinetic coupling between these processes in neuronal tissues, we investigated the transport and phosphorylation of [2-3H]glucose in two neuronal cell models: a stable neuroblastoma cell line (NCB20), and a primary culture of isolated rat dorsal root ganglia cells. When transport and phosphorylation were measured in series, phosphorylation was the limiting step, because intracellular glucose concentrations were the same as those outside of cells, and because the apparent Km for glucose utilization was lower than expected for the transport step. However, the apparent Km was still severalfold higher than the Km of hexokinase I. When [2-3H]glucose efflux and phosphorylation were measured from the same intracellular glucose pool in a parallel assay, rates of glucose efflux were three- to-fivefold greater than rates of phosphorylation. With the parallel assay, we observed that activation of glucose utilization by the sodium channel blocker veratridine caused a selective increase in glucose phosphorylation and was without effect on glucose transport. In contrast to results with glucose, both cell types accumulated 2-deoxy-D-[14C]glucose to concentrations severalfold greater than extracellular concentrations. We conclude from these studies that glucose utilization in neuronal cells is phosphorylation-limited, and that the coupling between transport and phosphorylation depends on the type of hexose used.

Role of the C-terminal tail of the GLUT1 glucose transporter in its expression and function in Xenopus laevis oocytes

Structural determinants for the glucose transport kinetics of the erythrocyte glucose transporter have not been established. In this work the role of the cytosolic carboxy-terminal tail in the expression and function of the human GLUT1 isoform in Xenopus oocytes was investigated. Oocyte plasma membrane expression of GLUT1 was a saturable function of the amount of mRNA injected. Transport activity increased as a linear function of the amount of immunoreactive transporter in the plasma membrane. Transport kinetics of human GLUT1 expressed in oocytes resembled those of human erythrocyte GLUT1. Addition of up to 31 extra amino acids to the carboxy-terminal tail of GLUT1 was without effect on its function in oocytes. Removal of the carboxy-terminal 21 amino acids also did not affect GLUT1 expression or transport kinetics in oocytes. Removal of the entire carboxy-terminal tail to Phe-450 resulted in a transporter that had moderately decreased plasma membrane expression compared to that of GLUT1. However, transport activity of this construct was less than 5% of that of GLUT1, and was associated with loss of its outward-facing inhibitor binding site. When the carboxy-terminal 29 amino acids of GLUT1 were replaced with the corresponding region of GLUT4, transporter expression in the plasma membrane and the transport Vmax fell to low levels, similar to those of native GLUT4. When the carboxy-terminal 29 or 73 amino acids of GLUT1 were swapped into the corresponding region of GLUT4, the transport Vmax markedly increased to about one-third to one-half that of GLUT1, although the affinity for substrate was halved. These results show that the carboxy-terminal tail of the GLUT1 is not critical for targeting of the protein to the plasma membrane, but that this region is an important determinant of transport function.

A "cysteineless" GLUT1 glucose transporter has normal function when expressed in Xenopus oocytes

To test the role of cysteines in the function of GLUT1 glucose transporter, site-directed mutagenesis was used to replace all six GLUT1 cysteines with serine residues. When the individual and combined Cys-->Ser mutants were expressed in Xenopus laevis oocytes, zero-trans uptake of 3-O-methylglucose was comparable to that seen in native GLUT1. The "cysteineless" construct also retained the kinetic features of GLUT1, including an asymmetric transport mechanism and similar substrate and inhibitor affinities. Whereas GLUT1 transport was inhibited by sulfhydryl reagents, that of the "cysteineless" construct was not. These results show that cysteines are not required for GLUT1 function or oligomer formation. The "cysteineless" construct may therefore serve as a template for reintroducing cysteines back into GLUT1 at sites useful for testing transporter structure and function.

Coexpression of glucose transporters and glucokinase in Xenopus oocytes indicates that both glucose transport and phosphorylation determine glucose utilization

A Xenopus oocyte expression system was used to examine how glucose transporters (GLUT 2 and GLUT 3) and glucokinase (GK) activity affect glucose utilization. Uninjected oocytes and low rates of both glucose transport and phosphorylation; expression of GLUT 2 or GLUT 3 increased glucose phosphorylation approximately 20-fold by a low Km, endogenous hexokinase at glucose concentrations < or = 1 mM, but not at higher glucose concentrations. Coexpression of functional GK isoforms with GLUT 2 or 3 increased glucose utilization approximately an additional two- to threefold primarily at the physiologic glucose concentrations of 5-20 mM. The Km for glucose of both the hepatic and beta cell isoforms of GK, determined in situ, was approximately 5-10 mM when coexpressed with either GLUT 2 or GLUT 3. The increase in glucose utilization by coexpression of GLUT 3 and GK was dependent upon glucose phosphorylation since two missense GK mutations linked with maturity-onset diabetes, 182: Val-->Met and 228:Thr-->Met, did not increase glucose utilization despite accumulation of both a similar amount of immunoreactive GK protein and glucose inside the cell. Coexpression of a mutant GK and a normal GK isoform did not interfere with the function of the normal GK enzyme. Since the coexpression of GK and a glucose transporter in oocytes resembles conditions in the hepatocyte and pancreatic beta cell, these results indicate that increases in glucose utilization at glucose concentrations > 1 mM depend upon both a functional glucose transporter and GK.

Identification and functional characterization of A1 and A2 adenosine receptors in the rat vas deferens: a comparison with A1 receptors in guinea pig left atrium and A2 receptors in guinea pig aorta

This study was undertaken to characterize the adenosine receptors in the rat vas deferens. Because adenosine receptors have been well characterized in the cardiovascular system of the guinea pig, antagonist dissociation constants (pKB values) in the rat vas deferens were compared with those from the left atrium (A1) and the aorta (A2) of the guinea pig. The A1-selective agonists (+/-)-N6-endonorbornan-2-yl-5'-N-hydroxy ethylcarboxamidoadenosine (N-0723) and N6-cyclohexyladenosine (CHA) and the nonselective agonist 5'-N-ethylcarboxamidoadenosine (NECA) inhibited the electrically evoked contractions of both the vas deferens and left atrium with a potency order of N-0723 > NECA = CHA. The A2a-selective agonist 2-[p-(2-carboxyethyl)-phenethylamino]5'-N-ethylcarboxamidoadenosin e (CGS21680) was equipotent to NECA in the vas deferens but was 500-fold less potent than NECA in the left atrium. In the aorta only NECA was a potent agonist. The nonselective adenosine receptor antagonist 8-phenyltheophylline antagonized the responses in all three tissues with approximately equal potency (pKB approximately 6.6). In the rat vas deferens, the A1-selective antagonists (+/-)-N6-endonorboman-2-yl-9-methyladenine (N-0861) and 1,3-dipropyl-8-cyclopentylxanthine (DPCPX) were more potent at antagonizing the responses to A1-selective agonists (pKB approximately 8.8 and 6.4, respectively) than they were at antagonizing the responses to NECA and CGS21680 (pKB = 6.3 and < 5, respectively). However, in the left atrium, N-0861 (pKB = 6.2) and DPCPX (pKB = 8.9) were no more potent in antagonizing responses to the A1-selective agonists than they were in antagonizing responses to NECA.(ABSTRACT TRUNCATED AT 250 WORDS)

N-0923, a selective dopamine D2 receptor agonist, is efficacious in rat and monkey models of Parkinson's disease

Certain aminotetralins are known to be potent dopamine D2 receptor agonists. N-0923, [-]2-(N-propyl-N-2-thienylethylamino)-5- hydroxytetralin HCl, recognizes the high and low affinity states of the D2 receptor in membranes from bovine caudate with a Klow of 79 nM. The selectivity ratio is D2/D1 = 15 and D2/alpha 2 = 1.4. N-0923 also inhibits dopamine uptake and prolactin secretion, and it is an antagonist at the alpha 2 receptor. N-0923 (3-300 nmol/kg, s.c.) induced dose-dependent contralateral turning behavior in rats with unilateral 6-hydroxydopamine lesions of the substantia nigra. The ED50 of 30 nmol/kg was effective for 1 h. The positive enantiomer (N-0924; 300 nmol/kg, s.c.) was without effect. A hemiparkinsonian syndrome was induced in four Macaca nemestrina monkeys by unilateral infusion of the neurotoxin MPTP into the right carotid artery. Video recordings of free-moving behavior revealed bradykinesia, disuse of the contralateral upper limb and turning in a direction ipsilateral to the lesion. N-0923 (3-300 nmol/kg, i.m.) induced contralateral turning behavior, exploratory activity, and contralateral limb usage. The ED50 for turning (30 nmol/kg) was effective for 0.5 h. The potency order for induction of contralateral rotations was (+)-PHNO > N-0923 > bromocriptine. N-0924 (300 nmol/kg, i.m.) was ineffective. We conclude that N-0923 may be useful as a therapeutic agent in the treatment of Parkinson's disease.

Coupling of glucose transport and phosphorylation in Xenopus oocytes and cultured cells: determination of the rate-limiting step

The initial events in glucose metabolism by all cells are the transport and phosphorylation of glucose. To quantify the relative contributions of these two processes to overall glucose utilization, we have developed an experimental approach for their in situ measurement as parallel processes. The method is based on the use of intracellular [2-3H]glucose as a substrate for both the transporter and hexokinase, and involves simultaneous measurement of [2-3H]glucose efflux and of 3H2O released by phosphorylation. The Xenopus oocyte expression system was used to test the method, since in these cells transport and phosphorylation activities can be regulated by expression of mRNA or injection of foreign protein. Oocytes microinjected with [2-3H]glucose showed no release of injected glucose, but did have saturable phosphorylation kinetics, with a Km of 40 microM and a Vmax of 0.1 nmol/min/oocyte. Co-injection of yeast hexokinase increased glucose phosphorylation by five-fold. Expression of human glucose transporter (GLUT1) mRNA resulted in a 25-30-fold increase in the rate of saturable efflux of microinjected glucose compared to control oocytes. The kinetics of transport and phosphorylation of [2-3H]glucose were analyzed by a multiple curve-fitting program that provided estimates of kinetic coefficients for both processes from a single time course. The analysis showed that expression of GLUT1 shifted the rate-limiting step in glucose utilization from transport to phosphorylation. A similar shift occurred at a three-fold lower extracellular concentration of 2-deoxyglucose. In a pancreatic beta cell line both transport and phosphorylation showed high Km values, with phosphorylation as the limiting step. The in situ measurement of glucose transport and phosphorylation as parallel processes should be useful in defining the relative contributions of each step to overall glucose metabolism in other cell and tissue models.

Ligand-induced conformational changes modify proteolytic cleavage of the adipocyte insulin-sensitive glucose transporter

The transport conformation of the human erythrocyte glucose transporter (GLUT1) modifies rates of proteolytic cleavage of this protein by a variety of enzymes. We investigated the effects of ligand-induced conformational change on the susceptibility to enzymic cleavage of the insulin-sensitive rat adipocyte glucose transporter (GLUT4). A GLUT4-enriched slow sedimenting microsomal fraction was prepared from basal adipocytes and subjected to PAGE and immunoblotting. The GLUT4 protein was detected in these immunoblots with a C-terminal-specific antiserum as an M(r)-46,000-50,000 doublet. GLUT1 protein was not detected by a GLUT1-specific antiserum in these membranes. Tryptic digestion caused loss of the GLUT4 signal in immunoblots in a time- and concentration-dependent fashion. Low-M(r) membrane-bound fragments were not observed in electrophoretic gels, whether detection was attempted by immunoblotting or by counting radioactivity in gel slices following photolabelling with [3H]cytochalasin B. Transport-specific ligands known to induce an outward-facing conformation in the human erythrocyte GLUT1 protein retarded cleavage of the GLUT4 protein by submaximal concentrations of trypsin, whereas ligands known to induce an inward-facing conformation increased the extent of cleavage. The transported substrate D-glucose retarded tryptic cleavage of GLUT4. This result contrasts with the known behaviour of GLUT1, in which D-glucose accelerates cleavage. Cleavage of GLUT4 by thermolysin was also retarded by the outward-binding analogue 4,6-O-ethylidene glucose. These results show that the conformational sensitivity to proteolysis of GLUT4 mirrors that of GLUT1, except that the glucose-loaded GLUT4 has a different steady-state configuration, which may reflect underlying kinetic differences between the two proteins.

Tryptic digestion of the human erythrocyte glucose transporter: effects on ligand binding and tryptophan fluorescence

The conformation of the human erythrocyte glucose transport protein has been shown to determine its susceptibility to enzymatic cleavage on a large cytoplasmic loop. We took the converse approach and investigated the effects of tryptic digestion on the conformational structure of this protein. Exhaustive tryptic digestion of protein-depleted erythrocyte ghosts decreased the affinity of the residual transporter for cytochalasin B by 3-fold but did not affect the total number of binding sites. Tryptic digestion also increased the affinity of the residual transporter for D-glucose and inward-binding sugar phenyl beta-D-glucopyranoside but decreased that for the outward-binding 4,6-O-ethylidene glucose. These results suggest that tryptic cleavage stabilized the remaining transporter in an inward-facing conformation, but one with decreased affinity for cytochalasin B. The steady-state fluorescence emission scan of the purified reconstituted glucose transport protein was unaffected by tryptic digestion. Addition of increasing concentrations of potassium iodide resulted in linear Stern-Volmer plots, which were also unaffected by prior tryptic digestion. The tryptophan oxidant N-bromosuccinimide was investigated to provide a more sensitive measure of tryptophan environment. This agent irreversibly inhibited 3-O-methylglucose transport in intact erythrocytes and cytochalasin B binding in protein-depleted ghosts, with a half-maximal effect observed for each activity at about 0.3-0.4 nM. Treatment of purified glucose transport protein with N-bromosuccinimide resulted in a time-dependent quench of tryptophan fluorescence, which was resolved into two components by nonlinear regression using global analysis. Tryptic digestion retarded the rate of oxidation of the more slowly reacting class of tryptophans. (ABSTRACT TRUNCATED AT 250 WORDS)

Serum angiotensin converting enzyme in diabetic patients

Serum angiotensin converting enzyme (ACE) levels are higher in patients with diabetes mellitus than in many others. Techniques are available to grade different degrees of diabetic retinopathy, which can demonstrate a relationship between ACE and diabetic retinopathy. In this study, patients with diabetic retinopathy had higher serum ACE levels (6.3 +/- 0.2) than nondiabetic patients (4.3 +/- 0.5) (p < 0.001). In addition, the mean serum ACE level in diabetic patients with nonproliferative retinopathy (5.55 +/- 0.4) was less than that in diabetic patients with proliferative retinopathy (6.63 +/- 0.25) (p = 0.02). Due to the variability in individual serum ACE levels and the frequent use of ACE inhibitors by hypertensive diabetics, these techniques are not suitable for retinopathy screening programs. However, the graded relationship demonstrated by these data may have relevance for the pathophysiology of diabetic retinopathy.

In vitro and in vivo pharmacological characterization of N6-cyclopentyl-9-methyladenine (N-0840): a selective, orally active A1 adenosine receptor antagonist

The pharmacological and biochemical profile of N6-cyclopentyl-9-methyladenine (N-0840) was elucidated in vitro and in vivo. In radioligand binding assays, N-0840 had 14- to 400-fold greater affinity for A1 than A2 adenosine receptors and did not inhibit radioligand binding to alpha-1, alpha-2, beta, 5-hydroxytryptamine 1a, muscarinic, D1 or D2 receptors at concentrations < or = 10,000 nM. In guinea pig tissues, N-0840 competitively antagonized A1 receptor-mediated, 5'-N-ethylcarboxamidoadenosine-induced negative inotropism (paced left atria, KB = 0.83 microM), chronotropism (spontaneously beating right atria, KB = 0.91 microM) and dromotropism (Langendorff heart; KB = 0.72 microM). However, at concentrations up to 100 microM, N-0840 did not antagonize A2 adenosine receptor-mediated, 5'-N-ethylcarboxamidoadenosine-induced relaxations of the guinea pig aorta. N-0840 was a poor inhibitor of total cyclic nucleotide phosphodiesterase activity and of adenosine uptake (IC50 > 200 microM), and it did not inhibit adenosine deaminase activity. In anesthetized rats, N-0840 selectively antagonized A1 adenosine receptor-mediated bradycardia, but generally failed to affect A2 adenosine receptor-mediated vasodilation in the in situ perfused hindquarters (A2/A1 selectivity: > or = 33-fold). The duration of action of N-0840 ranged from 1 min (after 3 mumol/kg i.v.) to 8 hr (after 100 mumol/kg p.o.). N-0840 (< or = 100 mumol/kg i.v.; < or = 1,000 mumol/kg p.o.) had little or no effect on blood pressure or heart rate and produced no adverse drug reactions.(ABSTRACT TRUNCATED AT 250 WORDS)

Monitoring conformational change in the human erythrocyte glucose carrier: use of a fluorescent probe attached to an exofacial carrier sulfhydryl

Several fluorescent sulfhydryl reagents were tested as probes for assessing substrate-induced conformational change of the human erythrocyte glucose carrier. Of these, 2-(4'-maleimidylanilino)-naphthalene-6-sulfonic acid (Mal-ANS) inhibited 3-O-methylglucose transport most strongly and specifically labeled a previously characterized exofacial sulfhydryl on the glucose carrier. Analysis of equilibrium cytochalasin B binding in cells treated with Mal-ANS suggested that the inhibition of transport was due to a partial channel-blocking effect, and not to competition for the substrate binding site or to hindrance of carrier conformational change. In purified glucose carrier prepared from cells labeled on the exofacial sulfhydryl with Mal-ANS, a blue shift in the peak of fluorescence indicated that the fluorophore was in a relatively hydrophobic environment. Mal-ANS fluorescence in such preparations was quenched by ligands with affinity for the outward-facing carrier (ethylidene glucose, D-glucose, and maltose), but not by inhibitors considered to bind to the inward-facing carrier conformation (cytochalasin B or phenyl beta-D-glucoside). The effect of ethylidene glucose appeared to be related to an interaction with the glucose carrier, since the concentration dependence of ethylidene glucose-induced quench correlated well with the ability of the sugar analog to inhibit cytochalasin B binding to intact cells. The hydrophilic quenchers iodide and acrylamide decreased carrier-bound Mal-ANS fluorescence, resulting in downward-curving Stern-Volmer plots. Whereas ethylidene glucose enhanced iodide-induced quench, it had no effect on that of acrylamide.(ABSTRACT TRUNCATED AT 250 WORDS)

The one-site model of human erythrocyte glucose transport: testing its predictions using network thermodynamic computer simulations

Network thermodynamic computer simulations were carried out using parameters experimentally derived by Lowe and Walmsley ((1987) Biochim. Biophys. Acta 903, 547-550) for two tests of the one-site model of human erythrocyte glucose transport. In the temperature-jump experiment, the simulations predicted the amplitude and relaxation time of accelerated uptake, but underestimated the net uptake due to an unexpectedly low measured basal rate. In the maltose-acceleration experiment, the dissociation constant of maltose was assessed at 0 degrees C by measuring the inhibitory effects of maltose on both cytochalasin B binding and on 3-O-methylglucose uptake, and using this value (52 mM) to calculate the dissociation constant (2.9 mM). The simulated experiment then did show a transient acceleration in uptake comparable in magnitude to that observed experimentally, except that the relaxation time was more than 10-fold longer in the simulations. Measurements of the temperature dependence of the inhibition of cytochalasin B binding by maltose and 3-O-methylglucose indicated that apparent sugar affinity is sensitive to carrier orientation at low temperatures, whereas at more physiologic temperatures the intrinsic dissociation constant predominated.

Localization of a reactive exofacial sulfhydryl on the glucose carrier of human erythrocytes

Tryptic digestion studies of the human erythrocyte glucose carrier have shown that a reactive and transport-sensitive exofacial sulfhydryl is located in the carboxy-terminal half of the molecule, corresponding to Cys347, Cys421, or Cys429. In the present studies, the erythrocyte glucose carrier labeled on the exofacial sulfhydryl with bis(maleimidomethyl) ether-L-[35S]cysteine was chemically cleaved, either at tryptophans by N-bromosuccinimide or at nonalkylated cysteines by 2-nitro-5-thiocyanobenzoic acid. The resulting fragments were separated by linear gradient polyacrylamide gel electrophoresis, and the labeled fragments were identified by their apparent molecular weight, and by immunoblotting with antibodies to specific regions of the carrier protein. All of the labeled fragments were recognized by an antibody to the carboxy terminus of the carrier, but not by an antibody to a cytoplasmic loop on the C-terminal half of the carrier. The labeled exofacial sulfhydryl was assigned to Cys429, since this is the only residue of the three possibilities which is beyond the expected cleavage sites of the two reagents in the carrier sequence. These results concur with the predictions of hydropathy analysis and will be relevant for studies of how modification of this sulfhydryl affects carrier function, particularly since several other known carrier isoforms lack a corresponding cysteine.

Thiol-fatty acylation of the glucose transport protein of human erythrocytes

Incubation of intact human erythrocytes with [3H]palmitate labeled a protein with electrophoretic characteristics of the glucose transporter. This labeling occurred via a thioester linkage, since it was unaffected by organic solvent extraction, but was substantially removed as the hydroxamate upon treatment with neutral hydroxylamine. Immunoprecipitation of the labeled protein with a monoclonal antibody to the glucose transporter confirmed its identity.

Differential labeling of the erythrocyte hexose carrier by N-ethylmaleimide: correlation of transport inhibition with reactive carrier sulfhydryl groups

Inhibition of hexose transport by N-ethylmaleimide was studied with regard to alkylation of different types of sulfhydryl group on the hexose carrier of the human erythrocyte. Uptake of 3-O-methylglucose was progressively and irreversibly inhibited by N-ethylmaleimide, with a half-maximal effect at 10-13 mM. A sulfhydryl group known to exist on the exofacial carrier was not involved in transport inhibition by N-ethylmaleimide, since reversible protection of this group by the impermeant sulfhydryl reagent 5,5'-dithiobis(2-nitrobenzoic acid) had no effect on the ability of N-ethylmaleimide to inhibit transport, or on its ability to decrease the affinity of the exofacial carrier for maltose. Nevertheless, the exofacial sulfhydryl was quite reactive with N-ethylmaleimide, since it was possible using a differential labeling technique to specifically label this group in protein-depleted ghosts with a half-maximal effect at 0.3 mM N-[3H]ethylmaleimide, and to localize it to the Mr 19,000 tryptic carrier fragment. Transport inhibition by N-ethylmaleimide correlated best with labeling of a single cytochalasin B-sensitive internal sulfhydryl group on the glycosylated Mr 23,000-40,000 tryptic fragment of the carrier, which was half-maximally labeled at about 4 mM reagent. Whereas N-ethylmaleimide readily alkylates the exofacial carrier sulfhydryl, it inhibits transport by reacting with at least one internal carrier sulfhydryl located on the glycosylated tryptic carrier fragment.