大鼠红细胞葡萄糖代谢的异头物选择性

为研究大鼠红细胞对葡萄糖利用的异头物选择性及其作用机制,应用大鼠红细胞,对葡萄糖的两种异头物作了异构化速率、乳酸生成量、内流速度和大鼠红细胞已糖激酶作用下的磷酸化速度等进行了测定．结果指出,３７℃时大鼠红细胞的Ｄ－葡萄糖β－异头物和α－异头物代谢成乳酸的速度分别是０．２７μｍｏｌ／ｇＨｂ（３ｍｉｎ）和０．２１μｍｏｌ／ｇＨｂ（３ｍｉｎ）,即前者快于后者３０％．同时β－Ｄ－葡萄糖向红细胞内转运速度也快于后者：分别是５．０和３．５μｍｏｌ／ｇＨｂ（３ｍｉｎ）．大鼠红细胞已糖激酶的葡萄糖磷酸化速率实验结果指出：β－异头物比α－异头物快３０％;对于该两种异头物已糖激酶的Ｋｍ值均为５３μｍｏｌ／Ｌ．红细胞与α－和β－Ｄ－葡萄糖保温１ｍｉｎ后,其葡萄糖浓度均达到１ｍｍｏｌ／Ｌ左右,说明至少在１ｍｉｎ内对于已糖激酶的磷酸化此两种异头物的葡萄糖浓度均已饱和．这些结果提示,大鼠红细胞葡萄糖利用的β－异头物优选性主要与其磷酸化速度有关,而与其转运速度关系不大．     

Anomeric specificity in the response of gut glucagon-like immunoreactive materials to glucose.

An anomeric specificity of the glucose sensors of A cells and B cells of the pancreas has been reported. In this context the present authors investigated, using the canine intestinal loop prepared from the terminal portion of the ileum, how glucagon-like immunoreactive materials (GLI) of the gut would respond to glucose anomers in an attempt to explore a possible anomeric specificity of glucose-stimulated gut GLI secretion. As a result GLI was found to be more readily released into the blood stream after an intestinal alpha-glucose load than following beta-gluocse during a 15-minute observation period. It is thus suggested that gut GLI-secreting cells have glucose sensors similar to those of pancreatic A or B cells which are specific for the alpha-glucose anomer.     

Anomeric preference of glucose phosphorylation and glycolysis in human erythrocytes

Lactate output from the alpha and beta anomers of glucose was measured in intact human erythrocytes at 37 degrees C; and glucose anomer phosphorylation, in human erythrocyte homogenates. The rates of both glucose metabolism (lactate output) and phosphorylation were higher in the presence of beta-D-glucose as distinct from alpha-D-glucose at three glucose concentrations used (2, 5, and 10 mM). Thus, the v beta/v alpha ratios of metabolism and phosphorylation of glucose at 2 mM were 1.24 and 1.22, respectively. The results indicate that the beta preference of hexokinase, a rate-limiting enzyme in glycolysis, is reflected in beta-preferential glycolysis.     

Anomeric preference of glucose utilization in human erythrocytes loaded with glucokinase

Human erythrocytes were loaded with homogeneous rat liver glucokinase by an encapsulation method based on hypotonic hemolysis and isotonic resealing. As assayed at 10 mM glucose, glucokinase and hexokinase activities in glucokinase-loaded erythrocytes were 218 and 384 nmol/min/gHb, respectively; whereas hexokinase activity in both intact and unloaded red cells, which contain no glucokinase activity, was about 400 nmol/min/gHb. No difference in the rate of lactate production from glucose anomers between intact and unloaded erythrocytes suggested that the encapsulation procedure itself did not affect glucose utilization in red cells. Alpha-anomeric preference in lactate production from glucose was observed in glucokinase-loaded erythrocytes, whereas the beta anomer of glucose was more rapidly utilized than the alpha anomer in intact and unloaded erythrocytes. The results indicate that the step of glucose phosphorylation determines the anomeric preference in glucose utilization by human erythrocytes, since glucokinase and hexokinase are alpha- and beta-preferential, respectively, in glucose phosphorylation.     

Insulin release and eflux of [32P]Phosphate from islets of rat Langerhans treated with iodoacetic acid and the anomers of D-glucose.

To clarify the insulin-releasing mechanism, we studied insulin release and the efflux of [32P]phosphate by glucose at 0.1 mM/min of gradient level or at 16.7 mM, and other metabolism in islets of rat Langerhans. When treated with 1 mM iodoacetic acid (IAA) plus the anomers of D-glucose at 2.8 mM for 6 min at 37 degrees C, islets elicited insulin at half the control rate under the step-wise stimulation by glucose and at the same rate as the control under the slow-rise stimulation by glucose. Using islets treated with IAA plus the alpha anomer at 16.7 mM, the step-wise stimulation secreted insulin at half a rate of the control and the slow-rise stimulation at the rate lower than the control, which was not significantly different from the control rate. Treatment with IAA plus the beta anomer at 16.7 mM inhibited insulin release under both types of stimulations by glucose. The step-wise stimulation caused the same rapid efflux of [32P]phosphate from IAA-treated islets as from the control islets, except for islets treated with IAA plus the beta anomer at 16.7 mM. The rate of glucose utilization in islets was inhibited by all IAA-treatments to the same extent, being merely half the control rate. Treatments with IAA plus the anomers at 16.7 mM significantly reduced the formation of [3H]-cAMP and the activity of protein phosphokinase in islets, while in the presence of the anomers at 2.8 mM IAA produced no significant effect. Neither IAA-treatments altered the uptake of 45Ca and the ATP content in islets. The uptake of [14C]IAA was significantly enhanced by the presence of the beta anomer at 16.7 mM to two times the control level. On the basis of these results, we suggested that the B cell might contain both glucoreceptors and rate-sensors of glucose controlling insulin release and the former might be less sensitive to IAA as compared with the latter.     

The reduction of CrVI to CrIII by the alpha and beta anomers of D-glucose in dimethyl sulfoxide. A comparative kinetic and mechanistic study

The reduction of CrVI by alpha-D-glucose and beta-D-glucose was studied in dimethyl sulfoxide in the presence of pyridinium p-toluensulfonate, a medium where mutarotation is slower than the redox reaction. The two anomers reduce CrVI by formation of an intermediate CrVI ester precursor of the slow redox step. The equilibrium constant for the formation of the intermediate chromic ester and the rate of the redox steps are different for each anomer. alpha-D-Glucose forms the CrVI-Glc ester with a higher equilibrium constant than beta-D-glucose, but the electron transfer within this complex is slower than for the beta anomer. The difference is attributed to the better chelating ability of the 1,2-cis-diolate moiety of the alpha anomer. The CrV species, generated in the reaction mixture, reacts with the two anomers at a rate comparable with that of CrVI. The EPR spectra show that the alpha anomer forms several linkage isomers of the five-coordinate CrV bis-chelate, while beta-D-glucose affords a mixture of six-coordinate CrV monochelate and five-coordinate CrV bis-chelate. The conversion of the CrV mono- to bis-chelate is discussed in terms of the ability of the 1,2-cis- versus 1,2-trans-diolate moieties of the glucose anomers to bind CrV.     

The crystal structure of Trypanosoma cruzi glucokinase reveals features determining oligomerization and anomer specificity of hexose-phosphorylating enzymes

Glucose is an essential substrate for Trypanosoma cruzi, the protozoan organism responsible for Chagas' disease. The glucose is intracellularly phosphorylated to glucose 6-phosphate. Previously, a hexokinase responsible for this phosphorylation has been characterized. Recently, we identified an ATP-dependent glucokinase in T. cruzi exhibiting a tenfold lower substrate affinity compared to the hexokinase. Both enzymes, which belong to very different groups of the same family, are located inside glycosomes, the peroxisome-like organelles of Kinetoplastida that are known to contain the first seven glycolytic steps as well as enzymes of the oxidative branch of the pentose phosphate pathway. Here, we present the crystallographic structure of T. cruzi glucokinase, in complex with glucose and ADP. The structure suggests a loose tetrameric assembly formed by the association of two tight dimers. TcGlcK was previously reported to exist in a concentration-dependent equilibrium of monomeric and dimeric states. Here, we used mass spectrometry analysis to confirm the existence of TcGlcK monomeric and dimeric states. The analysis of subunit interactions and comparison with the bacterial glucokinases give insights into the forces promoting the stability of the different oligomeric states. Each T. cruzi glucokinase monomer contains one glucose and one ADP molecule. In contrast to hexokinases, which show a moderate preference for the alpha anomer of glucose, the electron density clearly shows the d-glucose bound in the beta configuration in the T.cruzi glucokinase. Kinetic assays with alpha and beta-d-glucose further confirm a moderate preference of the T. cruzi glucokinase for the beta anomer. Structural comparison of the glucokinase and hexokinases permits the identification of a possible mechanism for anomer selectivity in these hexose-phosphorylating enzymes. The preference for distinct anomers suggests that in T. cruzi hexokinase and glucokinase are not directly competing for the same substrate and are probably both present because they exert distinct physiological functions.     

Glucose interactions with a model peptide

Molecular dynamics simulations have been conducted of the helical polypeptide melittin, in concentrated aqueous solutions of the alpha and beta anomers of D-glucopyranose. Glucose is an osmolyte, and it is expected to be preferentially excluded from the surfaces of proteins. This was indeed found to be the case in the simulations. The results indicate that the observed exclusion may have a contribution from an under-representation of hydrogen bonding interactions between glucose groups and exposed side chains, compared to water. However, glucose was found to bind quite specifically to melittin by stacking its hydrophobic face, consisting of aliphatic protons, against the flat hydrophobic face of the indole group of the tryptophan-19 side chain. Although the binding site for this interaction is localized, the binding is weak for both anomers, with a binding free energy estimated as only ～0.5 kcal/mol (i.e. near k(B)T). The face of the sugar stacked against the Trp indole ring is different for the two anomers of glucose, due to the disruption of the H1-H3-H5 hydrophobic triad of the beta anomer by the axial C1 hydroxyl group in the alpha anomer. The measurable affinity of the sugar for the Trp side chain is consistent with the very frequent occurrence of this group in the binding sites of proteins that complex with sugars.     

Partial purification and some properties of beta-phosphoglucomutase from Lactobacillus brevis

A phosphoglucomutase (beta-phosphoglucomutase) specific for beta-glucose 1-phosphate, which catalyzes the beta-glucose 1-phosphate:glucose 6-phosphate interconversion, was 560-fold purified from Lactobacillus brevis strain L6. The isoelectric point of beta-phosphoglucomutase was 3.8 and it had an apparent molecular weight of 29,000 estimated by gel chromatography. The enzyme required a divalent cation (Mn2+ greater than Mg2+ greater than Ni2+ greater than Co2+) and beta-glucose 1,6-bisphosphate for activity. The equilibrium constant Ke for the reaction beta-D-glucose 1-phosphate in equilibrium D-glucose 6-phosphate at 30 degrees C and pH 6.7 is 18.5. beta-phosphoglucomutase had a pH optimum between 6.3 and 6.8 and appeared to be quite specific: alpha-glucose 1-phosphate, alpha- or beta-galactose 1-phosphate and alpha- or beta-N-acetylglucosamine 1-phosphate did not substitute for beta-glucose 1-phosphate. Double reciprocal plots of the data from initial velocity studies at five beta-glucose 1-phosphate concentrations (10 to 100 microM) and four beta-glucose 1,6-bisphosphate concentrations (0.125 to 1.0 microM) showed that the apparent Michaelis constants for beta-glucose 1-phosphate and beta-glucose 1,6-bisphosphate were related to the concentrations of beta-glucose 1,6-bisphosphate and beta-glucose 1-phosphate, respectively, in such a way as to suggest a ping-pong mechanism. The same conclusion was obtained when substrate-velocity relationships were investigated at fixed ratio of both substrates: the Lineweaver-Burk plots showed linear lines and no parabolic ones. The "true" Km for beta-glucose 1-phosphate and beta-glucose 1,6-bisphosphate were found to be about 12 and 0.8 microM, respectively.     

Glucoamylase originating from Schwanniomyces occidentalis is a typical alpha-glucosidase

A starch-hydrolyzing enzyme from Schwanniomyces occidentalis has been reported to be a novel glucoamylase, but there is no conclusive proof that it is glucoamylase. An enzyme having the hydrolytic activity toward soluble starch was purified from a strain of S. occidentalis. The enzyme showed high catalytic efficiency (k(cat)/K(m)) for maltooligosaccharides, compared with that for soluble starch. The product anomer was alpha-glucose, differing from glucoamylase as a beta-glucose producing enzyme. These findings are striking characteristics of alpha-glucosidase. The DNA encoding the enzyme was cloned and sequenced. The primary structure deduced from the nucleotide sequence was highly similar to mold, plant, and mammalian alpha-glucosidases of alpha-glucosidase family II and other glucoside hydrolase family 31 enzymes, and the two regions involved in the catalytic reaction of alpha-glucosidases were conserved. These were no similarities to the so-called glucoamylases. It was concluded that the enzyme and also S. occidentalis glucoamylase, had been already reported, were typical alpha-glucosidases, and not glucoamylase.     

The anomeric form of D-fructose 1,6-bisphosphate used as substrate in the muscle and yeast aldolase reactions.

From a series of rapid quench kinetic experiments, it has been demonstrated that muscle D-fructose bisphosphate aldolase catalyzes the cleavage of beta-D-fructose 1,6-bisphosphate but not that of the alpha anomer, although the alpha anomer may be tightly bound. Yeast D-fructose bisphosphate aldolase appears to utilize both alpha and beta anomers of the substrate, with yeast apoaldolase catalyzing the interconversion of the alpha and beta forms.     

The synthesis and chemical properties of polyisoprenyl beta-D-mannopyranosyl phosphates.

2,3,4,6-Tetra-O-acetyl-beta-D-mannopyranosyl phosphate, free of the alpha anomer, was coupled with citronellol and dolichol in the presence of triisopropylbenzenesulfonyl chloride to give, after chromatographic purification and deacetylation, the respective polyisoprenyl beta-D-mannopyranosyl phosphates. These compounds were compared with the previously synthesized alpha anomers by means of their chromatographic properties, spectra, optical rotations, and hydrolysis reactions when treated with acid and alkali. To characterize the compounds resulting from these treatments, and to determine the mechanism of the alkaline hydrolysis, beta-D-mannopyranosyl phosphate was converted into beta-D-mannonpyranose 1,2-phosphate, and hence into D-mannose 2-phosphate, obtained as a mixture of alpha and beta anomers, characterized by infrared and nuclear magnetic resonance spectra and elemental analysis. Beta-D-Mannopyranosyl phosphate was readily separated by thin layer chromatography from the corresponding alpha anomer.     

B3LYP/6-311++G** geometry-optimization study of pentahydrates of alpha- and beta-D-glucopyranose

Five water molecules were placed in 37 different configurations around alpha- and beta-D-glucopyranose in the gt, gg, and tg conformational states, and the glucose-water complexes were geometry optimized using density functionals at the B3LYP/6-311++G** level of theory. The five water molecules were organized in space and energy minimized using an empirical potential, AMB02C, and then further geometry optimized using DFT algorithms to minimum energy positions. Electronic energy, zero point vibrational energy, enthalpy, entropy, stress energy on glucose and the water cluster, hydrogen-bond energy, and relative free energy were obtained for each configuration using thermodynamic procedures and an analytical Hessian program. The lowest energy complex was that of a clustering of water molecules around the 1- and 6-hydroxyl positions of the beta-gt anomer. Configurations in which the water molecules created a favorable network completely around and under glucose were found to have low energy for both alpha and beta anomers. Calculation of the alpha/beta anomeric ratio using the zero point corrected energy gave, approximately 32/68%, highly favoring the beta anomer in agreement with the experimental approximately 36/64% value. This ratio is better than the approximately 50/50% ratio found in our previous monohydrate study. An approximate hydroxymethyl population was obtained by noting average relative energies among the three conformational states, gt, gg, and tg. In the beta anomer complexes the gt conformation was favored over the gg state, while in the alpha anomer complexes the gg state was favored over the gt conformation, with the tg conformations all being of higher energy making little or no contribution to the rotamer population. Some geometry variances, found between glucose in vacuo and glucose after interaction with water molecules, are described and account for some observed C-5-C-6 bond length anomalies reported by us previously for the vacuum glucose structures.     

Preparation of alpha and beta anomers of 1,2,3,6-tetra-O-acetyl-4-chloro-4-deoxy-D-galactopyranose based upon anomerization and kinetic acetylation

4-Chloro-4-deoxy-alpha-d-galactopyranose, 1,2,3,6-tetra-O-acetyl-4-chloro-4-deoxy-alpha-d-galactopyranose and 1,2,3,6-tetra-O-acetyl-4-chloro-4-deoxy-beta-d-galactopyranose were readily prepared from 1,4:3,6-dianhydro-beta-d-fructofuranosyl 4-chloro-4-deoxy-alpha-d-galactopyranoside. In the study, we found an interesting anomerization phenomenon of 4-chloro-4-deoxy-d-galactose. The molar ratio of alpha and beta anomers in solution is about 1:2 when the anomerization reaches a dynamic equilibrium, and the beta anomer could completely convert to the alpha anomer in the process of crystallization and precipitation. The acetylation of 4-chloro-4-deoxy-d-galactopyranose is kinetically controlled, and the configuration of the starting galactose determines the configuration of the resulting acetates. The influence of the chloro group at C-4 and the O-acetyl group at the anomeric carbon on the galactopyranose ring conformations is discussed, based upon the crystallographic data for the alpha and beta anomers of 1,2,3,6-tetra-O-acetyl-4-chloro-4-deoxy-d-galactopyranose.     

Phosphorylated aminosugars: synthesis, properties, and reactivity in enzymatic reactions

A number of phosphorylated aminosugars have been prepared and tested as substrates for metabolic reactions. 6-Aminoglucose is a slow substrate for yeast hexokinase with a Vmax that is only 0.012% that for glucose. While Vmax is pH independent, V/K decreases below the pK of 9.0 of the amino group. 6-Aminoglucose is a competitive inhibitor vs glucose with a Ki value increasing below the pK of 9 but leveling off at 33 mM below pH 7.16. Thus, protonation decreases binding affinity by 2.4 kcal/mol and only the neutral amine is catalytically competent. 6-Aminoglucose-6-P was synthesized enzymatically with hexokinase. Its pK's determined by 31P NMR were 2.46 and 8.02 (alpha anomer) and 2.34 and 7.85 (beta anomer), with a beta:alpha ratio of 3.0. It is most stable at pH 12 (half-life 228 h at 22 degrees C), while as a monoanion its half-life is 3 h. The free energy of hydrolysis at 25 degrees C and pH 9.25 is -10.3 kcal/mol. The phosphorylated amino analogues of 6-P-gluconate, ribulose-5-P, fructose-6-P, fructose-1,6-bis-P (amino group at C-6 only), and glyceraldehyde-3-P were synthesized enzymatically. The 31P NMR chemical shifts of these analogues are 8-8.5 ppm at pH 9.5. Their relative stability is 6-aminogluconate-6-P greater than 3-aminoglyceraldehyde-3-P greater than 6-aminoglucose-6-P greater than 6-aminofructose-1,6-bis-P congruent to 6-aminofructose-6-P greater than 5-aminoribulose-5-P. These analogues were tested as substrates for their respective enzymes.(ABSTRACT TRUNCATED AT 250 WORDS)     

Affinity purification and glucose specificity of aldose reductase from bovine lens

Aldose reductase, a possible key enzyme of sugar-cataract formation in diabetes, has been purified from bovine lens by a five-step procedure including affinity chromatography with Mātrex gel red A. The enzyme was purified 12,600-fold and was apparently homogeneous by polyacrylamide gel electrophoresis. The glucose specificity of the purified enzyme was studied with d-glucose anomers and d-glucitol as substrates. The ratios of the reduction rate of α-d-glucose to that of β-d-glucose at 10, 13, and 20 mm were 1.90, 1.76, and 1.72, respectively. These values were in good agreement with the ratios (1.92, 1.81, and 1.66) calculated on the basis of the rate constants reported for d-glucose mutarotation equilibrium (J. M. Los, L. B. Simpson, and K. Wiesner, 1956, J. Amer. Chem. Soc.78, 1564–1568) and the assumption that aldose reductase acts on the aldehyde form of d-glucose. In addition, the composition of d-glucose produced from d-glucitol in the reverse reaction was 63% α anomer and 37% β anomer, which also agreed well with the values, 65 and 35%, respectively, calculated from the rate constants in reactions from the aldehyde form to both the α anomer and the β anomer. It was suggested from these kinetic analyses that aldose reductase acts on the aldehyde form of d-glucose (K_m = 0.66 μm) but not on either the α or the β anomer of d-glucose.     

Investigations of different carbohydrate anomers in copper(II) complexes with D-glucose, D-fructose, and D-galactose by Raman and EPR spectroscopy

With the aim of verifying different carbohydrate anomers coordinated to copper(II) ions, some copper(II) complexes with D-glucose (Glc), D-fructose (Fru), and D-galactose (Gal) were prepared and investigated by spectroscopic techniques. Their compositions were verified by elemental, ICP-AES and thermal analyses, in addition to conductivity measurements. The compounds isolated were consistent with the formula Na2[Cu2(carbohydrate)3].8H2O and Na[Cu2(carbohydrate)3].6H2O for the aldoses Glc and Gal, respectively, and Na2[Cu3(carbohydrate)4].8H2O in the case of the ketose, Fru. EPR spectra of these solids showed a rhombic environment around the metal center and suggested the presence of different anomers of the carbohydrates in each case. By Raman spectroscopy, it was possible to verify the predominance of the beta anomer of d-glucose in the corresponding copper complex, while in the free ligand the alpha anomer is predominant. In the case of the analogous complex with d-galactose, the spectrum of the complex shows bands of both anomers (alpha and beta) in approximately the same relative intensities as those observed in the isolated free ligand spectrum. On the other hand, for the complex with d-fructose a mixture of both furanose (five-membered ring) and pyranose (six-membered ring) structures was detected with prevalence of the furanose structure. Based on variations in the relative intensities of characteristic Raman bands, the binding site for copper in the fructose ligand was identified as most likely the 1-CH2OH and the anomeric 1-OH, while in beta-D-glucose it is presumably the anomeric 1-OH and the O-5 atom. These results indicated that EPR and Raman spectroscopy are suitable supporting techniques for the characterization of carbohydrate anomers coordinated to paramagnetic ions.     

Preference of glucokinase for the alpha anomer of hexose: relation to alpha-anomer preference in hexose-induced insulin release by pancreatic islets

The alpha anomers of glucose and mannose were more rapidly phosphorylated than the beta anomers by rat liver glucokinase, whose chromatographic and kinetic properties are known to be quite similar to those of pancreatic islet glucokinase. This result may explain our previous findings of the alpha-anomeric preference in glucose- and mannose-stimulated insulin release, and therefore suggests to support the hypothesis that glucokinase in islets functions as a crucial hexose sensing enzyme for insulin release induced by glucose and mannose.