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Polyhedron 26 (2007) 1269–1276

Monosaccharides and the VO(IV) metal ion: Equilibrium,thermal studies and hypoglycemic effect

Marcela Guiotoku a, Fatima Regina Mena Barreto Silva b, Jose Carlos Azzolini c,Ana Lucia Ramalho Merce d, Antonio S. Mangrich d, Luis F. Sala e, Bruno Szpoganicz a,*

a Departamento de Quımica, Universidade Federal de Santa Catarina, Florianopolis, SC 88040-900, Brazilb Departamento de Bioquımica, Centro de Ciencias Biologicas, Universidade Federal de Santa Catarina, Florianopolis, SC 88040-900, Brazil

c Centro Tecnologico, Universidade do Oeste de Santa Catarina, Joacaba, SC 89600-000, Brazild Departamento de Quımica, Universidade Federal do Parana, Curitiba, PR 81531-990, Brazil

e Departamento de Quımica-Fısica, Fac. Cs. Bioq. y Farm., UNR, 2000 Rosario, Argentina

Received 26 July 2006; accepted 19 October 2006Available online 25 October 2006

Abstract

Protonation and complexation equilibriums of monosaccharides and the VO(IV) metal ion in aqueous solution were studied as well astheir effect on the hyperglycemia of diabetic rats. The complexes formed were characterized by potentiometric titrations, paramagneticresonance spectroscopy (EPR) and thermogravimetric–differential scanning calorimetry (TGA–DSC). The system involving D-gluconicacid (HGlu) and oxovanadium(IV) (VO2+) was chosen to study the serum glucose levels in alloxan-induced diabetic rats. A binuclearspecies was detected in small quantities, which was formed by coordination of two HGlu molecules and two VO2+ ions through ahydroxide bridge. The mononuclear species formed by HGlu and VO2+ were confirmed by EPR. The anisotropic spectra obtained fromaqueous frozen solutions (77 K) are characteristic of mononuclear VO-hexoses. The cyclic sugars D-ribone-1,4-lactone (Riblac), D-galac-tone-1,4-lactone (Galac) and 2-deoxy-D-glucopyranose (dGlu) showed weak interactions with the metal ion and they are not able to holdthe metal in solution above pH 4.6 resulting in hydrolysis of the metal ion. Also, the acute treatment with sugar complexes of HGlu–VOled to a significant hypoglycemic effect (23% and 18% by intraperitoneal or oral gavage treatment, respectively) in diabetic rats. Theseresults show the potential effectiveness of VO–HGlu complexes as anti-hyperglycemic agents through intraperitoneal injection in alloxan-induced diabetic rats.� 2006 Elsevier Ltd. All rights reserved.

Keywords: Vanadyl complexes; Potentiometric titration; EPR; TGA–DSC; D-Gluconic acid; Hypoglycemic; Diabetes

1. Introduction

Vanadium is a trace metal ion present in higher animalsand it is known to be essential to some organisms, includingtunicates and mushrooms. Vanadium has the exceptionalability to interact with biomolecules in both cationic andanionic forms in its several oxidation states, the most com-

0277-5387/$ - see front matter � 2006 Elsevier Ltd. All rights reserved.

doi:10.1016/j.poly.2006.10.032

* Corresponding author. Tel.: +55 48 33316844x216; fax: +55 4833316850.

E-mail address: bruno@qmc.ufsc.br (B. Szpoganicz).

mon ones under physiological conditions being vanadate(V)H2VO4

� and oxovanadium(IV) (VO2+) [1–3].Diabetes mellitus (DM) is the most common endocrine

disorder characterized by high blood glucose levels due toabsolute or relative deficiency of insulin levels. DM affectsmore than 100 million persons worldwide and its incidenceis increasing steadily with changing lifestyles [4]. Inade-quate pancreatic insulin production (type 1 diabetes) [5],and defective insulin utilization (type 2 diabetes) [6], causesa carbohydrate, protein and fat metabolism derangement.Type 1 DM is controlled by daily subcutaneous injectionsof insulin and type 2 DM is treated by several types of

1270 M. Guiotoku et al. / Polyhedron 26 (2007) 1269–1276

synthetic therapeutics [6,7], or, not infrequently, by a com-bination therapy. There is therefore a need for researchefforts to seek effective active drugs that mimic or enhancethe properties of insulin.

Vanadium and organovanadium complexes have beenproposed as adjuncts in the treatment of diabetes mellitusand the efficacy of vanadium as an insulin-mimickingtransition metal has been reported [8,9]. Several clinicalstudies have been performed with organovanadium com-plexes using low doses of vanadium (2 mg/kg/day) inorder to improve the insulinomimetic efficacy of vana-dium without increasing its toxicity in diabetic patients[9,10].

Insulin mimetic VO compounds are described in the lit-erature and several of them are sugar-derived substances[11–15]. Different types of coordination involving the van-adyl ion have also been investigated [16,17]. However, poorabsorption from the gastrointestinal tract into the blood-stream and the optimal effectiveness in vivo are current lim-itations to administering vanadate to diabetic patients.Some vanadium complexes have been shown to increasepotency over inorganic vanadium in induced-diabetic rats[18]. One example is bis(maltolato)oxovanadium(IV) whichoffers significant water solubility, neutral charge, and lipo-philicity and may therefore enhance gastrointestinal tractabsorption [19,20].

Saccharides belong to an interesting class of chemicalcompounds that plays an important role in various formsof living systems by being present as small integral partsof diverse biological molecules, including nucleic acids,polysaccharides, antibiotics, glycoproteins, glycolipidsand aminosugars. Saccharide molecules posses severalfunctional hydroxyl groups that may play key roles in com-plexing with transition metal ions to produce a variety ofmetal–saccharide complexes differing in their nature andutility [21–23].

In this approach we characterized the species of sugarchelated with vanadium in the VO–HGlu system, at phys-iological pH, in order to study its anti-diabetic effect in anacute treatment of hyperglycemic rats by intraperitonealinjection or oral gavage. The aim of this study was to inves-tigate the use of sugars as a chelating agent in a vanadiumcomplex with the aim of presenting an insulin mimeticcompound, which would be expected to have a lowtoxicity.

2. Experimental

2.1. Materials

The monossaccharides D-Gluconic acid (potassium salt),D-ribose, D-ribono-1,4-lactone, D-galactone-1,4-lactoneand 2-deoxy-D-glucose and the metallic salts of analyticgrade (VOSO4) were purchased from commercial sources(Sigma and Aldrich) and used without further purification.The stock solution of VO(IV) (1 · 10�2 mol L�1) was stan-dardized by titration with a standard solution of KMnO4

(potassium permanganate) through the oxi-reductionmethod [24]. The exact amount of the acid added to pre-vent the cation precipitation was calculated using Gran’sPlot [25]. Purity of all the ligands was established by poten-tiometric titration. Carbonate-free solutions of 0.100mol L�1 KOH were prepared from Baker Dilut-It ampolesand were standardized by titration with potassium acidphthalate purchased from Sigma.

2.2. Animals

Male Wistar rats (160–190 g) were used. They were bredin our animal facility and housed in an air-conditioned room(approximately 22 �C) with controlled lighting 12:12 hlight/dark cycle (lights on from 06:00 to 18:00 h). The ani-mals were maintained with food pellets (Nuvital, NuvilabCR1, Curitiba, PR, Brazil), while tap water was availablead libitum. Fasted animals were deprived of food for atleast 16 h but allowed free access to water. All the animalswere carefully monitored and maintained in accordancewith ethical recommendations of the Brazilian Veteri-nary-Medicine Council (CMV) and the Brazilian Collegeof Animal Experimentation (COBEA). Rats fasted for16 h received 70 mg/kg body weight of alloxan (Sigma,St. Louis, MO, USA) by a single intravenous injection.The diabetic state was assessed, by measuring serumglucose levels 3 days later [26]. Rats that died duringthe experiment (around 8%) were excluded from theanalysis.

2.3. Synthesis of the VO–HGlu complex

The VO–HGlu complex was synthesized following amodified procedure describe in the literature [27]. The sol-ids were prepared by mixing concentrated solutions ofVOSO4 and D-Gluconic acid (potassium salt) in aqueousmedia at neutral pH value and the solvent was evaporateduntil a solid was formed. The solid was purified after suc-cessive recrystallizations.

2.4. Potentiometric titrations

The potentiometric titrations were carried out in asealed thermostated vessel at 25 �C, under inert atmo-sphere (Ar) with continuous magnetic stirring. The pHmeter (Corning) with glass and reference electrodes(Ingold) was calibrated with standard HCl and KOHsolutions to directly read �log[H+], designated as pH.The experimental solutions, adjusted to 0.1000 mol L�1

ionic strength by addition of KCl, were titrated with0.100 mol L�1 standard CO2-free KOH solution. Equilib-rium measurements of sugars were carried out in the pres-ence and absence of VO2+. The problem solutionscontained 0.100 mmol of saccharides and the metal sys-tem had a molar ratio of 1:1 and 1:2 (metal:ligand) with0.1000 mmol of VO2+. All the potentiometric studies were

Fig. 1. Potentiometric pH profiles of D-gluconic acid (HGlu) in thepresence and absence of VO2+, in the metal to ligand ratio of 1:1 (M:L), at25.0 �C and I = 0.100 mol/L (KCl). Negative values in the abscissarepresent the neutralization of mineral acid added to the system. In theabscissa a stands for number of moles of KOH per number of moles ofligand.

M. Guiotoku et al. / Polyhedron 26 (2007) 1269–1276 1271

carried out with 40.00 mL of experimental solution andeach system was titrated at least three times. The pHrange for accurate measurements was considered to be2.5–11.0.

All equilibrium constants were calculated using the pro-gram BEST7 [28a]. Mole and millimole units were used toexpress the quantities of reagents, and the hydrolysis con-stants in water for the metal ion were obtained from the lit-erature [28b] and were fully employed in all calculations.The species distribution curves were drawn with the SPE

and SPEPLOT programs [28a].

2.5. EPR

EPR spectra were obtained at 298 K and 77 K for themetal–ligand complex solution at pH 3.6, 7.25 and 10.6,on a Bruker ESP300E instrument operating at 9.5 GHz(X-band) with 100 kHz field modulation.

2.6. TGA–DSC

Thermal analysis was carried out using a Shimadzu sys-tem (DSC – 50 and TGA – 50) at a heating rate of 10 �Cmin�1 and under N2 atmosphere (50 mL min�1). The sam-ple mass used was 6.730 mg. Standard indium (156.6 �C)and hydrate calcium oxalate were used for DSC andTGA calibration, respectively.

2.7. Time-course of VO–HGlu complex effects on

hyperglycemia in alloxan-induced diabetic rats

Animals for which the development of hyperglycemiawas confirmed (around 98%) 72 h after the alloxan injec-tion were randomized into four groups of six rats: GroupI, normal rats that received a saline solution by oralgavages (0.5 mL); Group II, diabetic rats that receivedVO(IV) (0.0607 mol/L; 0.5 mL) intraperitoneally; GroupIII, diabetic rats that received 0.5 mL of VO–HGlu at100 mg/kg intraperitoneally and Group IV, diabetic ratsthat received 0.5 mL of VO–HGlu at 100 mg/kg by oralgavages. The VO–HGlu complex was prepared immedi-ately prior to its use and was kept in a physiological pHsolution to be administered to the animals. The serum glu-cose was measured before the animals received the complexsolution and at 1, 2 and 3 h subsequent to the treatment[29].

2.8. Determination of serum glucose concentration

Blood samples were collected from the cannula previ-ously implanted in the jugular vein as described by [30],and centrifuged. Serum glucose levels were determined bythe oxidase method [31]. The absorbencies were measuredwith a Pharmacia LKB-Ultrospec III spectrophotometer(Pharmacia LKB Biotechnology, Inc., Uppsala, Sweden)at 505 nm. A serum glucose range of 480–570 mg/dL wasused for the experiment.

2.9. Statistical analysis

Data were expressed as mean ± SEM. One-way analysisof variance (ANOVA) followed by the Bonferroni post-test, with the aid of the INSTAT program (version 1.121)was carried out. Differences were considered to be signifi-cant at p 6 0.05.

3. Results and discussion

3.1. Equilibrium of the D-gluconic acid and the vanadyl

cation VO2+

The potentiometric titration profile of HGlu in the pres-ence and absence of VO2+ in the 1:1 (metal:ligand) ratio, isgiven in Fig. 1. Complexation began around pH 3, requir-ing almost three moles of protons per mole of sugar toreach the break point. The buffer region above pH 7 showsthe consumption of over one addition mole of titrant(totally 4.5) per mole of the ligand (sugar). Only this systempresented a solubility feature up to pH 11, while the othersystems studied precipitated. The negative values in theabscissa represent the neutralization of the acid excess inthe metallic solution.

The equilibria in the VO–Hglu system are representedby Eqs. (1)–(7) where HL and L� are the protonated anddeprotonated species of the D-Gluconic acid, respectively;

Fig. 2. Potentiometric pH profiles of D galactone-1,4-lactone (Galac) andVO2+, in the metal to ligand ratio of 1:1 (M:L), at 25.0 �C andI = 0.100 mol/L (KCl). Negative values in the abscissa represent theneutralization of mineral acid added to the system. In the abscissa a standsfor number of moles of KOH per number of moles of ligand.

Table 2Values for the logarithms of the formation constants for the ligands Galac,Riblac, dGlu with VO2+; 25.0 �C and I = 0.100 mol/L

Species logK

Galac Riblac dGlu

[VOH�1 L] [H+]/[VO2+][L] �0.78 (6) �0.64 (6) �0.88 (6)[VOH2� L] [H+]/[VOH�1L] �4.92 (6) �4.93 (6) �5.32 (6)

1272 M. Guiotoku et al. / Polyhedron 26 (2007) 1269–1276

VO2+ is the aqueous oxovanadium and VOL+, VOH�1L,VOH�2L�, VOH�3L2� and VOH�4L3� are the mononu-clear species formed with none, one, two, three and fourhydroxyl groups from the D-gluconic acid, which weredeprotonated in the complexation by the metal ion. Thevalues for the equilibrium constants defined by Eqs. (1)–(7) are presented in Table 1.

L� þHþ�HL ð1ÞVO2þ þ L��VOLþ ð2ÞVOLþ�VOH�1LþHþ ð3ÞVOH�1L�VOH�2L� þHþ ð4ÞVOH�2L��VOH�3L�2 þHþ ð5ÞVOH�3L2�

�VOH�4L�3 þHþ ð6Þ2VOH�2L�ðVOÞ2H�5L2 þHþ ð7Þ

The potentiometric pH profile of D-galactone-1,4-lac-tone (Galac) in the presence of VO2+, in the 1:1 (metalligand) ratio is given in Fig. 2. The curves for Riblac anddGlu in the presence of the VO2+ ion are similar to thatshown in Fig. 2 and they are not shown here. The titrationcurves for Galac, Riblac and dGlu in the presence of theVO2+ ion were interrupted around pH 4.6 due to the pre-cipitation of the metal ion hydrolysis products.

The logarithms of the constants defined by Eqs. (8) and(9) are displayed in Table 2 and they refer to the sugars:Galac, Riblac, and dGlu in the presence of the VO2+ ion.

VO2þ þ L��VOH�1LþHþ ð8ÞVOH�1L�VOH2�LþHþ ð9Þ

It can be seen from the values in Table 2 that the closedchain sugars do not chelate well to the metal ion studied,although there is the formation of insoluble hydrolysisproducts of the metal species. Since the cyclic sugars com-plex the vanadyl ion weakly, the other analytical assayswere performed only with the HGlu sugar molecule.

The species distribution diagram of the HGlu and VO2+

system is given in Fig. 3. For this sugar, more species areformed than in the case of the others, because this is anopen chain sugar and its hydroxyl groups are in appropri-

Table 1Logarithms of stability constants (logK) for the acid–base and complexequilibriums for HGlu and the metal ion VO(IV)

Species logKa

[HL]/[H+] [L�] 3.60 (3) (3.46)b

[VOL+]/[VO2+] [L�] 3.90 (3)[VOH�1L] [H+]/[VOL+] �3.46 (5)[VOH2�L�] [H+]/[VOH�1L] �4.00 (5)[VOH�3L2�] [H+]/[VOH�2L�] �8.84 (5)[VOH�4L3�] [H+]/[VOH�3L2�] �11.18 (6)[(VO)2H�5L2] [H+]/[VOH�2L]2 �6.60 (5)

T = 25.0 �C, and l = 0.100 mol/L (KCl).a The numbers in parenthesis represent the absolute error of the last

digit of the value.b Ref. [27].

ate positions to complex with the metal ion yielding five-member rings, thus, inhibiting its hydrolysis and precipita-tion. We did not detect in solution complexes in a molarratio of 1:2 metal:ligand, as reported in the literature insolid state and characterized by IR and UV–Vis spectros-copies [32]. In that publication the authors detected thepresence of a deprotonated carboxylate molecule and alsoa deprotonated hydroxyl group coordinated to the metalion given rise to the [OV(gluconate)2]2� ion complex. Theformation of the 1:2 metal:ligand complex in the solidstate, is most probably due to some packing effects whichare contributing to the formation of this species and thusthe authors were able to isolate this compound in the solidstate when they used an excess of ligand.

The formation of 48% VOL species at pH 2.8 and 42%VOH�1L species at pH 3.8 can be seen in Fig. 3. Formationof the VOH�2L predominates above pH 4.0 until pH

Fig. 3. Species distribution diagram for the species formed in the HGlu–VO system, in the ligand to metal ratio of 1:1 according to pH variation. VO is thefree concentration of the oxocation, HL is the fully protonated sugar molecule and VOL is the complex in 1:1 stoichiometry, VOH�1L, VOH�2L,VOH�3L and VOH�4L are the mono-, di-, tri- and tetra- hydroxyl deprotonated species and VO2H�5L2 represents the dinuclear (VO)2H�5L2 species. Theligand concentration is 2.5 · 10�3 mol/L, T = 25.0 �C and I = 0.100 mol/L (KCl).

M. Guiotoku et al. / Polyhedron 26 (2007) 1269–1276 1273

values close to 9. This species has two deprotonated OHgroups complexed to the metal ion. This is the major spe-cies present at physiological pH (7.4). In this species theligand does not occupy all the coordination sites of themetal ion, leaving two positions where water moleculescan be coordinated to the octahedral vanadium(IV) ion(Fig. 4). At pH values above 7.0 there is the formation ofa small amount of the dinuclear complex species presentinga hydroxo bridge linking two metal ions. Above a pH valueof 8.8 VOH�3L and VOH�4L species predominate, as threeand four alcohol groups of the sugar molecule are deproto-nated, respectively.

Fig. 4. Proposed structure for the species represented as VOH�2L and(VO)2H�5L2, between the HGlu ligand and vanadyl ion.

3.2. EPR spectra

The Hglu–VO complexes are EPR-active in liquid(300 K) or in frozen (77 K) water solution. Experimentalspectra of the frozen water solutions of the complexes atpH 3.6, 7.25 and 10.6 are shown in Fig. 5. The g valuesand hyperfine splitting constants of the Hglu–VO com-plexes were obtained from computer simulation (Simfo-nia�, Bruker). According to the species distributiondiagram (Fig. 3) at pH 3.6 there are two main types of com-plexes, VOL and VOH-1 L. The best g and hyperfine split-ting constant values for the two complexes are: gi = 1.9405,

2500 3000 3500 4000 4500

pH = 3.6

pH = 7.25

pH = 10.6

Magnetic Field / G

Fig. 5. EPR spectra at X-band (m � 9.5 GHz) of the frozen water solutionof the Hglu–VO complexes at pH 3.6, 7.25 and 10.6. Instrument settingsfor the experimental spectra: power, 20 mW; modulation 100 kHz; sweepcentre, 3450 G; sweep width, 2000 G.

1274 M. Guiotoku et al. / Polyhedron 26 (2007) 1269–1276

g^ = 1.9827, Ai = 180 · 10�4 cm�1, A^ = 70 · 10�4 cm�1.The orders (gi < g^ and Ai� A^) are consistent with theoxovanadium(IV) site of C4v symmetry with a 2B2 groundstate (unpaired electron on a dxy vanadium orbital). Themagnitude of the EPR parameters gi and Ai are consistentwith four oxygen atoms as equatorial ligands. These couldbe one from the Hglu carboxylate group and three fromwater molecules or, alternatively, two from water moleculesand two from the Hglu molecule. VO(acac)2 has been previ-ously study and it was found that this compound has fouroxygen atoms as equatorial ligands, gi = 1.944 and Ai =174 · 10�4 cm�1 [33a]. This result is consistent with thosefor the VOL and VOH�1L complexes. The EPR parametersfor the Hglu–VO complexes at pH 7.25 and 10.6 are analo-gous (gi = 1.9495, g^ = 1.9810, Ai = 156 · 10�4 cm�1,A^ = 58 · 10�4 cm�1) and support a different set of equato-rial donor atoms for the VO2+ group, with a strongercoordination ligand field than those for the complexesformed at pH 3.6, using two or three hydroxyl groups ofthe Hglu ligand as donor atoms (VOH�2L and VOH�3L)and perhaps with a little distortion of the C4v symmetryaround the vanadyl centre [33b]. The different degree ofdistortion for the two complexes could bring the EPRparameters for the two complexes to the same values.

Fig. 6 represents the DSC curve for the VO–HGlucomplex isolated at neutral pH showing one endothermic

Fig. 6. TGA–DSC of HGlu–VO complex showing one endothermic transitendothermic transition at 345.77 �C, corresponding to the decomposition of t

transition at 118.82 �C, corresponding to 12.5% of amolecular weight of 296 g mol�1, corresponding to twowater molecules coordinated to the metal ion in the com-plex as shown in Fig. 4. The two water molecules can bereplaced during the association of the complex with areceptor site of the insulin. In other words, it has two freepositions for coordination. The second endothermic tran-sition (at 345.77 �C) is assigned to a break in the sugarchain [34,35].

3.3. Biological assays

Taking into account our previous results performed withthis experimental approach [36,37] we chose the optimaltime-course procedure to investigate the short-term actionof this new complex. Table 3 shows the glycemia profilesfor normal animals (control group) and diabetic animalstreated with VO(IV) solution (0.0607 mol/L) intraperitone-ally or a single dose of VO–HGlu complex (100 mg/kg)intraperitoneally or by oral gavage, over time (from zeroto 3 h). This dose of VO–HGlu complex showed a signifi-cant hypoglycemic effect from 0 to 3 h when compared tothe respective zero time when the animals were treatedintraperitoneally, but after oral administration of the com-plex only a slight lowering in glucose serum levels wasobserved.

ion at 118.82 �C, with a 12.5% water weight loss (2 mol) and anotherhe complex.

Table 3Time-course for the effect of VO–HGlu on glycemia in diabetic ratsa

Time (h) Group I Group II Group III Group IV

Normal + salinesolution (oral gavage)

Diabetic + VO(IV) (0.0607 mol/L)(intraperitoneal)

Diabetic + VO–Hglu (100 mg/kg)(intraperitoneal)

Diabetic + VO–Hglu (100 mg/kg)(oral gavage)

0 130 ± 2 505 ± 3 516 ± 8 502 ± 241 117 ± 2 434 ± 32b 459 ± 8b 410 ± 12b

2 115 ± 6 418 ± 16 b 413 ± 9b 497 ± 63 130 ± 4 430 ± 10b 397 ± 12b 487 ± 20

a Values are expressed as mean ± SEM; n = 6 in duplicate for each group.b Statistically significant difference in relation to the corresponding zero time value; p 6 0.05.

M. Guiotoku et al. / Polyhedron 26 (2007) 1269–1276 1275

According to the literature, results from glycemia stud-ies in diabetic animals, using acute screening trials of thecomplexes bis(maltolato)oxovanadium(IV); (BMOV),bis(maltolato)-zinc(II) dihydrate, cis-bis(maltolato)dioxo-molybdenum(IV), tris(maltolato)chromium(III) monohy-drate, bis(maltolato)copper(II) and bis(maltolato)cobalt(II), indicate that only vanadyl maltol, when administeredby oral gavage, lower plasma glucose levels compared tountreated diabetic animals (zero time) [38]. BMOV hasbeen experimentally used as a representative standard inseveral biological studies due to its approval for clinicalexperiments [18]. However, a pharmacologically beneficialdose of BMOV is associated with some toxicity and henceattempts are underway to evolve vanadium complexes withfewer side effects while retaining their enhanced therapeuticactivities [39]. Additionally, besides the BMOV complex,many vanadium complexes have been reported to be moreeffective when introduced by intraperitoneal administrationthan by oral gavage treatment [40].

Vanadium alone, in the form of vanadyl, was effective inlowering glucose levels in a similar concentration(0.0607 mol/L) to that used to prepare the sugar complex(0.0788 mol/L). The maximum percentage reduction inthe case of simultaneous administration, HGlu–VO com-plex, at a dose of 100 mg/kg administered intraperitoneallyin diabetic animals was 23%. As demonstrated in this pre-liminary study the HGlu–VO(IV) complex has a pharma-cological potential to be efficient as an organicallychelated vanadium complex. One of the advantages of thissugar complex is that, unlike the cyclic sugar complexes, itkeeps the metal ion in solution at physiological pH at theconcentrations used in the potentiometric studies. Thereis no metal ion hydrolysis and undesirable precipitatesare therefore not formed. Increasing the vanadium orHGlu doses with the aim of observing synergy is unlikelyto be advantageous due to the potential toxicity of highdoses of vanadium. However, studies on the efficiency ofthis new complex toward hyperglycemia, as well as investi-gations into its toxicity, are in progress, using the chronictreatment approach in diabetic animals.

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