Recombinant Human Aldose Reductase Research Articles

Exploring the Potential of Isoindole‐1,3‐Dione Derivatives as Novel Inhibitors of Aldose Reductase: An In Silico and In Vitro Insight into Therapeutic Strategies for Diabetic Complications

AbstractThis study explores the potential of isoindole‐1,3‐dione derivatives as novel inhibitors of aldose reductase (AR), focusing on their in silico and in vitro effects for therapeutic strategies against diabetic complications. Aldose reductase, a critical enzyme in the polyol pathway, plays a significant role in glucose metabolism and has been linked to diabetic complications. In this comprehensive study, isoindole‐1,3‐dione derivatives were synthesized and evaluated for their inhibitory effects on the recombinant human AR enzyme. The compounds’ inhibitory activities were measured both in vitro and through in silico techniques, employing molecular docking and free binding energy calculations and ADME studies. The newly synthesized compounds demonstrated varied inhibitory effects, with ethyl and phenyl substituents at specific positions enhancing inhibitory activity. Notably, compounds with carboxylic acid derivatives exhibited potent inhibitory effects, especially compound 6 with an IC50 value of 1.649 μM. In conclusion, this study provides valuable insights into the inhibitory potential of isoindole‐1,3‐dione derivatives against AR, suggesting their potential therapeutic application in mitigating diabetic complications. The combination of experimental and computational approaches offers a comprehensive understanding of the compounds’ interaction mechanisms and pharmacokinetic profiles, supporting their further exploration as antidiabetic agents.

Prunin from Poncirus trifoliata (L.) Rafin Inhibits Aldose Reductase and Glucose-Fructose-Mediated Protein Glycation and Oxidation of Human Serum Albumin.

Diabetes complications are associated with aldose reductase (AR) and advanced glycation end products (AGEs). Using bioassay-guided isolation by column chromatography, 10 flavonoids and one coumarin were isolated from Poncirus trifoliata Rafin and tested in vitro for an inhibitory effect against human recombinant AR (HRAR) and rat lens AR (RLAR). Prunin, narirutin, and naringin inhibited RLAR (IC50 0.48-2.84 μM) and HRAR (IC50 0.68-4.88 μM). Docking simulations predicted negative binding energies and interactions with the RLAR and HRAR binding pocket residues. Prunin (0.1 and 12.5 μM) prevented the formation of fluorescent AGEs and nonfluorescent Nε-(carboxymethyl) lysine (CML), as well as the fructose-glucose-mediated protein glycation and oxidation of human serum albumin (HSA). Prunin suppressed the formation of the β-cross-amyloid structure of HSA. These results indicate that prunin inhibits oxidation-dependent protein damage, AGE formation, and AR, which may help prevent diabetes complications.

An Arylbenzofuran, Stilbene Dimers, and Prenylated Diels-Alder Adducts as Potent Diabetic Inhibitors from Morus bombycis Leaves.

Morus bombycis has a long history of usage as a treatment for metabolic diseases, especially, diabetes mellitus (DM). Thus, we aimed to isolate and evaluate bioactive constituents derived from M. bombycis leaves for the treatment of DM. According to bioassay-guided isolation by column chromatography, eight compounds were obtained from M. bombycis leaves: two phenolic compounds, p-coumaric acid (1) and chlorogenic acid methyl ester (2), one stilbene, oxyresveratrol (3), two stilbene dimers, macrourin B (4) and austrafuran C (6), one 2-arylbenzofuran, moracin M (5), and two Diels-Alder type adducts, mulberrofuran F (7) and chalcomoracin (8). Among the eight isolated compounds, the anti-DM activity of 3-8 (which possess chemotaxonomic significance in Morus species) was evaluated by inhibition of α-glucosidase, protein tyrosine phosphatase 1B (PTP1B), human recombinant aldose reductase (HRAR), and advanced glycation end-product (AGE) formation as well as by scavenging peroxynitrite (ONOO-), which are crucial therapeutic targets of DM and its complications. Compounds 4 and 6-8 significantly inhibited α-glucosidase, PTP1B, and HRAR enzymes with mixed-type and non-competitive-type inhibition modes. Furthermore, the four compounds had low negative binding energies in both enzymes according to molecular docking simulation, and compounds 3-8 exhibited strong antioxidant capacity by inhibiting AGE formation and ONOO- scavenging. Overall results suggested that the most active stilbene-dimer-type compounds (4 and 6) along with Diels-Alder type adducts (7 and 8) could be promising therapeutic and preventive resources against DM and have the potential to be used as antioxidants, anti-diabetic agents, and anti-diabetic complication agents.

6-Formyl Umbelliferone, a Furanocoumarin from Angelica decursiva L., Inhibits Key Diabetes-Related Enzymes and Advanced Glycation End-Product Formation.

Over the years, great attention has been paid to coumarin derivatives, a set of versatile molecules that exhibit a wide variety of biological activities and have few toxic side effects. In this study, we investigated the antidiabetic potential of 6-formyl umbelliferone (6-FU), a novel furanocoumarin isolated from Angelica decursiva. Numerous pharmacological activities of 6-FU have been previously reported; however, the mechanism of its antidiabetic activity is unknown. Therefore, we examined the action of 6-FU on a few candidate-signaling molecules that may underlie its antidiabetic activity, including its inhibition of protein tyrosine phosphatase 1B (PTP1B), α-glucosidase, human recombinant aldose reductase (HRAR), and advanced glycation end-product (AGE) formation (IC50 = 1.13 ± 0.12, 58.36 ± 1.02, 5.11 ± 0.21, and 2.15 ± 0.13 μM, respectively). A kinetic study showed that 6-FU exhibited mixed-type inhibition against α-glucosidase and HRAR and competitive inhibition of PTP1B. Docking simulations of 6-FU demonstrated negative binding energies and close proximity to residues in the binding pockets of those enzymes. We also investigated the molecular mechanisms underlying 6-FU’s antidiabetic effects. 6-FU significantly increased glucose uptake and decreased PTP1B expression in insulin-resistant C2C12 skeletal muscle cells. Moreover, 6-FU (0.8–100 μM) remarkably inhibited the formation of fluorescent AGEs in glucose-fructose-induced human serum albumin glycation over the course of 4 weeks. The findings clearly indicate that 6-FU will be useful in the development of multiple target-oriented therapeutic modalities for the treatment of diabetes and diabetes-related complications.

Inhibition of Aldose Reductase by Ginsenoside Derivatives via a Specific Structure Activity Relationship with Kinetics Mechanism and Molecular Docking Study.

This present work is designed to evaluate the anti-diabetic potential of 22 ginsenosides via the inhibition against rat lens aldose reductase (RLAR), and human recombinant aldose reductase (HRAR), using DL-glyceraldehyde as a substrate. Among the ginsenosides tested, ginsenoside Rh2, (20S) ginsenoside Rg3, (20R) ginsenoside Rg3, and ginsenoside Rh1 inhibited RLAR significantly, with IC50 values of 0.67, 1.25, 4.28, and 7.28 µM, respectively. Moreover, protopanaxadiol, protopanaxatriol, compound K, and ginsenoside Rh1 were potent inhibitors of HRAR, with IC50 values of 0.36, 1.43, 2.23, and 4.66 µM, respectively. The relationship of structure–activity exposed that the existence of hydroxyl groups, linkages, and their stereo-structure, as well as the sugar moieties of the ginsenoside skeleton, represented a significant role in the inhibition of HRAR and RLAR. Additional, various modes of ginsenoside inhibition and molecular docking simulation indicated negative binding energies. It was also indicated that it has a strong capacity and high affinity to bind the active sites of enzymes. Further, active ginsenosides suppressed sorbitol accumulation in rat lenses under high-glucose conditions, demonstrating their potential to prevent sorbitol accumulation ex vivo. The findings of the present study suggest the potential of ginsenoside derivatives for use in the development of therapeutic or preventive agents for diabetic complications.

Screening inhibitory effects of selected flavonoids on human recombinant aldose reductase enzyme: in vitro and in silico study

Aldose reductase (AR) is the first enzyme of the polyol pathway that has physiological importance under hyperglycaemic conditions. The article has been focussed on AR enzyme inhibition by selected compounds. For this purpose, the in vitro inhibitory effects of various compounds on commercially available recombinant human AR (rAR) enzyme activity were investigated. The IC50 values of compounds on rAR inhibition effect were found for 6-hydroxy flavone, syringic acid, diosmetin, 6-fluoroflavone, 7-hydroxy-4′-nitroisoflavone, myricetin as 2.05, 2.97, 15.75, 16.1, 49.5, and 63 µM, respectively. 6-Hydroxy flavone and syringic acid competitively inhibited rAR with respect to the NADPH with Ki values 0.509 ± 0.036 and 0.842 ± 0.012 µM. In addition, docking studies were performed to evaluate the potential enzyme binding positions of the compounds. Our in vitro and in silico results indicated that the 6-hydroxy flavone may be a good lead compound in the development of AR inhibitors to prevent diabetic complications.

Vitamin K1 prevents diabetic cataract by inhibiting lens aldose reductase 2 (ALR2) activity

This study investigated the potential of vitamin K1 as a novel lens aldose reductase inhibitor in a streptozotocin-induced diabetic cataract model. A single, intraperitoneal injection of streptozotocin (STZ) (35 mg/kg) resulted in hyperglycemia, activation of lens aldose reductase 2 (ALR2) and accumulation of sorbitol in eye lens which could have contributed to diabetic cataract formation. However, when diabetic rats were treated with vitamin K1 (5 mg/kg, sc, twice a week) it resulted in lowering of blood glucose and inhibition of lens aldose reductase activity because of which there was a corresponding decrease in lens sorbitol accumulation. These results suggest that vitamin K1 is a potent inhibitor of lens aldose reductase enzyme and we made an attempt to understand the nature of this inhibition using crude lens homogenate as well as recombinant human aldose reductase enzyme. Our results from protein docking and spectrofluorimetric analyses clearly show that vitamin K1 is a potent inhibitor of ALR2 and this inhibition is primarily mediated by the blockage of DL-glyceraldehyde binding to ALR2. At the same time docking also suggests that vitamin K1 overlaps at the NADPH binding site of ALR2, which probably shows that vitamin K1 could possibly bind both these sites in the enzyme. Another deduction that we can derive from the experiments performed with pure protein is that ALR2 has three levels of affinity, first for NADPH, second for vitamin K1 and third for the substrate DL-glyceraldehyde. This was evident based on the dose-dependency experiments performed with both NADPH and DL-glyceraldehyde. Overall, our study shows the potential of vitamin K1 as an ALR2 inhibitor which primarily blocks enzyme activity by inhibiting substrate interaction of the enzyme. Further structural studies are needed to fully comprehend the exact nature of binding and inhibition of ALR2 by vitamin K1 that could open up possibilities of its therapeutic application.

Synthesis of new arylsulfonylspiroimidazolidine-2ʹ,4ʹ-diones and study of their effect on stimulation of insulin release from MIN6 cell line, inhibition of human aldose reductase, sorbitol accumulations in various tissues and oxidative stress

A novel class of spiroimidazolidine-2ʹ,4ʹ-diones substituted with aryl sulfonyl group at different positions was designed and synthesized. The target compounds were evaluated for their potential to release insulin from MIN6 cell line derived from in-vivo immortalized insulin-secreting pancreatic cells. The MIN6 cells represent an important model of beta cells, which as passage numbers increases, lose the first phase but retain partial second phase glucose stimulated insulin secretion (GSIS), similar to patients in early type 2 diabetes onset. Some of the compounds exhibited high potency. Compound 2d and 3f exhibited excellent insulin release activity from MIN6 cells when compared with standard drug, tolbutamide. Some of these compounds had a potent inhibitory activity for human recombinant aldose reductase (ALR2), an enzyme which converts glucose into sorbitol and plays a key role in development of complications arising from diabetes, such as retinopathy, nephropathy, neuropathy and cataract formation. Against human recombinant ALR2, compounds 2a, 3a-d, and 3f-h displayed effective inhibition activities. The results were augmented by the ability of the compounds to prevent sorbitol accumulation in the isolated rat lenses, sciatic nerves and erythrocytes. Some of the compounds were found to possess excellent dual activity, hence they may be promising candidates to modify and evaluate their dual action, i.e., insulin release to combat diabetes and ALR2 inhibition to prevent/treat diabetic complications. The compounds were also found to possess good antioxidant efficacy. Furthermore, most of the compounds lack toxicity as determined on human embryonic kidney cell lines 293 (HEK293).

Correction to: Dihydroxanthyletin-type coumarins from Angelica decursiva that inhibits the formation of advanced glycation end products and human recombinant aldose reductase.

The author would like to include conflict of interest statement of the online published article. The correct conflict of interest statement should read as: Conflict of interest The authors declare no conflict of interest.

Soyasaponins from Zolfino bean as aldose reductase differential inhibitors

Seven triterpenoid saponins were identified in methanolic extracts of seeds of the Zolfino bean landrace (Phaseolus vulgaris L.) by HPLC fractionation, revealing their ability to inhibit highly purified human recombinant aldose reductase (hAKR1B1). Six of these compounds were associated by MS analysis with the following saponins already reported in different Phaseolus vulgaris varieties: soyasaponin Ba (V), soyasaponin Bb, soyasaponin Bd (sandosaponin A), soyasaponin αg, 3-O-[R-l-rhamnopyranosyl(1 → 2)-α-d-glucopyranosyl(1 → 2)-α-d-glucuronopyranosyl]olean-12-en-22-oxo-3α,-24-diol, and soyasaponin βg. The inhibitory activity of the collected fractions containing the above compounds was tested for hAKR1B1-dependent reduction of both l-idose and 4-hydroxynonenal, revealing that some are able to differentially inhibit the enzyme. The present work also highlights the difficulties in the search for aldose reductase differential inhibitors (ARDIs) in mixtures due to the masking effect on ARDIs exerted by the presence of conventional aldose reductase inhibitors. The possibility of differential inhibition generated by a different inhibitory model of action of molecules on different substrates undergoing transformation is also discussed.

Isolation and identification of α-glucosidase inhibitory constituents from the seeds of Vigna nakashimae: Enzyme kinetic study with active phytochemical

The α-glucosidase inhibition effects of the 80% ethanol extracts in the seeds of five Vigna species (V. nakashimae, V. nipponensis, V. umbellate, V. radiate, and V. angularis) were evaluated and their half-maximal inhibitory concentration (IC50) values showed considerable differences (p < 0.05) ranging from 7.3 to >900 μg/ml. V. nakashimae exhibited the most potent inhibition with IC50 value of 7.3 ± 1.1 μg/ml, followed by V. nipponensis (184.0 ± 9.5 μg/ml) and V. umbellata (520.0 ± 8.1 μg/ml). Bioactivity-guided fractionation of V. nakashimae seeds yielded three phenolics by silica gel chromatography and their structures were elucidated as gambiriin D (1), luteoliflavan-7-O-glucopyranoside (2), and catechin-7-O-glucopyranoside (3) using nuclear magnetic resonance (NMR) spectroscopy. In particular, gambiriin D (1) possessed strong inhibition activity with IC50 of 36.8 ± 2.3 μM through simple reversible slow-binding inhibition (kinetic parameters: k4 = 0.0048 μM−1s−1; Kiapp = 48 μM). Furthermore, this compound inhibited recombinant human aldose reductase with IC50 value of 12.0 ± 0.7 μM. Results suggest that V. nakashimae may be a potent α-glucosidase inhibition for health products.

Enzymological Studies on the Mechanisms of Pathogenesis of Diabetic Complications

Aldose reductase (AR) is involved in the pathogenesis of complications in diabetes. In this study, the enzymatic properties of AR isolated from various sources and a recombinant human AR (rh-AR) were analyzed in detail. The sensitivity of different forms of AR to several AR inhibitors (ARIs) was compared. Our findings enabled us to propose that human AR should be used as the target enzyme in the development of ARIs. An enzyme-linked immunosorbent assay (ELISA) for human AR which employed monoclonal antibodies against rh-AR was created, and this method was used to demonstrate the distribution of AR in human tissues. AR was widely distributed in various organs and blood cell components. The levels of erythrocyte AR (e-AR) were 10.1±1.9 ng/mg Hb and 10.5±3.0 ng/mg Hb in healthy volunteers and diabetic patients, respectively, and thus there was no significant difference between them. The e-AR levels of diabetic patients were assayed using the ELISA developed to investigate the potential correlation between AR levels and the onset of diabetic complications. There were significant correlations between the incidence of diabetic neuropathy and e-AR levels in patients with disease duration of less than 10 years, and between the incidence of diabetic retinopathy and e-AR levels in patients with disease duration of 10-20 years. Our results suggest that measurement of e-AR levels in patients could help optimize drug therapy with ARIs and be a useful method to predict the onset of complications due to the upregulation of the polyol pathway.

Dihydroxanthyletin-type coumarins from Angelica decursiva that inhibits the formation of advanced glycation end products and human recombinant aldose reductase.

The formation of advanced glycation end-products (AGE) and aldose reductase activity have been implicated in the development of diabetic complications. The present study was aimed to evaluate human recombinant aldose reductase (HRAR) and AGE inhibitory activity of seven natural dihydroxanthyletin-type coumarins, 4-hydroxy Pd-C-III (1), 4'-methoxy Pd-C-I (2), Pd-C-I (3), Pd-C-II (4), Pd-C-III (5), decursidin (6), and (+)-trans-decursidinol (7) from Angelica decursiva. Coumarins 1-7 showed potent HRAR and AGE inhibitory activities with ranges of IC50 values of 1.03-21.31 and 0.41-5.56µM, respectively. In the kinetic study for HRAR enzyme inhibition, coumarins 1, 3, 4, and 7 were competitive-type inhibitors, 6 was a mixed-type inhibitor, whereas 2 and 5 were noncompetitive-type inhibitors. Furthermore, we also predicted the docking interactions of HRAR with coumarins 1-7 using AutoDock Vina, and as a result, the simulated enzyme-inhibitor complexes exhibited negative binding energies (Autodock Vina=-9.6 to -8.1kcal/mol for HRAR), indicating a high affinity and tight binding capacity for the HRAR active site. Our results clearly indicate the potential HRAR and AGE formation inhibitory activities of dihydroxanthyletin-type coumarins, which could be further explored to develop therapeutic modalities for the treatment of diabetes and related complications.

Sterols from the Green Alga Ulva australis.

Three new sterols, (24R)-5,28-stigmastadiene-3β,24-diol-7-one (1), (24S)-5,28-stigmastadiene-3β,24-diol-7-one (2), and 24R and 24S-vinylcholesta-3β,5α,6β,24-tetraol (3), together with three known sterols (4–6) were isolated from the green alga Ulva australis. The structures of the new compounds (1–3) were elucidated through 1D and 2D nuclear magnetic resonance spectroscopy as well as mass spectrometry. Compounds 4–6 were identified as isofucoterol (4), 24R,28S and 24S,28R-epoxy-24-ethylcholesterol (5), and (24S)-stigmastadiene-3β,24-diol (6) on the basis of spectroscopic data analyses and comparison with those reported in the literature. Compounds 4–6 were isolated from U. australis for the first time. These compounds, together with the previously isolated secondary metabolites of this alga, were investigated for their inhibitory effects on human recombinant aldose reductase in vitro. Of the compounds, 24R,28S and 24S,28R-epoxy-24-ethylcholesterol (5), 1-O-palmitoyl-3-O-(6′-sulfo-α-d-quinovopyranosyl) glycerol, (2S)-1-O-palmitoyl-3-O-[α-d-galactopyranosyl(1→2)β-d-galactopyranosyl] glycerol, 4-hydroxybenzoic acid, 4-hydroxyphenylacetic acid, and 8-hydroxy-(6E)-octenoic acid weakly inhibited the enzyme, while the three new sterols, 1–3, were almost inactive.

Phytochemical Analysis of Agrimonia pilosa Ledeb, Its Antioxidant Activity and Aldose Reductase Inhibitory Potential.

The aim of this study was to determine aldose reductase (AR) inhibitory activity and 1,1-diphenyl-2-picrylhydrazyl (DPPH) free radical scavenging activity of compounds from Agrimonia pilosa Ledeb (AP). We isolated agrimoniin (AM), four flavonoid glucosides and two flavonoid glucuronides from the n-butanol fraction of AP 50% methanol extract. In addition to isolated compounds, the AR-inhibitory activity and the DPPH free radical scavenging activity of catechin, 5-flavonoids, and 4-flavonoid glucosides (known components of AP) against rat lens AR (RLAR) and DPPH assay were measured. AM showed IC50 values of 1.6 and 13.0 μM against RLAR and DPPH scavenging activity, respectively. Additionally, AM, luteolin-7-O-glucuronide (LGN), quercitrin (QU), luteolin (LT) and afzelin (AZ) showed high inhibitory activity against AR and were first observed to decrease sorbitol accumulation in the rat lens under high-sorbitol conditions ex vivo with inhibitory values of 47.6%, 91.8%, 76.9%, 91.8% and 93.2%, respectively. Inhibition of recombinant human AR by AM, LGN and AZ exhibited a noncompetitive inhibition pattern. Based on our results, AP and its constituents may play partial roles in RLAR and oxidative radical inhibition. Our results suggest that AM, LGN, QU, LT and AZ may potentially be used as natural drugs for treating diabetic complications.

Green fluorescent protein chromophore derivatives as a new class of aldose reductase inhibitors

A number of (Z)-4-arylmethylene-1H-imidazol-5(4H)-ones, which are related to the fluorescent chromophore of the Aequorea green fluorescent protein (GFP), have been synthesized and evaluated their in vitro inhibitory activity against recombinant human aldose reductase for the first time. The GFP chromophore model 1a, with a p-hydroxy group on the 4-benzylidene and a carboxymethyl group on the N1 position, exhibited strong bioactivity with an IC50 value of 0.36 μM. This efficacy is higher than that of sorbinil, a known highly potent aldose reductase inhibitor. Compound 1h, the 2-naphtylmethylidene analogue of 1a, exhibited the best inhibitory effect among the tested copounds with an IC50 value of 0.10 μM. Structure-activity relationship studies combined with docking simulations revealed the interaction mode of the newly synthesized inhibitors toward the target protein as well as the structural features required to gain a high inhibitory activity. In conclusion, the GFP chromophore model compounds synthesized in this study have proved to be potential drugs for diabetic complications.

In Vitro Antidiabetic and Antioxidant Potential of the Ethanolic Extract of Skipjack Tuna (K atsuwonus Pelamis ) Heart

Skipjack tuna, Katsuwonus pelamis, are distributed throughout the Pacific Ocean in the tropical and subtropical areas, including South Korea, Japan and Indonesia. The antidiabetic and antioxidant potential of 70% ethanol (EtOH) extract of skipjack tuna heart were investigated via protein tyrosine phosphatase 1B (PTP1B), α-glucosidase, human recombinant aldose reductase (HRAR), 1,1-diphenyl-2-picrylhydrazyl (DPPH) radical, peroxynitrite (ONOO−), 2,2′-azino-bis-3-ethylbenzothiazoline-6-sulfonic acid (ABTS) radical, and total reactive oxygen species (ROS). The 70% EtOH tuna heart extract exhibited potent inhibitory activity against PTP1B, α-glucosidase and HRAR with inhibition percentages of 85.42, 82.70 and 51.1%, respectively, at a concentration range of 1–2 mg/mL. In addition, it was a potent inhibitor against DPPH, ABTS, ONOO−, and ROS with inhibition percentages of 69.45, 58.31, 96.20 and 34.02%, respectively, at a concentration of 1 mg/mL. The total phenolic content present in tuna extract was 15.80 mg/g GAE. The results demonstrate the potential antidiabetic and antioxidant activities of tuna heart extract. Practical Application Tuna has been consumed as a healthy protein source for hundreds of years. As a food, skipjack is a very good source of protein, vitamins, minerals and omega-3 fatty acids and can lower blood pressure and cholesterol. Generally, the meat of skipjack tuna is used as food and other parts such as the heart are used as fertilizer. Recent studies have identified a number of bioactive components from fish muscle protein, collagen, peptides, gelatin, oil, bone and internal organs that remain after processing. These fish components showed antioxidant, antihypertensive, antiproliferative, antimicrobial and antianemic activities. The findings demonstrated antidiabetic and antioxidant potential of tuna heart extract.

Aldose Reductase Inhibitory and Antiglycation Activities of Four Medicinal Plant Standardized Extracts and Their Main Constituents for the Prevention of Diabetic Complications

The present study was intended to evaluate, highlight and explore the role of four medicinal plant ( Morus alba L., Phyllanthus amarus Schum. & Thonn., Punica granatum L., and Stevia rebaudiana Bertoni) standardized extracts and their major constituents (morusin, phyllanthin, punicalagin and stevioside) in the treatment of long-term diabetic complications by inhibition of aldose reductase (AR) enzyme and advanced glycation end products (AGEs) formation. Rat lens and kidney homogenates, which contain rat lens AR (RLAR) and rat kidney AR (RKAR) crude enzymes, respectively, prepared in the laboratory and commercially available human recombinant AR (HRAR) were used to carry out in vitro bioassays. AR inhibitory activity and inhibition of AGEs formation was done by using ultraviolet-visible (UV-Vis) and fluorescence spectroscopy with aminoguanidine as a standard. In vivo AR inhibitory activity, which involves determination of rat lens galactitol levels in galactosemic condition by using reverse phase high pressure liquid chromatography (RP-HPLC) and gas liquid chromatography (GLC) was determined. In the in vitro bioassays, punicalagin, the major constituent of P. granatum was found to have the most potent AR inhibitory activity with IC 50 values of 5.28, 6.22 and 4.70 μM in RLAR, RKAR, and HRAR assays, respectively, among the tested standardized extracts and pure compounds. Punicalagin (IC 50 = 5.11 μM), also inhibited generation of AGEs at a much lower concentration than that of the reference standard aminoguanidine (IC 50 = 50.47 μM). The effect of punicalagin in suppressing rat lens galactitol levels in galactose-fed rat model was also superior to all other studied compounds including the positive control quercetin. Standardized extracts of the plants and their major components have the potential to be used as capable shielding agents in therapy of diabetic complications; however, further studies are warranted to explain and explore their use in the treatment of diabetic complications. Keywords: Standardized Extracts, Aldose Reductase, Non-enzyme Maillard Reaction, Punicalagin, Galactitol Accumulation

Modulation of aldose reductase activity by aldose hemiacetals

BackgroundGlucose is considered as one of the main sources of cell damage related to aldose reductase (AR) action in hyperglycemic conditions and a worldwide effort is posed in searching for specific inhibitors of the enzyme. This AR substrate has often been reported as generating non-hyperbolic kinetics, mimicking a negative cooperative behavior. This feature was explained by the simultaneous action of two enzyme forms acting on the same substrate. MethodsThe reduction of different aldoses and other classical AR substrates was studied using pure preparations of bovine lens and human recombinant AR. ResultsThe apparent cooperative behavior of AR acting on glucose and other hexoses and pentoses, but not on tethroses, glyceraldehyde, 4-hydroxynonenal and 4-nitrobenzaldehyde, is generated by a partial nonclassical competitive inhibition exerted by the aldose hemiacetal on the reduction of the free aldehyde. A kinetic model is proposed and kinetic parameters are determined for the reduction of l-idose. ConclusionsDue to the unavoidable presence of the hemiacetal, glucose reduction by AR occurs under different conditions with respect to other relevant AR-substrates, such as alkanals and alkenals, coming from membrane lipid peroxidation. This may have implications in searching for AR inhibitors. The emerging kinetic parameters for the aldoses free aldehyde indicate the remarkable ability of the enzyme to interact and reduce highly hydrophilic and bulky substrates. General significanceThe discovery of aldose reductase modulation by hemiacetals offers a new perspective in searching for aldose reductase inhibitors to be developed as drugs counteracting the onset of diabetic complications.

Triple aldose reductase/α-glucosidase/radical scavenging high-resolution profiling combined with high-performance liquid chromatography–high-resolution mass spectrometry–solid-phase extraction–nuclear magnetic resonance spectroscopy for identification of antidiabetic constituents in crude extract of Radix Scutellariae

In this work, development of a new microplate-based high-resolution profiling assay using recombinant human aldose reductase is presented. Used together with high-resolution radical scavenging and high-resolution α-glucosidase assays, it provided the first report of a triple aldose reductase/α-glucosidase/radical scavenging high-resolution inhibition profile – allowing proof of concept with Radix Scutellariae crude extract as a polypharmacological herbal drug. The triple bioactivity high-resolution profiles were used to pinpoint bioactive compounds, and subsequent structure elucidation was performed with hyphenated high-performance liquid chromatography–high-resolution mass spectrometry–solid-phase extraction–nuclear magnetic resonance spectroscopy. The only α-glucosidase inhibitor was baicalein, whereas main aldose reductase inhibitors in the crude extract were baicalein and skullcapflavone II, and main radical scavengers were ganhuangemin, viscidulin III, baicalin, oroxylin A 7-O-glucuronide, wogonoside, baicalein, wogonin, and skullcapflavone II.