Inclusion of Theobromine Modifies Uric Acid Aggregation with Possible Changes in Melamine–Uric Acid Clusters Responsible for Kidney Stones

7-Methylxanthine Inhibits the Formation of Monosodium Urate Crystals by Increasing Its Solubility

Gout is characterized by the formation of monosodium urate crystals in peripheral joints. We carried out laboratory studies to investigate the effect of adding nine different methylxanthines and two different methylated uric acid derivatives on the development of these crystals over the course of 96 h in a medium whose composition was similar to that of synovial fluid. Our results showed that 7-methylxanthine reduced or totally prevented crystal formation; 1-methylxanthine, 3-methylxanthine, 7-methyluric acid, and 1,3-dimethyluric acid had weaker effects, and the other molecules had no apparent effect. The presented results indicate that a 7-methylxanthine concentration of about 6 × 10-5 M (10 mg/L) prevented the formation of crystals for an initial urate concentration of 1.78 × 10-3 M (300 mg/L) in the presence of 0.4 M of Na+ for 96 h at 25 °C and a pH of 7.4. We attribute these results to alterations in thermodynamics, not kinetics. Our results suggest that prevention of crystallization in vivo could be achieved by direct oral administration of 7-methylxanthine or other methylxanthines that are metabolized to 7-methylxanthine. For example, the hepatic metabolism of theobromine leads to significant plasma levels of 7-methylxanthine (14% of the initial theobromine concentration) and 3-methylxanthine (6% of the initial theobromine concentration); however, 7-methyluric acid is present at very low concentrations in the plasma. It is important to consider that several of the specific molecules we examined (theobromine, caffeine, theophylline, dyphylline, etophylline, and pentoxifylline) did not directly affect crystallization.

Semiempirical Approach to the Fukui Function Analysis of Uric Acid under Different pH Conditions

Analytic Fukui functions calculated at a first-principles level are combined with experimental pKa values and the calculation of tautomerization energies to obtain the effective regioselectivity of uric acid toward electron-transfer reactions under different pH conditions. Second-order electron binding energies are also computed to determine which of the tautomers is more likely to participate in the electron transfer. A comparison of vertical and adiabatic proton detachment energies allows us to conclude that tautomerization is not mediating deprotonation and that two monoanionic species are of comparable relevance. The main difference between these monoanionic species is the ring that has been deprotonated. Both monoanionic species are produced from a single neutral tautomer and mainly produce a single dianionic tautomer. As a method for the analysis of systems affected by pH such as uric acid, we propose to plot condensed Fukui functions versus pH, allowing us to draw the effect of pH on the regioselectivity of electron transfer in a single image.

Electro-optical behaviour of CuFe2 O4 @rGO nanocomposite for nonenzymatic detection of uric acid via the electrochemical method

Uric acid (UA) is a blood and urine component obtained as a metabolic by-product of purine nucleotides. Abnormalities in UA metabolism cause crystal deposition as monosodium urate and lead to various diseases such as gout, hyperuricemia, Lesch-Nyhan syndrome, etc. Monitoring these diseases requires a rapid, sensitive, selective, and portable detection approach. Therefore, this study demonstrates the hydrothermal synthesis of CuFe2 O4 /reduced graphene oxide (rGO) nanocomposite for selective detection of UA. After the nanocomposite synthesis, characterization was performed by X-ray diffraction spectroscopy, Fourier transform infrared spectroscopy, Raman spectroscopy, X-ray photoelectron spectroscopy, UV-visible spectrometry, atomic force spectroscopy, scanning electron microscopy, and electrochemical analysis. Furthermore, from the electrochemical analysis using cyclic voltammetry (CV), kinetic studies were carried out by varying the scan rate to obtain the diffusion coefficient, surface concentration, and rate of charge transfer to achieve a calibration curve that indicates the quasi reversible nature of the fabricated electrode with a linear regression coefficient of oxidation (R2 : 0.9992) and reduction (R2 : 0.9971) peaks. Moreover, the fabricated nonenzymatic amperometric sensor to detect UA with a linearity (R2 : 0.9989) of 1-400 μM was highly sensitive (2.75 × 10-4  mAμM-1  cm-2 ) and had a lower limit of detection (0.01231 μM) at pH 7.5 in phosphate-buffered saline solution. Therefore, the CuFe2 O4 /rGO/ITO-based nonenzymatic sensor could detect interfering agents and spiked real bovine serum samples with higher sensitivity and selectivity for UA detection.

Efficacy of Theobromine and Its Metabolites in Reducing the Risk of Uric Acid Lithiasis

Uric acid lithiasis accounts for about 10% of all types of renal lithiasis. The most common causes of uric acid lithiasis are low urinary pH, followed by high concentration of urinary uric acid, and low diuresis. Treatment of patients consists of alkalinization of urine, reducing the consumption of purine-rich foods, and administration of xanthine oxidase inhibitors, because there are no established therapeutic inhibitors of uric acid crystallization. We recently found that theobromine inhibited uric acid crystallization in vitro, and that the increased urinary level of theobromine following its oral consumption was associated with the prevention of uric acid crystallization. In this study, we evaluated the inhibitory effects of theobromine metabolites and other methylxanthine-related compounds on uric acid crystallization. We also measured the urinary concentrations of theobromine and its metabolites in samples from healthy individuals and patients with uric acid stones and compared the extent of uric acid supersaturation and uric acid crystal formation in these different samples. Theobromine and other methylxanthines that lacked a substituent at position 1 inhibited uric acid crystallization, but other methylxanthines did not have this effect. Individuals with clinical parameters that favored uric acid crystallization did not develop uric acid crystals when theobromine and its metabolites were in the urine at high levels. Thus, theobromine and its metabolites reduced the risk of uric acid lithiasis.

Investigations on melamine-based uric acid kidney stone formation and its prevention by inhibitors

Melamine (Mel) as a milk powder adulterant came to light in September 2008, when a kidney stone disease (KSD) outbreak struck China. The mechanism of the formation of Mel-associated uric acid (UA) stones is relatively unknown. Therefore, in the present study, Mel's influence was explored at comparatively higher and lower concentrations in artificial urine. The parameter optimization performed when the Mel concentration in artificial urine was low, which revealed that higher pH values and lower UA concentration considerably delayed the induction of UA crystallization. When Mel concentration was increased relative to UA concentration, the induction time of UA crystallization decreased dramatically. At the highest concentration of Mel investigated (at UA-Mel molar ratio 1:1), PXRD analysis and SEM revealed a change in crystalline structure of the samples. Based on FTIR analysis, it was determined that UA-Mel interactions are essentially physical, because no new characteristic bands developed. Two inhibitors, namely tri-potassium citrate (TPC) and 3, 7-dimethylxanthine (DMX), were investigated for their inhibitory action on UA crystallization in the presence of Mel. DMX was observed to be more promising than TPC in delaying the induction of crystallisation and hence inhibiting crystal formation.

Effect of theobromine on dissolution of uric acid kidney stones

Purpose

Uric acid renal lithiasis has a high prevalence and a high rate of recurrence. Removal of uric acid stones can be achieved by several surgical techniques (extracorporeal shock wave lithotripsy, endoscopy, laparoscopy, open surgery). These stones can also be eliminated by dissolution within the kidneys, because the solubility of uric acid is much greater when the pH is above 6. At present, N-acetylcysteine with a urinary basifying agent is the only treatment proposed to increase the dissolution of uric acid stones. In this paper, we compare the effect of theobromine and N-acetylcysteine on the in vitro dissolution of uric acid calculi in artificial urine at pH 6.5.

Methods

The dissolution of uric acid renal calculi was performed in a temperature-controlled (37 °C) chamber. A peristaltic pump was used to pass 750 mL of synthetic urine (pH 6.5) through a capsule every 24 h. Stone dissolution was evaluated by measuring the change in weight before and after each experiment.

Results

N-acetylcysteine increased the dissolution of uric acid calculi, but the effect was not statistically significant. Theobromine significantly increased the dissolution of uric acid calculi. Both substances together had the same effect as theobromine alone. The addition of theobromine to a basifying therapy that uses citrate and/or bicarbonate is a potential new strategy for the oral chemolysis of uric acid stones.

Conclusion

Theobromine may prevent the formation of new stones and increase the dissolution of existing stones.

Appraising the potency of small molecule inhibitors and their graphene surface-mediated organizational attributes on uric acid-melamine clusters

Uric acid (UA) and melamine (MM) crystallization in humans is associated with adverse medical conditions, including the germination of kidney stones, because of their low solubility. The growth of kidney stones, usually formed on renal papillary facades, is accomplished on the matrix-coated surface by the aggregation of preformed crystals or secondary crystal nucleation. Therefore, the effects of inhibitors such as theobromine (TB) and allopurinol (AP) on MM-UA aggregation are investigated by employing classical molecular dynamics simulations on a graphene surface. This impersonates the exact essence of the precipitation of kidney stones. The interaction between MM-UA is very intense and, thus, large clusters are formed on the surface. The presence of TB and AP will, however, substantially inhibit their aggregation. TB and AP significantly impede UA aggregation in particular. Therefore, lower order UA clusters are formed. These smaller UA clusters then pull a lower number of MM towards themselves, resulting in a smaller order UA-MM cluster. MM and UA aggregation on a 2D graphene surface is found to be spontaneous. There is no difference in these molecules' adsorption with a change in the force field parameters (i.e., GAFF and OPLS-AA) for graphene. Moreover, the greater the surface area of graphene, the more molecules are absorbed. The solute-surface van der Waals interaction energy plays a driving force in the adsorption of solute molecules on the surface. In addition, interactions like hydrogen bonding and π-stacking over the graphene surface involve binding all like molecules. These aggregated solute molecules strongly attract more like molecules until all solute molecules are adsorbed on the graphene surface, as estimated by enhanced sampling. The molecular origin of graphene exfoliation by MM is also described here. The present work helps to design novel kidney stone inhibitors.

Underlying Mechanisms of Allopurinol in Eliminating Renal Toxicity Induced by Melamine-Uric Acid Complex Formation: A Computational Study

Using molecular dynamics, we address uric acid (UA) replacement by a model small-molecule inhibitor, allopurinol (AP), from its aggregated cluster in a columnar fashion. Experimentally it has been affirmed that AP is efficient in preventing UA-mediated renal stone formation. However, no study has presented the underlying mechanisms yet. Hence, a theoretical approach is presented for mapping the AP, which binds to melamine (MM) and UA clusters. In AP's presence, the higher-order cluster of UA molecules turns into a lower-order cluster, which "drags" fewer MM to them. Consequently, the MM-UA composite structure gets reduced. It is worth noting that UA-AP and AP-MM hydrogen-bonding interactions often play an essential role in reducing the UA-MM cluster size. Interestingly, an AP around UA makes a pillar-like structure, confirmed by defining the point-plane distribution function. The decomposition of the preferential interaction by Kirkwood-Buff integral into different angles like 0°-30°, 30°-60°, and 60°-90° firmly establishes the phenomenon mentioned above. However, the structural order for such π-stacking interactions between AP and UA molecules is not hierarchical but rather more spontaneous. The driving force behind UA-AP-MM composite formation is the favorable complexation energy that can be inferred by computing pairwise binding free energies for all possible combinations. Performing enhanced sampling and quantum calculations further confirms the evidence for UA degradation.

Controlling the self-assembly of human calcitonin: a theoretical approach using molecular dynamics simulations

Human calcitonin (hCT) is a 32-residue amino acid poly-peptide hormone which is secreted by the C-cells (also known as parafollicular cells) of thyroid glands. It acts to inhibit osteoclast cell hormones by reducing the cell function and regulating calcium and phosphate in blood. hCT has a high tendency to assemble into protofilaments with β-sheet conformations. Amyloid fibril formation of hCT reduces its bio-activity and limits its application as a therapeutic drug. Salmon calcitonin (sCT), which also carries the same disulfide bridge at the N and C-terminus, but differs at the 16 residue position from hCT, has less propensity to aggregate than hCT. Human calcitonin has much higher bio-activity than sCT if its aggregation propensity is reduced. Substituting the key residues which are responsible for the aggregation of hCT, is one of the ways to reduce its aggregation and fibril formation. hCT analogues with less aggregation tendency can be exploited as therapeutic drugs. In this work, we study the amyloidogenic behavior of hCT and its peptide based derivatives i.e., sCT, phCT, N17H hCT, Y12L hCT and DM hCT, through classical molecular dynamics (MD) simulations. Our study reveals that sCT is the least aggregation prone derivative, and the double mutation at position 12 and 17 can reduce the aggregation propensity of this peptide. Also, we have applied these mutant variants of hCT as peptide inhibitors in the self-aggregation of hCT. This study could help in understanding and preparing peptide-based inhibitors for hCT fibrillation and their applications as therapeutic drugs.

The miscibility and solubility of uric acid and vitamin C in the solution phase and their structural alignment in the solid-liquid interface

The crystallization of uric acid (UA) in humans is correlated with unpropitious medical predicaments, including gout and kidney stone germination. Its comparatively low solubility in physiological solutions is a significant contributory factor to UA biomineralization. The inhibition of UA aggregation is investigated as a reasonable approach for reducing kidney and gout-related problems. Therefore, we examine the role of vitamin C (Vit-C), a water-soluble vitamin, in the aggregation of UA, and its potency in solubilizing UA has been confirmed experimentally. We notice that Vit-C encapsulates the aggregated UA. Moreover, it can dismantle the assemblies of UA. We have proffered comprehensive molecular mechanisms of the interplay between the aggregated UA and Vit-C. Vit-C molecules are interspersed in solution due to its non-aggregating nature. We perceive that, through hydrogen bonding and aromatic stacking interactions, Vit-C molecules interact with UA molecules. The determination of the Flory-Huggins interaction parameters suggests that the presence of Vit-C enhances the solubility of UA aggregates. In addition, UA molecules are conformed on a monolayer graphene sheet, where they are assembled to create a 2D self-assembly. Vit-C, however, encapsulates and disseminates itself within the aggregated UA molecules on the surface. Therefore, the molecular mechanisms of the impact of Vit-C on UA aggregation can provide relevant insights into drug design against chronic diseases.

Synthesis of a novel Ce(iii)/melamine coordination polymer and its application for corrosion protection of AA2024 in NaCl solution

We present the synthesis of a new cerium(iii)-melamine coordination polymer (CMCP) by a mixed-solvothermal method and its characterization. Characterization techniques included Raman, Fourier Transformation Infra-Red (FTIR), X-Ray Diffraction (XRD) and Scanning Electron Microscope (SEM), in which the change in the electronic environment and the crystallinity were tracked. The characterization results confirm the coordination of cerium(iii) with melamine through -NH2 groups, instead of the N atoms of the triazine ring, for which we propose a mechanism of interaction. In addition, Biovia Materials Studio package was applied to determine and investigate the molecular structure of the CMCP. All simulations were done using COMPASS force-field theory and atom-based method for summation of electrostatic and van de Waals forces. The application of the CMCP for the corrosion inhibition of AA2024 in 3.5% NaCl solution was tested using the potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) techniques. The results point out that the presence of cerium as cerium(iii) in the CMCP structure plays the fundamental role of inhibition, whereby the inhibition mechanism occurs by cathodic oxidation of Ce(iii) to Ce(iv) and cyclic reduction of Ce(iv) to Ce(iii) by melamine part of CMCP.

Investigation on the Mechanisms of Synchronous Interaction of K3Cit with Melamine and Uric Acid That Avoids the Formation of Large Clusters

Uric acid (UA) has an enormous competence to aggregate over melamine (Mel), producing large UA clusters that "drag" Mel toward them. Such a combination of donor-acceptor pairs provides a robust Mel-UA composite, thereby denoting a high complexity. Thus, a straightforward but pragmatic methodology might indeed require either destruction of the aggregation of UA or impediment of the hydrogen-bonded cluster of Mel and UA. Here, potassium citrate (K₃Cit) is used as a potent inhibitor for a significant decrease of large UA-Mel clusters. The underlying mechanisms of synchronous interactions between K₃Cit and the Mel-UA pair are examined by the classical molecular dynamics simulation coupled with the enhanced sampling method. K₃Cit binds to the Mel-UA pair profoundly to produce a Mel-UA-K₃Cit complex with favorable complexation energy (as indicated by the reckoning of pairwise ΔG_bind° employing the molecular mechanics Poisson-Boltzmann surface area (MM-PBSA) method). The strength of interaction follows the order UA-K₃Cit > Mel-K₃Cit > Mel-UA, thus clearly demonstrating the instability caused by upsetting the π-stacking of UA and hydrogen bonding of Mel-UA simultaneously. The comprehensive, strategically designed "direct approach" and "indirect approach" cluster structure analysis shows that K₃Cit reduces the direct approach Mel-UA cluster size significantly irrespective of ensemble variation. Furthermore, the estimation of potentials of mean force (PMFs) reveals that the (UA)_decamer-Mel interaction prevails over (UA)_tetramer-Mel. The dynamic property (dimer existence autocorrelation functions) proves the essence of dimerization between Mel and UA in the absence and presence of K₃Cit. Moreover, the calculation of the preferential interaction parameter provides the concentration at which Mel-K₃Cit and UA-K₃Cit interactions are predominant over the interaction of Mel and UA.

A Copper(II) Macrocycle Complex for Sensing Biologically Relevant Organic Anions in a Competitive Fluorescence Assay: Oxalate Sensor or Urate Sensor?

Fluorescence sensing of oxalate has garnered some attention in the past two decades as a result of this anion's prominence and impact on society. Previous work on oxalate sensors and other divalent anion sensors has led to the conclusion that the sensors are selective for the anion under investigation. However, sensor selectivity is often determined by testing against a relatively small array of "guest" molecules or analytes and studies often exclude potentially interfering compounds. For example, studies on oxalate sensors have excluded compounds such as citrate and urate, which are anions in the biological matrices where oxalate is measured (e.g., urine, blood, and bacterial lysate). In the present study, we reassessed the selectivity of a dinuclear copper(II) macrocycle (Cu₂L) in an eosin Y displacement assay using biologically relevant anions. Although previously reported as selective for oxalate, we found greater indicator displacement (fluorescence response) for urate and oxaloacetate and a significant response to citrate. These anions are larger than oxalate and do not appear to fit into the putative binding pocket of Cu₂L. Consistent with previous reports, Cu₂L did not release eosin Y in the presence of several other dicarboxylates, including adipate, glutarate, malate (except at 10 mM), fumarate, succinate, or malonate (except at 10 mM), and the monocarboxylate acetate. This was demonstrated by the failure of the anions to reverse eosin Y quenching by Cu₂L. We also assessed, for the first time, other monocarboxylates, including butyrate, pyruvate, lactate, propionate, and formate. None of these anions were able to displace eosin Y, indicating no interaction with Cu₂L that interfered with the eosin Y binding site. Single-crystal X-ray crystallography revealed that nonselective binding of the anions is likely partly caused by readily accessible copper(II) ions on the external surface of Cu₂L. In addition, π-π stacking of urate with the aromatic groups of Cu₂L cannot be ruled out as a contributor to binding. We conclude that Cu₂L is not suitable for oxalate sensing in a biological matrix unless interfering compounds are selectively removed or masked.