Rapid and sustainable HPLC method for the determination of uremic toxins in human plasma samples

Protein-bound uremic toxins, mainly indoxyl sulfate (3-INDS), p-cresol sulfate (pCS), and indole-3-acetic acid (3-IAA) but also phenol (Pol) and p-cresol (pC), are progressively accumulated during chronic kidney disease (CKD). Their accurate measurement in biomatrices is demanded for timely diagnosis and adoption of appropriate therapeutic measures. Multianalyte methods allowing the establishment of a uremic metabolite profile are still missing. Hence, the aim of this work was to develop a rapid and sensitive method based on high-performance liquid chromatography with fluorescence detection for the simultaneous quantification of Pol, 3-IAA, pC, 3-INDS, and pCS in human plasma. Separation was attained in 12 min, using a monolithic C18 column and isocratic elution with acetonitrile and phosphate buffer containing an ion-pairing reagent, at a flow rate of 2 mL min^-1. Standards were prepared in plasma and quantification was performed using the background subtraction approach. LOQ values were ≤ 0.2 µg mL^-1 for all analytes except for pCS (LOQ of 2 µg mL^-1). The method proved to be accurate (93.5-112%) and precise (CV ≤ 14.3%). The multianalyte application of the method, associated to a reduced sample volume (50 µL), a less toxic internal standard (eugenol) in comparison to the previously applied 2,6-dimethylphenol and 4-ethylphenol, and a green extraction solvent (ethanol), resulted in the AGREE score of 0.62 which is in line with the recent trend of green and sustainable analytical chemistry. The validated method was successfully applied to the analysis of plasma samples from control subjects exhibiting normal levels of uremic toxins and CKD patients presenting significantly higher levels of 3-IAA, pC, 3-INDS, and pCS that can be further investigated as biomarkers of disease progression.

Sample preparation and chromatographic methods for the determination of protein-bound uremic retention solutes in human biological samples: An overview

Protein-bound uremic retention solutes, such as indole-3-acetic acid, indoxyl sulfate, p-cresol and p-cresol sulfate, are associated with the development of several pathologies, namely renal, cardiovascular, and bone toxicities, due to their potential accumulation in the human body, thus requiring analytical methods for monitoring and evaluation. The present review addresses conventional and advanced sample treatment procedures for sample handling and the chromatographic analytical methods developed for quantification of these compounds in different biological fluids, with particular focus on plasma, serum, and urine. The sample preparation and chromatographic methods coupled to different detection systems are critically discussed, focusing on the different steps involved for sample treatment, namely elimination of interfering compounds present in the sample matrix, and the evaluation of their environmental impact through the AGREEprep tool. There is a clear trend for the application of liquid-chromatography coupled to tandem mass spectrometry, which requires protein precipitation, solid-phase extraction and/or dilution prior to analysis of biological samples. Furthermore, from a sustainability point of view, miniaturized methods resorting to microplate devices are highly recommended.

Correlation of inflammatory biomarkers with the diversity of Bacteroidaceae, Bifidobacteriaceae, Prevotellaceae and Lactobacillaceae families in the intestinal microbiota of patients with end stage renal disease

PURPOSE

Serum levels of inflammatory cytokines and uremic toxins, and their inter-correlations with the diversity of Bacteroidaceae, Bifidobacteriaceae, Prevotellaceae and Lactobacillaceae families in intestinal microbiota were investigated in patients with end stage renal disease (ESRD).

METHODS

Stool and blood samples from 20 ESRD patients on maintenance hemodialysis were collected. DNA genome of the bacterial composition of the stool samples was extracted and evaluated by the sequencing analysis of 16S rRNA genes. Serum levels of inflammatory cytokines and uremic toxins were then analyzed.

RESULTS

The mean serum concentrations of TNF-α, IL-6, indoxyl sulfate (IS) and p-cresol (PC) were 305.99 ​± ​12.03 ​ng/L, 159.95 ​± ​64.22 ​ng/L, 36.76 ​± ​5.09 ​μg/mL and 0.39 ​± ​0.15 ​μg/mL, respectively. The most significant positive correlation was observed between Prevotellaceae family and total antioxidant capacity (TAC), Lactobacilli species and CRP and PC, as well as Scardovia wiggsiae and IS (p ​< ​0.001). A negative correlation was also found between Bacteroides clarus and PC. Patients with ESRD on maintenance hemodialysis had elevated levels of PC and IS and increased levels of the inflammatory markers. The most positive correlation was found between microbiota and CRP and PC, while the most negative one was between microbiota and IL-1 and TAC.

CONCLUSIONS

The abundance and diversity of Bacteroidaceae, Bifidobacteriaceae, Prevotellaceae and Lactobacillaceae families and their correlations with clinical parameters could provide benefits in the ESRD patients but they could not promote the symptoms.

Correlation between microbial communities and flavor compounds during the fifth and sixth rounds of sauce-flavor baijiu fermentation

Sauce-flavor baijiu is a representative of Chinese traditional fermented baijiu using grains as the raw materials through the co-fermentation of microorganisms. The whole manufacturing process includes 7 times of distillation and generates 7 kinds of base baijius. The final product is a mixture of the 7 kinds of base baijius. Thus the base baijius greatly affect the quality of the final product. The quality of the base baijiu obtained by the sixth distillation is obviously poorer than that of the fifth one. However, the reason is still unclear and limits the quality control of baijiu fermentation. In this study, the flavor substances and microbiota in the up, middle and bottom layers of fermented grains in the fifth and sixth rounds were compared. Some flavor esters showed obviously decreased concentrations in the sixth round, including ethyl benzoneacetic acid, ethyl hexanoic acid, ethyl dodecanoic acid, diethyl butanedioic acid, and ethyl 2-hydroxyl-propanoic acid. Meanwhile, an off-flavor p-cresol was detected in the sixth round. Correlation analysis of flavor chemicals and microbiota indicated that fungi in the fifth round played an important role for ester synthesis. Some bacterial and fungal species were both positively correlated with p-cresol synthesis, and the related p-cresol metabolic pathways were proposed for the first time. These results revealed flavor divergences of fermented grains between the fifth and sixth rounds, and will ultimately help to improve baijiu quality.

Effects of the L-tyrosine-derived bacterial metabolite p-cresol on colonic and peripheral cells

Specific families of bacteria present within the intestinal luminal content produce p-cresol from L-tyrosine. Although the hosts do not synthesize p-cresol, they can metabolize this compound within their colonic mucosa and liver leading to the production of co-metabolites including p-cresyl sulfate (p-CS) and p-cresyl glucuronide (p-CG). p-Cresol and its co-metabolites are recovered in the circulation mainly conjugated to albumin, but also in their free forms that are excreted in the urine. An increased dietary protein intake raises the amount of p-cresol recovered in the feces and urine, while fecal excretion of p-cresol is diminished by a diet containing undigestible polysaccharides. p-Cresol in excess is genotoxic for colonocytes. In addition, in these cells, this bacterial metabolite decreases mitochondrial oxygen consumption, while increasing the anion superoxide production. In chronic kidney disease (CKD), marked accumulation of p-cresol and p-CS in plasma is measured, and in renal tubular cells, p-cresol and p-CS increase oxidative stress, affect mitochondrial function, and lead to cell death, strongly suggesting that these 2 compounds act as uremic toxins that aggravate CKD progression. p-Cresol and p-CS are also suspected to play a role in the CKD-associated adverse cardiovascular events, since they affect endothelial cell proliferation and migration, decrease the capacity of endothelial wound repair, and increase the senescence of endothelial cells. Finally, the fact that concentration of p-cresol is transiently increased in young autistic children biological fluids, and that intraperitoneal injection of p-cresol in animal models induces some behavioral characteristics observed in the autism spectrum disorders (ASD), raise the view that p-cresol may possibly represent one of the components involved in ASD etiology. Further pre-clinical and clinical studies are obviously needed to determine if the lowering of p-cresol and/or p-CS circulating concentrations, by dietary and/or pharmacological means, would allow, by itself or in combination with other interventions, to improve CKD progression and associated cardiovascular outcomes, as well as some neurological outcomes in children with an early diagnosis of autism.

The microbial metabolite p-Cresol induces autistic-like behaviors in mice by remodeling the gut microbiota

BACKGROUND

Autism spectrum disorders (ASD) are associated with dysregulation of the microbiota-gut-brain axis, changes in microbiota composition as well as in the fecal, serum, and urine levels of microbial metabolites. Yet a causal relationship between dysregulation of the microbiota-gut-brain axis and ASD remains to be demonstrated. Here, we hypothesized that the microbial metabolite p-Cresol, which is more abundant in ASD patients compared to neurotypical individuals, could induce ASD-like behavior in mice.

RESULTS

Mice exposed to p-Cresol for 4 weeks in drinking water presented social behavior deficits, stereotypies, and perseverative behaviors, but no changes in anxiety, locomotion, or cognition. Abnormal social behavior induced by p-Cresol was associated with decreased activity of central dopamine neurons involved in the social reward circuit. Further, p-Cresol induced changes in microbiota composition and social behavior deficits could be transferred from p-Cresol-treated mice to control mice by fecal microbiota transplantation (FMT). We also showed that mice transplanted with the microbiota of p-Cresol-treated mice exhibited increased fecal p-Cresol excretion, compared to mice transplanted with the microbiota of control mice. In addition, we identified possible p-Cresol bacterial producers. Lastly, the microbiota of control mice rescued social interactions, dopamine neurons excitability, and fecal p-Cresol levels when transplanted to p-Cresol-treated mice.

CONCLUSIONS

The microbial metabolite p-Cresol induces selectively ASD core behavioral symptoms in mice. Social behavior deficits induced by p-Cresol are dependant on changes in microbiota composition. Our study paves the way for therapeutic interventions targeting the microbiota and p-Cresol production to treat patients with ASD. Video abstract.

Interference of dietary polyphenols with potentially toxic amino acid metabolites derived from the colonic microbiota

Each day, varying amounts of undigested or partially digested proteins reach the colon where they are metabolized by the microbiota, resulting in the formation of compounds such as ammonia, p-cresol, skatole, phenol, indole, and hydrogen sulfide (H₂S). In farm animals, the excessive production of these metabolites can affect the quality of meat and milk and is a source of contaminating emissions from animal manure. In humans, their accumulation is potentially harmful, and it has been proposed that they could be involved in the development of pathologies such as colorectal cancer and ulcerative colitis, among others. This review assesses the evidence supporting the use of dietary polyphenols to reduce the production of these metabolites. Most studies have used condensed (proanthocyanidins) or hydrolyzable (ellagitannins and gallotannins) tannins, and have been carried out in farm animals. Several show that the administration of tannins in pigs, chicken, and ruminants decreases the levels of ammonia, p-cresol, skatole, and/or H₂S, improving meat/milk quality and reducing manure odor. Direct application of tannins to manure also decreases ammonia emissions. Few studies were carried out in rats and humans and their results confirm, to a lesser extent, those reported in farm animals. These effects would be due to the capacity of tannins to trap ammonia and H₂S, and to modify the composition of the microbiota, reducing the bacterial populations producing metabolites. In addition, PACs prevent p-cresol and H₂S-induced alterations on intestinal cells in vitro. Tannins, therefore, appear as an interesting tool for improving the quality of animal products, human health, and the harmful emissions associated with breeding.

Near real-time analysis of para-cresol in wastewater with a laccase-carbon nanotube-based biosensor

Para-Cresol is a water-soluble organic pollutant, which is harmful to organisms even at low concentrations. Therefore, it is important to rapidly detect the p-cresol in wastewater as well as natural water. In this work, a new, simple and stable biosensor was developed for on-site quantitatively determination and near real-time monitoring p-cresol in wastewater. The new biosensor was designed and fabricated using a screen-printed carbon electrode (SPCE) modified by waste-derived carbon nanotubes (CNTs) immobilized with laccase (LAC). The fabrication processes and performance of the biosensors were systematically characterized and optimized by Fourier transform infrared spectroscopy (FTIR), field emission scanning electron microscope (FESEM), transmission electron microscopy (TEM) and electrochemical methods. With improved conductivity, the proposed biosensor could provide the direct quantitation of p-cresol. The linear range of the biosensor is 0.2-25 ppm of p-cresol with a detection limit of 0.05 ppm. Additionally, the biosensor exhibited high reproducibility, stability and reusability during the validation. More importantly, the biosensor was successfully applied for the rapid detection of p-cresol in environmental lab wastewater under the interference of metal ions and other organics, and the results were consistent with high-performance liquid chromatography (HPLC). Finally, the biosensor with a portable potentiostat was approved as an easy-to-use, sensitive and inexpensive platform that could provide near real-time monitoring of p-cresol concentration in wastewater during Fenton oxidation treatment process.

Enhanced visible-light driven multi-photoredox Cr(VI) and p-cresol by Si and Zr interplay in fibrous silica-zirconia

Multiple contaminants including heavy metals and phenolic compounds are normally co-exist in wastewater, which caused the treatment process is rather complicated. Herein, the synergistic photoredox of Cr(VI) and p-cresol (pC) by innovative fibrous silica zirconia (FSZr) photocatalyst was reported. The high surface area of FSZr comprised of microspheres with a bicontinuous concentric lamella structure morphology consisted of silica, while its core consisted of ZrO₂ structure. The rearrangement of FSZr framework increased the crystallinity, formed Si-O-Zr bonds and narrowed the band gap of ZrO₂ for enhanced of photoredox of Cr(VI) and pC. Compared to the reaction, the photoredox efficiency of FSZr for removing Cr(VI) and pC in simultaneous system was found to be 96 % and 59 %, respectively which are higher than that in its single system owing to the efficient electron-hole charge separation. Phenolic compound with high degree of electron donating group gave beneficial effect to photoreduction of Cr(VI). Consequently, a proposed mechanism involving multi-photoredox pathway were proposed based on photoredox reaction and scavengers studies. FSZr sustained the simultaneous photoredox activities after five runs demonstrating its possibility to be use in the wastewater treatment of various pollutants.

Removal of p-cresol and tylosin from water using a novel composite of alginate, recycled MnO2 and activated carbon

For sustainable production, there is an urgent need to minimize the adverse environmental impacts of swine farming, which is a major contributor of the pollutants p-cresol and tylosin. Novel reactive composite alginate beads (CAB-MOACs) were fabricated by combining alginate with activated carbon (AC) and MnO₂ recovered from spent battery waste and used for efficient removal of p-cresol and tylosin from water. Batch experiments were carried out under varying pH (3-11), temperature (15-50 °C), and agitation speed (50-200 rpm) to understand their effects on removal efficiency. The CAB-MOACs had better removal efficiency for p-cresol and tylosin than alginate beads alone or beads containing only AC or MnO₂. Adsorption to CAB-MOACs followed pseudo-second-order kinetics (R²≥0.98) and Langmuir isotherm models (R²≥0.95). CAB-MOACs showed higher removal efficiency (∼99.9% after 10 h) compared to beads containing only immobilized MnO₂ (60-70%) or AC (94-96%). Regeneration and reuse performance of the CAB-MOACs was excellent through five cycles, although slightly better for p-cresol than tylosin. With low-cost manufacturing and beneficial utilization of hazardous waste such as spent batteries, the newly developed composite beads show potential as an effective adsorbent for treating wastewater effluent containing emerging contaminants like p-cresol and tylosin. Future studies may focus on product refinement and large-scale testing on actual wastewaters.

Development of the LC-MS/MS method for determining the p-cresol level in plasma

p-Cresol is a protein-bound uremic retention solute that originates in the intestine through bacterial metabolism and accumulates throughout the body in case of kidney failure. To date, there has been no method to analyze unconjugated p-cresol concentration in the blood with a limit of detection lower than 75 pg. Thus, the aim of this study was to develop and validate a novel liquid chromatography-tandem mass spectrometry method for the determination of unconjugated p-cresol in plasma with a lower detection limit than what has been determined using previously described methods. Sample preparation included derivatization of p-cresol with dansyl chloride (derivatization reagent) showed to be a better approach to analyze the compound. The method optimization involved various pH, time of the reaction, and concentration of derivatization reagent. The validation process was performed according to the procedures prescribed by the European Medicines Agency. All analyzed validation criteria were fulfilled. The novel validated method was applied to compare the level of p-cresol in patients with chronic renal failure before and after dialysis (n = 24). Additionally, the concentration of p-cresol was determined in patients with multiple organ dysfunction syndrome (n = 23). The established method can be used for determination of p-cresol in the plasma in further clinical research.

CYP1A1 activity in rainbow trout is inhibited by the environmental pollutant p-cresol

Naturally- and anthropogenically-produced cresols could pose serious risks to fish health. In this study, three piscine CYP isoforms were investigated for their abilities to interact with p-cresol. Therefore, the activity of 7-ethoxyresorufin-O-deethylase (EROD), 7-benzyloxy-4-trifluoromethylcoumarin O-debenzylase (BFCOD), and p-nitrophenol hydroxylase (PNPH) were evaluated in the hepatic microsomes of juvenile rainbow trout. Results showed that EROD activity was inhibited in a competitive manner, BFCOD activity was inhibited in presence the highest tested p-cresol concentration and PNPH activity was not affected. These results indicate that p-cresol might affect the ability of fish to metabolize numerous aromatic hydrocarbons and dioxin compounds, which are present in the aquatic environment.

Melatonin Rescues Mesenchymal Stem Cells from Senescence Induced by the Uremic Toxin p-Cresol via Inhibiting mTOR-Dependent Autophagy

p-Cresol, found at high concentrations in the serum of chronic kidney failure patients, is known to cause cell senescence and other complications in different parts of the body. p-Cresol is thought to mediate cytotoxic effects through the induction of autophagy response. However, toxic effects of p-cresol on mesenchymal stem cells have not been elucidated. Thus, we aimed to investigate whether p-cresol induces senescence of mesenchymal stem cells, and whether melatonin can ameliorate abnormal autophagy response caused by p-cresol. We found that p-cresol concentration-dependently reduced proliferation of mesenchymal stem cells. Pretreatment with melatonin prevented pro-senescence effects of p-cresol on mesenchymal stem cells. We found that by inducing phosphorylation of Akt and activating the Akt signaling pathway, melatonin enhanced catalase activity and thereby inhibited the accumulation of reactive oxygen species induced by p-cresol in mesenchymal stem cells, ultimately preventing abnormal activation of autophagy. Furthermore, preincubation with melatonin counteracted other pro-senescence changes caused by p-cresol, such as the increase in total 5′-AMP-activated protein kinase expression and decrease in the level of phosphorylated mechanistic target of rapamycin. Ultimately, we discovered that melatonin restored the expression of senescence marker protein 30, which is normally suppressed because of the induction of the autophagy pathway in chronic kidney failure patients by p-cresol. Our findings suggest that stem cell senescence in patients with chronic kidney failure could be potentially rescued by the administration of melatonin, which grants this hormone a novel therapeutic role.

p-Cresyl sulfate decreases peripheral B cells in mice with adenine-induced renal dysfunction

Infection is a major cause of mortality in chronic kidney disease (CKD) patients. Although immune dysfunction is a risk factor for infection in CKD patients, its causes are not fully elucidated. In the present study, we evaluated whether p-cresyl sulfate (pCS), an intestinal bacteria-derived uremic toxin, was involved in immune dysfunction in CKD. We used osmotic pumps to establish adenine-induced renal dysfunction mice with a chronically high blood pCS concentration. Analysis of lymphocyte subsets revealed that pCS significantly reduced peripheral B cells in renal dysfunction mice. In vitro, pCS inhibited interleukin (IL)-7-induced proliferation of CD43⁺ B-cell progenitors and suppressed IL-7-induced phosphorylation of signal transducer and activator of transcription 5 (STAT5) in these cells. Cell cycle analysis showed that pCS significantly decreased the percentage of CD43⁺ B-cell progenitors in S phase and increased that in G1 phase. These results suggest that pCS suppressed IL-7-induced STAT5 signaling and inhibited B-cell progenitor proliferation, leading to reduction of peripheral B cells in adenine-induced renal dysfunction mice. Therefore, pCS decreases peripheral B cells by inhibiting proliferation of CD43⁺ B-cell progenitors and is a likely cause of immune dysfunction in CKD patients.

Analytical study of effective biodegradation of p-cresol using Serratia marcescens ABHI001: application in bioremediation

This study evaluated the capability of Serratia marcescens ABHI001 to effectively degrade p-cresol through different techniques. The molecular identity of the laboratory isolate S. marcescens ABHI001 was confirmed through the 16S ribosomal DNA gene pattern, and its morphological features were investigated through field-emission scanning electron microscopy. In addition, the degradation behavior of the isolate for cresol was verified using several techniques, including UV-visible spectroscopy, followed by high-performance liquid chromatography (HPLC), gas chromatography, and Fourier transform infrared spectroscopy. The maximum degradation percentage of 85% for p-cresol could be achieved after 18 h of incubation with S. marcescens ABHI001. The formation of p-hydroxybenzaldehyde, p-hydroxybenzoate, and protocatechuate metabolites was confirmed through HPLC. The study results indicate that S. marcescens ABHI001 may have applications in the bioremediation of organic pollutants, such as p-cresol.

Plasma p-cresol lowering effect of sevelamer in non-dialysis CKD patients: evidence from a randomized controlled trial

BACKGROUND

The accumulation of p-cresol, a metabolic product of aromatic amino acids generated by intestinal microbiome, increases the cardiovascular risk in chronic kidney disease (CKD) patients. Therefore, therapeutic strategies to reduce plasma p-cresol levels are highly demanded. It has been reported that the phosphate binder sevelamer (SEV) sequesters p-cresol in vitro, while in vivo studies on dialysis patients showed controversial results. Aim of our study was to evaluate the effect of SEV on p-cresol levels in non-dialysis CKD patients.

METHODS

This was a single-blind, randomized placebo-controlled trial (Registration number NCT02199444) carried on 69 CKD patients (stage 3-5, not on dialysis), randomly assigned (1:1) to receive either SEV or placebo for 3 months. Total p-cresol serum levels were evaluated at baseline (T0), and 1 (T1) and 3 months (T3) after treatment start. The primary end-point was to evaluate the effect of SEV on p-cresol levels.

RESULTS

Compared to baseline (T0, 7.4 ± 2.7 mg/mL), p-cresol mean concentration was significantly reduced in SEV patients after one (- 2.06 mg/mL, 95% CI - 2.62 to - 1.50 mg/mL; p < 0.001) and 3 months of treatment (- 3.97 mg/mL, 95% CI - 4.53 to - 3.41 mg/mL; p < 0.001); no change of plasma p-cresol concentration was recorded in placebo-treated patients. Moreover, P and LDL values were reduced after 3 months of treatment by SEV but not placebo.

CONCLUSIONS

In conclusion, our study represents the first evidence that SEV is effective in reducing p-cresol levels in CKD patients in conservative treatment, and confirms its beneficial effects on inflammation and lipid pattern.

Effect of a Short-Course Treatment with Synbiotics on Plasma p-Cresol Concentration in Kidney Transplant Recipients

OBJECTIVE

We evaluated whether a short-term course with synbiotics may lower plasma p-Cresol concentrations in kidney transplant patients (KTRs) who accumulate this uremic toxin both because of increased production by their dysbiotic gut microbiome and because of reduced elimination by the transplanted kidneys.

METHODS

Thirty-six KTRs (29 males, mean age 49.6 ± 9.1 years) with transplant vintage > 12 months, stable graft function, and no episode of acute rejection or infection in the last 3 months were enrolled in this single-center, parallel-group, double-blinded, randomized (2:1 synbiotic to placebo) study. Synbiotic (Probinul Neutro, CadiGroup, Rome, Italy) or placebo was taken at home for 30 days, as 5 g powder packets dissolved in water three times a day far from meals. The main outcome measure was the decrease in total plasma p-Cresol measured by high-performance liquid chromatography at baseline and after 15 and 30 days of placebo or synbiotic treatment.

RESULTS

After 15 and 30 days of treatment, plasma p-Cresol decreased by 40% and 33% from baseline (both p < 0.05), respectively, in the synbiotic group, whereas it remained stable in the placebo group. After 30 days of treatment, no significant change was observed in either group in renal function, glycemia, plasma lipids, or albumin concentration. Treatment was well tolerated and did not induce any change in stool characteristics.

CONCLUSION

The results of this pilot study suggest that treatment with synbiotics may be effective to lower plasma p-Cresol concentrations in KTRs. Prospective larger scale, longer term studies are needed to establish whether cardiovascular prognosis could also be improved with this nutritional intervention.

Performance of electrochemical oxidation and photocatalysis in terms of kinetics and energy consumption. New insights into the p-cresol degradation

This work reports the comparative performance of two Advanced Oxidation Processes (AOPs), electrochemical oxidation and photocatalysis, as individual technological alternatives for the treatment of effluents containing p-cresol. First, the influence of operating parameters in the oxidation and mineralization yield was carried out together with kinetic analysis. Boron Doped Diamond (BDD), RuO₂ and Pt as anodic materials, Na₂SO₄ and NaCl as supporting electrolytes and different current densities were evaluated in electrochemical oxidation whereas the effect of TiO₂ concentration and radiation was studied in the photocatalytic degradation. Then, the parameter Electrical Energy per Order (E_EO) was calculated to compare the energy consumption in both AOPs, concluding that under the studied conditions the electrochemical treatment with BDD, Na₂SO₄ and 125 A m^-2 showed the best energy efficiency, with an E_EO of 5.83 kW h m^-3 order^-1 for p-cresol and 58.05 kW h m^-3 order^-1 for DOC removal, respectively.

Evaluation of p-cresol degradation with polyphenol oxidase (PPO) immobilized in various matrices

p-Cresol is an environmental pollutant due to its vast use, toxicity and persistence, nevertheless, its degradation in an enzyme is unclear. In this study, we used Pleurotus sp. isolate VLECK02 polyphenol oxidase (PPO) for the determination of p-cresol degradation. On the basis of UV, FT-IR and chromatographic (HPLC and GC–MS) analysis, 4-methylcatechol was identified as the main metabolite of p-cresol catabolism. In addition, batch and semi-continuous degradation of p-cresol (10 and 20 mM) were studied and compared by free and immobilized PPO in different matrices like sodium alginate (SA), sodium alginate–polyvinyl alcohol (SA–PVA) and sodium alginate–polyvinyl alcohol–silver nanoparticles (SA–PVA–AgNPs). The experimental data showed that an enzyme (PPO) immobilized in SA–PVA–AgNPs was completely degraded p-cresol at initial concentrations of 10 and 20 mM within 30 h. These results suggest that the enzyme immobilized in SA–PVA–AgNPs has achieved higher degradation rates at a given time than free PPO and PPO immobilized in SA–PVA and SA. The SA–PVA–AgNPs and SA–PVA immobilized enzyme could be reused for more than 12 and 8 cycles, respectively, without losing any degradation capacity. Moreover, the immobilized PPO showed higher tolerance to various temperatures and pH than free PPO. Hence, immobilized PPO could be useful for the bioremediation of environment contaminated with phenolic compounds like p-cresol.

Homologous expression and biochemical characterization of the arylsulfatase from Kluyveromyces lactis and its relevance in milk processing

The industrial manufacturing process of lactose-free milk products depends on the application of commercial β-galactosidase (lactase) preparations. These preparations are often obtained from Kluyveromyces lactis. There is a gene present in the genome of K. lactis which should encode for an enzyme called arylsulfatase (EC 3.1.6.1). Therefore, this enzyme could also be present in β-galactosidase preparations. The arylsulfatase is suspected of being responsible for an unpleasant "cowshed-like" off-flavor resulting from the release of p-cresol from milk endogenous alkylphenol sulfuric esters. So far, no gene/functionality relationship is described. In addition, no study is available which has shown that arylsulfatase from K. lactis is truly responsible for the flavor generation. In this study, we cloned the putative arylsulfatase gene from K. lactis GG799 into the commercially available vector pKLAC2. The cloning strategy chosen resulted in a homologous, secretory expression of the arylsulfatase. We showed that the heretofore putative arylsulfatase has the desired activity with the synthetic substrate p-nitrophenyl sulfate and with the natural substrate p-cresol sulfate. The enzyme was biochemically characterized and showed an optimum temperature of 45-50 °C and an optimum pH of 9-10. Additionally, the arylsulfatase was activated by Ca(2+) ions and was inactivated by Zn(2+) ions. Moreover, the arylsulfatase was inhibited by p-cresol and sulfate ions. Finally, the enzyme was added to ultra-heat treated (UHT) milk and a sensory triangle test verified that the arylsulfatase from K. lactis can cause an unpleasant "cowshed-like" off-flavor.

Uremic toxin p-cresol induces Akt-pathway-selective insulin resistance in bone marrow-derived mesenchymal stem cells

We reported a functional incompetence in mesenchymal stem cells (MSCs) under uremia, but the mechanisms have not been explored. To study the mechanisms of dysfunctional MSCs induced by uremia, we characterized insulin signaling in MSCs and investigated the effect of uremic toxin, p-cresol, on the proangiogenic actions of insulin. In MSCs, insulin induced hypoxia-inducible factor (HIF)-1α, vascular endothelial growth factor, and stromal cell-derived factor 1α expressions via PI3K/Akt-dependent pathway. MSCs treated with p-cresol exhibited altered insulin signaling in a selective manner for insulin receptor substrate-1/PI3K/Akt pathway, whereas ERK pathway remained active. The insulin-induced increase of HIF-1α was blunted by p-cresol treatment. This Akt-selective insulin resistance was also observed in MSCs isolated from chronic kidney disease (CKD) mice. In mice model of hindlimb ischemia, blood flow recovery, capillary density, and local production of angiogenic factors in the ischemic limb treated with CKD MSCs were significantly inferior to those promoted by control MSCs. However, modifying CKD MSCs by overexpression of HIF-1α restored all of these changes. Taken together, these data suggest that p-cresol contributes to insulin resistance in a selective manner for Akt pathway. This might be a biological explanation for the functional incompetence of MSCs under uremia through defects in the insulin-induced elevation of HIF-1α protein expression.

Comparative vibrational spectroscopic studies, HOMO-LUMO, NBO analyses and thermodynamic functions of p-cresol and 2-methyl-p-cresol based on DFT calculations

In the present study structural properties of p-cresol, and 2-methoxy-p-cresol have been studied by using B3LYP/cc-pvdz and B3PW91/cc-pvdz of Density Functional Theory (DFT) utilizing Becke three exchange functional and Lee Yang Paar correlation functional. The Fourier transform infrared and Fourier transform Raman spectra of title molecules were recorded (solid phase). Optimized geometry, harmonic vibrational frequencies and various thermodynamic parameters of the title compounds were calculated with B3LYP/cc-pvdz, and B3PW91/cc-pvdz basis sets. Non-linear optical (NLO) behavior of the p-cresol and 2-methoxy-p-cresol were investigated by determining of electric dipole moment, polarizability α, and hyperpolarizability β using the above mentioned basis sets. The molecular properties such as ionization potential, electronegativity, chemical potential, electrophilicity have been deduced from HOMO-LUMO analysis employing the same basis sets. A detailed interpretation of the infrared and Raman spectra of title molecules were reported. UV spectrum was measured in different solvent. The energy and oscillator strength are calculated by Time Dependant Density Functional Theory (TD-DFT) results. The calculated HOMO and LUMO energies also confirm that charge transfer occurs within the molecule. The complete assignments were performed on the basis of the potential energy distribution (PED) of vibrational modes, calculated with scaled quantum mechanics (SQM) method. Finally the theoretical FT-IR, FT-Raman, and UV spectra of the title molecules have also been constructed.

Potential of activated carbon from waste rubber tire for the adsorption of phenolics: effect of pre-treatment conditions

Rubber tire activated carbon modification (RTACMC) and rubber tire activated carbon (RTAC) were prepared from waste rubber tire by microwave assisted chemical treatment and physical heating respectively. A greater improvement in porosity and total pore volume was achieved in RTACMC as compared to that of RTAC. But both have a predominantly mesoporous structure. Under identical operating conditions, an irradiation time of 10 min, chemical impregnation ratio of 1.50 and a microwave power of 600 W resulted in maximizing the efficiency of RTACMC for p-cresol (250 mg/g) at a contact time of 90 min while RTAC showed a 71.43 mg/g adsorption capacity at 150 min. Phenol, due to its higher solubility was adsorbed to a lesser extent by both adsorbents. Physical nature of interactions, pore diffusion mechanism and exothermicity of the adsorption process was operative in both adsorbents. The outcomes support the feasibility of preparing high quality activated carbon from waste rubber tire by microwave assisted chemical activation.

Uremic toxins impair human bone marrow-derived mesenchymal stem cells functionality in vitro

Mesenchymal stem cells (MSCs) are becoming therapeutic agents of interest in many areas of medicine, including renal diseases and kidney transplantations. However, the effect of uremia on cell properties is still unclear. Therefore, we examined the in vitro influence of uremic toxins, p-cresol (PC) and indoxyl sulfate (IS), on human bone marrow-derived MSC functionality. Cultured MSCs were treated with PC and IS at concentrations corresponding to subsequent stages of chronic kidney disease. Cell viability was characterized by metabolic activity (MTT assay) and proliferation rate (BrdU assay). Apoptosis (Annexin V test) and cell membrane damage (LDH assay) were also tested. MSC secretory properties were determined by measuring cytokine/growth factor levels in media from toxin-treated cells (ELISA). Uremic concentrations of PC and IS resulted in significant inhibition of MSC metabolic activity and proliferation. Toxins did not induce apoptosis, but damaged cell membranes. MSC paracrine activity was also altered - a decrease of VEGF and TGF-β1 levels and an increase in IGF-1 and IL-8 secretion was detected. Presented data indicate a negative influence of uremic toxins on functional characteristics of human bone marrow-derived MSCs. Therefore, their use as autologous therapeutic agents for kidney disease may be questionable and requires further investigations. The observed phenomenon may be attributable to many other MSC therapies, because of the high prevalence of chronic kidney disease in adult population.

Effects of uremic toxin p-cresol on proliferation, apoptosis, differentiation, and glucose uptake in 3T3-L1 cells

Malnutrition is a common feature seen in chronic dialysis patients, and the survival rate of obese patients receiving such treatment is higher than that of lean patients. Irrespective of obesity or diabetes, dialysis patients commonly have insulin resistance, and the leading cause of death is cardiovascular (CV) disease. It has been reported that the concentration of p-cresol, a uremic toxin, is highly associated with CV events. As uremic toxin levels are high in dialysis patients, they may be involved in the pathogenesis of insulin resistance and CV disease in this population. However, little is known so far. Thus, we focused on this uremic toxin to examine its effects on adipocytes and their precursors. 3T3-L1 cells, a mouse preadipocyte cell line, were cultured until 90% confluency. The cells were then differentiated with 500 μM 3-isobutyl-methylxanthine, 250 nM dexamethasone, and 10 μg/mL insulin. Cell proliferation was evaluated by cell counting and bromodeoxyuridine (Brd-U) incorporation assay. Glucose uptake was estimated using radiolabeled 2-deoxyglucose. The range of concentrations of p-cresol used in the experiments was from 2 to 200 μM. The investigation of cell proliferation by cell counting revealed that, compared with control, 3T3-L1 cells treated with 100 and 200 μM p-cresol were significantly decreased in number at day 3 and day 7 of culture. The Brd-U incorporation assay also demonstrated similar inhibitory effects on cell proliferation, suggesting that p-cresol affected the normal cell cycle. Oil Red O staining at day 7 showed that the number of mature adipocytes was decreased by treatment with 200 μM p-cresol. Consistent with that finding, the number of apoptotic cells at day 7 was increased by treatment with 100 and 200 μM p-cresol. Peroxisome proliferator-activated receptor γ (PPARγ) mRNA expression increased time-dependently during the differentiation process of 3T3-L1 cells. p-Cresol dose-dependently decreased differentiation-induced mRNA expression of PPARγ. Uptake of 3H-labeled 2-deoxyglucose was markedly decreased by 200 μM p-cresol in the presence or in the absence of insulin, mainly because of the decreased number of mature adipocytes. High concentrations of p-cresol disturbed the cell cycle, induced apoptosis, inhibited the differentiation of preadipocytes into mature adipocytes, and decreased glucose uptake at baseline and after insulin stimulation. These findings indicate that accumulated p-cresol may induce reduction in adipose tissue, insulin resistance, and malnutrition, eventually leading to poor outcomes in chronic dialysis patients.

Effects of intestinal bacteria-derived p-cresyl sulfate on Th1-type immune response in vivo and in vitro

Protein fermentation by intestinal bacteria generates various compounds that are not synthesized by their hosts. An example is p-cresol, which is produced from tyrosine. Patients with chronic kidney disease (CKD) accumulate high concentrations of intestinal bacteria-derived p-cresyl sulfate (pCS), which is the major metabolite of p-cresol, in their blood, and this accumulation contributes to certain CKD-associated disorders. Immune dysfunction is a CKD-associated disorder that frequently contributes to infectious diseases among CKD patients. Although some studies imply pCS as an etiological factor, the relation between pCS and immune systems is poorly understood. In the present study, we investigated the immunological effects of pCS derived from intestinal bacteria in mice. For this purpose, we fed mice a tyrosine-rich diet that causes the accumulation of pCS in their blood. The mice were shown to exhibit decreased Th1-driven 2, 4-dinitrofluorobenzene-induced contact hypersensitivity response. The concentration of pCS in blood was negatively correlated with the degree of the contact hypersensitivity response. In contrast, the T cell-dependent antibody response was not influenced by the accumulated pCS. We also examined the in vitro cytokine responses by T cells in the presence of pCS. The production of IFN-γ was suppressed by pCS. Further, pCS decreased the percentage of IFN-γ-producing Th1 cells. Our results suggest that intestinal bacteria-derived pCS suppressesTh1-type cellular immune responses.