A thermodynamic atlas of carbon redox chemical space

Carbon redox chemistry plays a fundamental role in biology. However, the thermodynamic and physicochemical principles underlying the rise of metabolites involved in redox biochemistry remain poorly understood. Our work introduces the theory and techniques that allow us to quantify and understand the global energy landscape of carbon redox biochemistry. We analyze the space of all possible oxidation states of linear-chain molecules with two to five carbon atoms and generate a detailed atlas of the thermodynamic stability of metabolites in comparison to nonbiological molecules. Although the emergence of life required the underlying chemistry to bootstrap itself out of equilibrium, a quantitative understanding of the environment-dependent thermodynamic landscape of prebiotic molecules will be extremely valuable for future origins of life models.

Redox biochemistry plays a key role in the transduction of chemical energy in living systems. However, the compounds observed in metabolic redox reactions are a minuscule fraction of chemical space. It is not clear whether compounds that ended up being selected as metabolites display specific properties that distinguish them from nonbiological compounds. Here, we introduce a systematic approach for comparing the chemical space of all possible redox states of linear-chain carbon molecules to the corresponding metabolites that appear in biology. Using cheminformatics and quantum chemistry, we analyze the physicochemical and thermodynamic properties of the biological and nonbiological compounds. We find that, among all compounds, aldose sugars have the highest possible number of redox connections to other molecules. Metabolites are enriched in carboxylic acid functional groups and depleted of ketones and aldehydes and have higher solubility than nonbiological compounds. Upon constructing the energy landscape for the full chemical space as a function of pH and electron-donor potential, we find that metabolites tend to have lower Gibbs energies than nonbiological molecules. Finally, we generate Pourbaix phase diagrams that serve as a thermodynamic atlas to indicate which compounds are energy minima in redox chemical space across a set of pH values and electron-donor potentials. While escape from thermodynamic equilibrium toward kinetically driven states is a hallmark of life and its origin, we envision that a deeper quantitative understanding of the environment-dependent thermodynamic landscape of putative prebiotic molecules will provide a crucial reference for future origins-of-life models.

Glucose Degradation Products in Peritoneal Dialysis Fluids May Have Both Local and Systemic Effects: A Study of Residual Fluid and Mesothelial Cells

Objective

When peritoneal dialysis (PD) fluids are heat sterilized, glucose is degraded to carbonyl compounds. These compounds are known to interfere with many cellular functions and to promote the formation of advanced glycation end-products. However, little is known about what actually happens with glucose degradation products (GDPs) after infusion into the peritoneal cavity. The aim of the present study was to investigate possible targets for GDPs in the peritoneal cavity.

Design

In vitro reactions between residual fluid and GDPs were studied by incubating unused PD fluid with overnight dialysate. Confluent monolayer cultures of human mesothelial cells were used as a model to study the reactions of GDPs with the cells lining the peritoneal cavity.

Methods

Samples were analyzed, using high pressure liquid chromatography, for the presence of formaldehyde, acetaldehyde, 5-hydroxymethyl-2-furaldehyde (5-HMF), methylglyoxal, and 3-deoxyglucosone (3-DG). Cytotoxicity was determined as inhibition of proliferation of cultured fibroblasts.

Results

None of the analyzed GDPs reacted with overnight dialysate. Formaldehyde and methylglyoxal, in contrast to 3-DG and 5-HMF, reacted with the cultured mesothelial cells.

Conclusions

Low molecular weight carbonyls such as formaldehyde and methylglyoxal most probably react with the mesothelial cells lining the peritoneal cavity, and could be responsible for the disappearance of these cells during long-term treatment. 3-Deoxyglucosone showed remarkably low reactivity and was most probably transported within the patient.

Evidence for Less Irritation to the Peritoneal Membrane in Rats Dialyzed with Solutions Low in Glucose Degradation Products

Background

Acidic pH and the presence of glucose degradation products (GDP) are believed to compromise the biocompatibility of peritoneal dialysis fluids (PDF). The present study examines the effects of long-term exposure to GDP and low pH by comparing conventional PDF and a new, neutral pH, low GDP solution.

Methods

All experiments were performed using a chronic infusion model of dialysis in nonuremic rats. The animals were treated for 6 weeks with 2 daily injections of 4.25% glucose-containing PDF. The following PDF were tested: CAPD3 (single-chamber bag, low pH, high GDP), CAPD3 pH 7.4 (single-chamber bag, neutral pH, high GDP), CAPD3-Balance (double-chamber bag, neutral pH, low GDP). All test solutions were obtained from Fresenius Medical Care, Bad Homburg, Germany.

Results

After 6 weeks of exposure, peritoneal permeability to water, urea, creatinine, glucose, and sodium, assessed by peritoneal equilibration test, was similar in all groups. However, compared to other PDF, dialysis with CAPD3-Balance was associated with reduced concentrations of protein and hyaluronan in the dialysate, decreased peritoneal eosinophilia, and reduced dialysate levels of chemokines CCL2/MCP-1 and CCL5/RANTES. Morphologic changes in the peritoneal membrane of CAPD3-Balance-treated animals were much less pronounced and included reduced vascular density, preservation of the mesothelial monolayer and intercellular junction, and no reduplication of the submesothelial basement membrane.

Conclusions

A new generation of PDF with physiologic pH and low GDP level produce less irritation to the peritoneal membrane and better preserve its structural integrity. This effect seems to be related predominantly to minimized GDP concentrations.

Investigation of hallmarks of carbonyl stress and formation of end products in feline chronic kidney disease as markers of uraemic toxins

OBJECTIVES

Cats are commonly affected by chronic kidney disease (CKD). Many reactive carbonyl intermediates and end products originating from the oxidative stress pathways are recognised as uraemic toxins and may play a role in CKD progression. The aim of the present study is to confirm whether carbonyl end-product formation is higher in cats affected by CKD and to assess whether an angiotensin-converting enzyme inhibitor (ACEi) might affect these hallmarks.

METHODS

Twenty-two cats were divided into three groups: a control group (CG), cats with CKD and cats with CKD treated with an ACEi. Serum levels of pentosidine, carboxymethyllysine, advanced oxidation protein products, malondialdehyde, methylglyoxal and hexanoyl-lysine were measured. In addition, biochemical parameters and systolic blood pressure were evaluated. After checking for normality, comparisons between groups were performed followed by multiple comparison tests. P values ⩽0.05 were considered significant. Correlations between concentrations of the considered biomarkers and of the other metabolic parameters were investigated.

RESULTS

Advanced oxidation protein products, malondialdehyde and hexanoyl-lysine concentrations were significantly higher in CKD and ACEi-treated groups compared with the CG ( P <0.05). Carboxymethyllysine increased in the ACEi-treated group when compared with the CG, whereas intermediate values of these biomarkers were found in the CKD group ( P <0.05). The ACEi-treated group showed the highest values of carboxymethyllysine, advanced oxidation protein products and hexanoyl-lysine. By contrast, the CKD group had the highest concentration of malondialdehyde. No statistically significant difference was found in the levels of pentosidine or methylglyoxal. End products correlated with creatinine and urea and with each other.

CONCLUSIONS AND RELEVANCE

Significantly high concentrations of both intermediate and end products of carbonyl/oxidative stress were detected in CKD cats. This is the first study to have concurrently taken into account several uraemic toxins and biochemical parameters in cats affected by CKD.

Olive leaf extract concentrated in hydroxytyrosol attenuates protein carbonylation and the formation of advanced glycation end products in a hepatic cell line (HepG2)

Glycation takes place both at the cellular level and at the extracellular matrix level and generates, consequently, advanced glycation end-products (AGEs) associated with chronic diseases and the aging process. Two olive leaf extracts concentrated in (i) oleuropein (OLE-A; 93.9 mg oleuropein g^-1) and (ii) hydroxytyrosol (OLE-B; 54.5 mg hydroxytyrosol g^-1) were evaluated according to their antiglycative and antioxidant capacity in vitro. OLE-B exerted the highest anti-AGE effect in different glycation models (IC₅₀: 0.25-0.29 mg mL^-1). OLE-B showed the highest antioxidant capacity and methylglyoxal-trapping capacity (IC₅₀ 0.16 mg mL^-1). OLE-B showed a significant inhibitory effect against protein carbonylation (21%) and generation of argpyrimidine (26%) in a hepatocyte cellular carbonyl stress model evoked by methylglyoxal (MGO). OLE-B was further fractionated by solid phase-extraction, and the protective effect against protein carbonylation was only exerted by the fraction containing hydroxytyrosol. However, hydroxytyrosol standard, at the same concentration in the extract, inhibited the protein carbonylation below 10% but not significantly. The results indicate that the antiglycative activity of OLE in cells could be due to a synergic effect of hydroxytyrosol and other minor compounds with similar polarity. The research of the antiglycative activity in vivo could confirm these promising results and to propose OLE as a natural anti-AGE agent.

Effect of Scavenging Circulating Reactive Carbonyls by Oral Pyridoxamine in Uremic Rats on Peritoneal Dialysis

Pyridoxamine, a reactive carbonyl (RCO) scavenger, can ameliorate peritoneal deterioration in uremic peritoneal dialysis (PD) rats when given via dialysate. We examined the effects of scavenging circulating RCOs by oral pyridoxamine. Rats underwent nephrectomy and 3 weeks of twice daily PD either alone or with once daily oral pyridoxamine. PD solution was supplemented with methylglyoxal, a major glucose-derived RCO, to quench intraperitoneal pyridoxamine. Oral pyridoxamine achieved comparable blood and dialysate pyridoxamine concentrations, suppressed pentosidine accumulation in the blood but not in the mesenterium or dialysate, and reduced the increases in small solute transport and mesenteric vessel densities, with no effects on submesothelial matrix layer thickening or serum creatinine. Thus, reducing circulating RCOs by giving oral pyridoxamine with PD provides limited peritoneal protection. However, orally given pyridoxamine efficiently reaches the peritoneal cavity and would eliminate intraperitoneal RCOs. Oral pyridoxamine is more clinically favorable and may be as protective as intraperitoneal administration.

Amine oxidases as important agents of pathological processes of rhabdomyolysis in rats

In this study we have tested an idea on the important role of amine oxidases (semicarbazide-sensitive amine oxidase, diamine oxidase, polyamine oxidase) as an additional source of oxidative/carbonyl stress under glycerol-induced rhabdomyolysis, since the enhanced formation of reactive oxygen species and reactive carbonyl species in a variety of tissues is linked to various diseases. In our experiments we used the sensitive fluorescent method devised for estimation of amine oxidases activity in the rat kidney and thymus as targeted organs under rhabdomyolysis. We have found in vivo the multiple rises in activity of semicarbazide-sensitive amine oxidase, diamine oxidase, polyamine oxidase (2-4.5 times) in the corresponding cell fractions, whole cells or their lysates at the 3-6th day after glycerol injection. Aberrant antioxidant activities depended on rhabdomyolysis stage and had organ specificity. Additional treatment of animals with metal chelator ‘Unithiol’ adjusted only the activity of antioxidant enzymes but not amine oxidases in both organs. Furthermore the in vitro experiment showed that Fenton reaction (hydrogen peroxide in the presence of iron) products alone had no effect on semicarbazide-sensitive amine oxidase activity in rat liver cell fraction whereas supplementation with methylglyoxal resulted in its significant 2.5-fold enhancement. Combined action of the both agents had additive effect on semicarbazide-sensitive amine oxidase activity. We can assume that biogenic amine and polyamine catabolism by amine oxidases is upregulated by oxidative and carbonyl stress factors directly under rhabdomyolysis progression, and the increase in catabolic products concentration contributes to tissue damage in glycerol-induced acute renal failure and apoptosis stimulation in thymus.

Renal accumulation and clearance of advanced glycation end-products in type 2 diabetic nephropathy: effect of angiotensin-converting enzyme and vasopeptidase inhibition

AIMS/HYPOTHESIS

Renal accumulation of AGEs may contribute to the progression of diabetic nephropathy. We evaluated the effect of ramipril (a pure ACE inhibitor) and AVE7688 (a dual inhibitor of ACE and neutral endopeptidase) on renal accumulation of the advanced glycation end-product (AGE) 3-deoxyglucosone-imidazolone, carboxymethyllysine (CML) and pentosidine, and on clearance of CML in type 2 diabetes.

METHODS

Male Zucker diabetic fatty rats (ZDF, Gmi-fa/fa) rats were treated from age 10 to 37 weeks with ramipril (1 mg.kg(-1).day(-1)), AVE7688 (45 mg.kg(-1).day(-1)) or without drug. Ramipril and AVE7688 reduced albuminuria by 30 and 90%, respectively.

RESULTS

ZDF rats showed increased renal accumulation of the AGE subtypes 3-deoxyglucosone-imidazolone, pentosidine and CML by about 40, 55 and 55%, respectively compared with heterozygous, non-diabetic control animals at the age of 37 weeks. AVE7688 but not ramipril attenuated the renal accumulation of 3-deoxyglucosone-imidazolone, pentosidine and CML and improved CML clearance in ZDF rats. During glycation reactions in vitro, AVE7688 also demonstrated potent chelating activity and inhibited metal-catalysed formation of pentosidine and CML.

CONCLUSIONS/INTERPRETATION

Improved AGE clearance and direct inhibition of AGE formation by chelation may contribute to reduced accumulation of renal AGEs and to the nephroprotective effects of vasopeptidase inhibition in type 2 diabetes.

Possible link between glycated hemoglobin and lipid peroxidation in hyperthyroidism

BACKGROUND

Glycated hemoglobin (HbA1C) levels are enhanced by elevated glucose concentrations. Glycation of hemoglobin is also modulated by lipid peroxides, ascorbic acid and reduced glutathione (GSH). We determined the strength of the relationships among these variables in a group of hyperthyroid patients.

METHODS

Twenty-two untreated hyperthyroid patients and 17 healthy controls were recruited for the study. Whole blood GSH, HbA1C, plasma lipid peroxides, ascorbic acid and fasting glucose were analyzed in both the groups. Direct and partial correlation analysis was performed to explore the possible relationships between these variables.

RESULTS

In hyperthyroid patients, HbA1C and lipid peroxides levels were found to be significantly increased than the controls. Ascorbic acid and GSH were decreased significantly in the test group when compared with the healthy control group. With partial correlation analysis, fasting glucose and lipid peroxides were found to have a significant positive correlation with HbA1C. Ascorbic acid and GSH showed no significant association with HbA1C levels.

CONCLUSION

These data suggest that HbA1C levels are closely associated with fasting glucose and lipid peroxides in hyperthyroid patients. Therefore, serum lipid peroxides level should be kept in mind while interpreting HbA1C as a long-term glycemic index in hyperthyroid cases.

In vitro biocompatibility performance of Physioneal

In vitro biocompatibility performance of Physioneal. toneal dialysis (PD) has been a successful and effective form of chronic renal replacement therapy since its introduction over 20 years ago. Despite its overall success, there is a growing body of evidence that suggests shortcomings in the preservation of membrane integrity. This has led to the development of several second-generation PD solutions that demonstrate improved biocompatibility. Physioneal, a neutral pH, bicarbonate/lactate-buffered solution, was one of the first of these new PD solutions to become commercially available. This review will focus on one of the first preclinical stages in the development of Physioneal: studies on in vitro biocompatibility testing. Studies in leukocyte, mesothelial cell, and fibroblast populations demonstrated significantly improved biocompatibility of neutral pH, bicarbonate/lactate-based solutions compared to conventional solutions. The solutions contributed to improved leukocyte viability and response to bacterial infection (e.g., phagocytosis, superoxide radical generation, and endotoxin-stimulated cytokine release). Studies on peritoneal mesothelial cells demonstrate improved cell viability, proliferation, and response to proinflammatory stimuli, and a reduced potential for angiogenesis and peritoneal fibrosis, all suggesting a better preservation of membrane structure and function. The bicarbonate/lactate-based solutions demonstrated decreased cytotoxicity and preserved cell growth in fibroblast cultures as well. In vitro biocompatibility testing has clearly demonstrated that neutral pH, bicarbonate/lactate-buffered Physioneal solutions are superior to conventional solutions in preserving cell viability and function in cell populations that contribute to peritoneal homeostasis. This positive assessment now provides a foundation and rationale for moving forward with the next stages in preclinical testing: in vivo animal models and human ex vivo studies.

Acute oxidative stress induces peritoneal hyperpermeability, mesothelial loss, and fibrosis

We explored the acute and long-term effects of short-lived, intense oxidative stress on peritoneal permeability and structure, induced with intraperitoneal injection of the oxidant agent deoxycholate, in rats. Ten minutes after the experimental intervention, peritoneal dialysis, performed over an exposure time of 60 minutes, revealed an increased urea dialysate/plasma ratio, greater glucose absorption, increased albumin losses in the effluent dialysate, and a reduced ultrafiltration rate. Mesothelial-cell imprints taken from the anterior liver surface indicated a substantially decreased density in the cell population. After the recovery period of 30 days, all alterations were still evident. Additionally, macroscopic and histologic observations made at this time interval detected peritoneal fibrosis and sclerosis, characterized by peritoneal adhesions, wrapping of intestinal loops, and the presence of a layer of fibrous tissue dressing the cavitary aspect of the liver peritoneal envelope. This report describes a reproducible experimental model of peritoneal fibrosis induced by acute oxidative injury. On the basis of these findings, it may be speculated that functional and structural alterations observed in patients are related to long-term continuous exposure of the monolayer to oxidative injury resulting from the high concentrations of d-glucose present in peritoneal dialysis solutions.

An evaluation of level of oxidative stress and protein glycation in nondiabetic undialyzed chronic renal failure patients

BACKGROUND

Chronic renal failure (CRF) patients on prolonged dialysis and with diabetes have been found to have significant alteration in their antioxidant status and protein glycation, but little is known about the same in nondiabetic undialyzed CRF patients. This study was performed to evaluate (a) the levels of oxidative stress and protein glycation in nondiabetic undialyzed CRF patients and (b) the possible influence of oxidative stress on protein glycation.

DESIGN AND METHODS

A case control study was performed on 23 nondiabetic undialyzed CRF patients and 17 control subjects. The concentrations of total plasma ascorbic acid, whole blood reduced glutathione (GSH) and plasma lipid peroxides were measured to evaluate the antioxidant status and oxidative stress. The protein glycation was measured from plasma fructosamine normalized for albumin concentrations. A partial correlation analysis was carried out to analyze the effect of oxidative stress parameters on protein glycation.

RESULTS

A compromised nonenzymatic defense against free-radical generation was evidenced by decreased concentrations of whole blood GSH and plasma ascorbic acid. The concentrations of lipid peroxides, fructosamine and fructosamine/albumin ratio were significantly (p<0.05) higher in the CRF group when compared with controls. The partial correlation analysis revealed that oxidative stress influences protein glycation in CRF patients.

CONCLUSIONS

Increased oxidative stress might have a role in promoting protein glycation in nondiabetic undialyzed CRF patients. The reduction of oxidative stress and protein glycation might open new therapeutic approaches in treating CRF patients.

3,4-Dideoxyglucosone-3-ene (3,4-DGE): a cytotoxic glucose degradation product in fluids for peritoneal dialysis

BACKGROUND

Bioincompatible glucose degradation products (GDPs) in fluids for peritoneal dialysis (PD) develop during sterilization and storage. Their biological activity has successfully been monitored through the use of various in vitro methods but their molecular and chemical nature is less well understood. Many GDPs are highly reactive carbonyl compounds. Although some of the identified GDPs are extremely cytotoxic, none of them actually possess cytotoxicity at the concentrations found in PD fluids. Thus, the GDP responsible for the toxicity in PD fluids has not yet been identified. The intention of the present work was to investigate to what extent the unsaturated dicarbonyl compound, 3,4-dideoxyglucosone-3-ene (3,4-DGE) was present in PD fluids, and if it could be responsible for the in vitro effects on L-929 fibroblast cells.

METHODS

A commercial preparation of 3,4-DGE and two different liquid chromatography methods were used for the chemical identification and quantification. In vitro bioincompatibility was determined as inhibition of cell growth using the L-929 fibroblast cell line.

RESULTS

3,4-DGE was present in conventionally manufactured PD fluids at a concentration of 9 to 22 micromol/L. In the newly developed PD fluid, Gambrosol trio, the concentrations were 0.3 to 0.7 micromol/L. When added as synthetic 3,4-DGE to cell growth media at the concentrations measured in conventional PD fluids, the inhibition of cell growth was significantly lower than for that seen with the conventional fluids. However, in the conventional PD fluids the total amount of 3,4-DGE available for toxic reactions most probably was higher than that measured, because 3,4-DGE was freshly recruited from a molecular pool when consumed. The speed of this recruitment was high enough to explain most of the growth inhibition seen for heat-sterilized PD fluids.

CONCLUSION

3,4-DGE is present in conventional PD fluids at a concentration between 9 and 22 micromol/L, and is the most biologically active of all GDPs identified to date. Thus, it is the main candidate to be held responsible for the clinical bioincompatibility caused by conventionally manufactured PD fluids.

Uremic toxins and peritoneal dialysis

Uremic toxicity is related in part to the accumulation of toxic substances, the nature of which has only partly been characterized. Because of the use of a highly permeable membrane and better preservation of the residual renal function, it could be anticipated that some of these uremic toxins are more efficiently cleared across the peritoneal membrane, and that the plasma and tissue levels of these compounds are lower than in hemodialysis patients. This article analyzes the generation and removal of several uremic toxins in peritoneal dialysis patients. The following uremic toxins are discussed: beta2-microglobulin, advanced glycation end products, advanced oxidation protein products, granulocyte inhibitory proteins, p-Cresol, and hyperhomocysteinemia. Some recent studies are reviewed suggesting that uremic toxins are involved in the progression of renal failure and are at least partially removed by peritoneal dialysis. We conclude that, although the plasma levels of some of these compounds are lower in peritoneal dialysis versus hemodialysis patients, it does not mean that the peritoneal dialysis patient is "better" protected against the numerous disturbances caused by these toxins.