The effect of delta-gluconolactone, an oxidised analogue of glucose, on the nonenzymatic glycation of human and rat haemoglobin

Paraoxonase 1: evolution of the enzyme and of its role in protecting against atherosclerosis

PURPOSE OF REVIEW

To review the discoveries which led to the concept that serum paraoxonase 1 (PON1) is inversely related to atherosclerotic cardiovascular disease (ASCVD) incidence, how this association came to be regarded as causal and how such a role might have evolved.

RECENT FINDINGS

Animal models suggest a causal link between PON1 present on HDL and atherosclerosis. Serum PON1 activity predicts ASCVD with a similar reliability to HDL cholesterol, but at the extremes of high and low HDL cholesterol, there is discordance with PON1 being potentially more accurate. The paraoxonase gene family has its origins in the earliest life forms. Its greatest hydrolytic activity is towards lactones and organophosphates, both of which can be generated in the natural environment. It is active towards a wide range of substrates and thus its conservation may have resulted from improved survival of species facing a variety of evolutionary challenges.

SUMMARY

Protection against ASCVD is likely to be the consequence of some promiscuous activity of PON1, but nonetheless has the potential for exploitation to improve risk prediction and prevention of ASCVD.

Paraoxonase 1 and atherosclerosis

Paraoxonase 1 (PON1), residing almost exclusively on HDL, was discovered because of its hydrolytic activity towards organophosphates. Subsequently, it was also found to hydrolyse a wide range of substrates, including lactones and lipid hydroperoxides. PON1 is critical for the capacity of HDL to protect LDL and outer cell membranes against harmful oxidative modification, but this activity depends on its location within the hydrophobic lipid domains of HDL. It does not prevent conjugated diene formation, but directs lipid peroxidation products derived from these to become harmless carboxylic acids rather than aldehydes which might adduct to apolipoprotein B. Serum PON1 is inversely related to the incidence of new atherosclerotic cardiovascular disease (ASCVD) events, particularly in diabetes and established ASCVD. Its serum activity is frequently discordant with that of HDL cholesterol. PON1 activity is diminished in dyslipidaemia, diabetes, and inflammatory disease. Polymorphisms, most notably Q192R, can affect activity towards some substrates, but not towards phenyl acetate. Gene ablation or over-expression of human PON1 in rodent models is associated with increased and decreased atherosclerosis susceptibility respectively. PON1 antioxidant activity is enhanced by apolipoprotein AI and lecithin:cholesterol acyl transferase and diminished by apolipoprotein AII, serum amyloid A, and myeloperoxidase. PON1 loses this activity when separated from its lipid environment. Information about its structure has been obtained from water soluble mutants created by directed evolution. Such recombinant PON1 may, however, lose the capacity to hydrolyse non-polar substrates. Whilst nutrition and pre-existing lipid modifying drugs can influence PON1 activity there is a cogent need for more specific PON1-raising medication to be developed.

Global and targeted metabolomics of synovial fluid discovers special osteoarthritis metabolites

To identify special metabolites in synovial fluid of osteoarthritis (OA) via a metabolomics approach. Synovial fluid of 35 participants (25 OA patients and 10 controls) was detected by GC-TOF/MS and multivariate data analysis was applied to analyze correlation among the observations. Different metabolites were screened by VIP value (VIP > 1), student t-test (p < 0.05), and fold change (fold >1.5), and verified with the standard metabolites in the synovial fluid of 24 OA patients and 11 controls by LC/MS. The classification performance of different metabolites was analyzed by receiver operating characteristic (ROC) analysis. The results showed that six different metabolites (glutamine, 1,5-anhydroglucitol, gluconic lactone, tyramine, threonine, and 8-aminocaprylic acid) were strongly associated with OA in global metabolomics. Verified results of the first three metabolites were the same as the identified results using targeted metabolomics. ROC curve analysis demonstrated that their concentrations in synovial fluid were strongly correlated to OA. In addition, the concentrations of gluconic lactone were significantly different between OA and RA. Metabolites with altered levels may be contributors to OA pathogenesis and can be used as potential diagnosis criteria for OA. Gluconic lactone may prove to be a novel criterion for differential diagnosis of OA from RA. © 2017 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 35:1973-1981, 2017.

How HDL protects LDL against atherogenic modification: paraoxonase 1 and other dramatis personae

PURPOSE OF REVIEW

To summarize the current evidence about how HDL impedes the oxidative and glycative atherogenic modification of LDL.

RECENT FINDINGS

Paraoxonase 1 (PON1) is located on HDL. Meta-analysis of clinical epidemiological investigations reveals a substantial association of low serum PON1 activity with coronary heart disease incidence independent of other risk factors including HDL cholesterol and apolipoprotein AI (apoAI). Transgenic animal models also indicate an antiatherosclerotic role for PON1. However, highly purified and recombinant PON1 do not retain their antioxidant properties.

SUMMARY

The therapeutic potential of PON1 should be recognized in preventing atherosclerosis and combating infection and organophosphate toxicity. In unleashing this potential, it is important to consider that both highly purified and recombinant PON1 are dissociated from the lipid phase and other components of HDL, such as apoAI and apoM, all of which may be required for HDL (through its PON1 component) to hydrolyze more lipophilic substrates.

Systemic alterations in the metabolome of diabetic NOD mice delineate increased oxidative stress accompanied by reduced inflammation and hypertriglyceremia

Nonobese diabetic (NOD) mice are a commonly used model of type 1 diabetes (T1D). However, not all animals will develop overt diabetes despite undergoing similar autoimmune insult. In this study, a comprehensive metabolomic approach, consisting of gas chromatography time-of-flight (GC-TOF) mass spectrometry (MS), ultra-high-performance liquid chromatography-accurate mass quadruple time-of-flight (UHPLC-qTOF) MS and targeted UHPLC-tandem mass spectrometry-based methodologies, was used to capture metabolic alterations in the metabolome and lipidome of plasma from NOD mice progressing or not progressing to T1D. Using this multi-platform approach, we identified >1,000 circulating lipids and metabolites in male and female progressor and nonprogressor animals (n = 71). Statistical and multivariate analyses were used to identify age- and sex-independent metabolic markers, which best differentiated metabolic profiles of progressors and nonprogressors. Key T1D-associated perturbations were related with 1) increases in oxidation products glucono-δ-lactone and galactonic acid and reductions in cysteine, methionine and threonic acid, suggesting increased oxidative stress; 2) reductions in circulating polyunsaturated fatty acids and lipid signaling mediators, most notably arachidonic acid (AA) and AA-derived eicosanoids, implying impaired states of systemic inflammation; 3) elevations in circulating triacylglyercides reflective of hypertriglyceridemia; and 4) reductions in major structural lipids, most notably lysophosphatidylcholines and phosphatidylcholines. Taken together, our results highlight the systemic perturbations that accompany a loss of glycemic control and development of overt T1D.

New sputum metabolite markers implicating adaptations of the host to Mycobacterium tuberculosis, and vice versa

In this study, a metabolomics research approach was used to identify new tuberculosis (TB) markers from sputum, in an attempt to better characterise the disease as well as the metabolic response of the host to Mycobacterium tuberculosis infection. After GCxGC-TOFMS analyses, various multivariate and univariate statistical methods were implemented to identify those compounds best describing the variation between the TB-positive and TB-negative patient groups. The interpretation of these new metabolite markers led to a number of new hypotheses, including: 1) support of the previously proposed citramalate cycle in M. tuberculosis; 2) the interaction of this cycle with an up-regulated glyoxylate cycle during pulmonary M. tuberculosis infection; 3) the increased utilisation of fatty acids and glutamate as alternative carbon sources by M. tuberculosis during pulmonary infection; 4) an alternative mechanism by which the host produces hydrogen peroxide via glucose oxidation, in order to eliminate the bacterial infection; 5) inhibition of the ETC due to pronounced oxidative stress during an active TB disease state, resulting in increased concentrations of various neurotransmitters and other metabolites previously associated with an inborn error of metabolism (MADD/GA type II); and 6) elevated concentrations of neurotransmitters associated with a number of previously described symptoms of TB.

High-density lipoprotein impedes glycation of low-density lipoprotein

Glycation of low-density lipoprotein (LDL) increases its atherogenicity, but whether high-density lipoprotein (HDL) can protect LDL against glycation is not known. LDL and HDL were isolated from 32 volunteers with serum HDL cholesterol concentrations ranging from 0.76 to 2.01 (mean = 1.36) mmol/L. Glycation of LDL was induced by incubation with 0-80 mmol/L glucose for 7 days at 37°C under nitrogen in the presence of and absence of human HDL. Glycation of LDL apolipoprotein B (apoB) doubled at glucose 50 and 80 mmol/L (both p < 0.001), and this increase was ameliorated by HDL. In the absence of glucose, 0.11 (0.01) [mean (standard error, SE)] mg apoB/mg LDL protein was glycated increasing to 0.22 (0.02) mg/mg at glucose 80 mmol/L in the absence of HDL, but remaining at 0.13 (0.01) mg/mg when autologous HDL was present. Heterologous HDL from a further study of 12 healthy participants was similarly effective in impeding LDL apoB glycation. HDL impeded not only glycation but also the lipid peroxidation, free amino group consumption and increased electrophoretic mobility of LDL which accompanied glycation. HDL from participants with higher serum paraoxonase1 (PON1) was more effective in impeding glycation and the related processes. In conclusion, HDL can impede the glucose-induced glycoxidation of LDL. PON1 may be important for this function of HDL.

Cranberry phytochemicals inhibit glycation of human hemoglobin and serum albumin by scavenging reactive carbonyls

Protein glycation caused by sugars and reactive carbonyls is a contributing factor to diabetic complications, aging, and other chronic diseases. The objective of this study was to investigate the inhibitory effects of cranberry phytochemicals on protein glycation. Cranberries, purified to yield sugar-free phytochemical powder, were fractionated into ethyl acetate and water fractions. Water fraction was further separated into water fraction I, II, and III on a Sephadex LH-20 column. Cranberry phytochemical powder and its fractions significantly inhibited the formation of glycated hemoglobin. The concentrations of cranberry phytochemicals required to inhibit 50% of albumin glycation (EC(50)) in albumin-glucose assay were lower than that of aminoguanidine except for water fraction I. Cranberry phytochemicals inhibited glycation of human serum albumin mediated by methylglyoxal, but the EC(50) were higher than that of aminoguanidine. Carbonyl scavenging assay showed that water fraction II scavenged 89.3% of methylglyoxal at 6 h of reaction. Fractions enriched with procyanidins showed higher antiglycation activities, suggesting procyanidins were the major active components. The hypothesis whether cranberry procyanidins scavenged reactive carbonyls by forming adducts was tested. Epicatechin was used as a model compound to react with methylglyoxal and glyoxal at pH 7.4. Five adducts were detected and their structures were tentatively identified using HPLC-ESI-MS/MS.

Suppressing posttranslational gluconoylation of heterologous proteins by metabolic engineering of Escherichia coli

Minimization of chemical modifications during the production of proteins for pharmaceutical and medical applications is of fundamental and practical importance. The gluconoylation of heterologously expressed protein which is observed in Escherichia coli BL21(DE3) constitutes one such undesired posttranslational modification. We postulated that formation of gluconoylated/phosphogluconoylated products of heterologous proteins is caused by the accumulation of 6-phosphogluconolactone due to the absence of phosphogluconolactonase (PGL) in the pentose phosphate pathway. The results obtained demonstrate that overexpression of a heterologous PGL in BL21(DE3) suppresses the formation of the gluconoylated adducts in the therapeutic proteins studied. When this E. coli strain was grown in high-cell-density fed-batch cultures with an extra copy of the pgl gene, we found that the biomass yield and specific productivity of a heterologous 18-kDa protein increased simultaneously by 50 and 60%, respectively. The higher level of PGL expression allowed E. coli strain BL21(DE3) to satisfy the extra demand for precursors, as well as the energy requirements, in order to replicate plasmid DNA and express heterologous genes, as metabolic flux analysis showed by the higher precursor and NADPH fluxes through the oxidative branch of the pentose phosphate shunt. This work shows that E. coli strain BL21(DE3) can be used as a host to produce three different proteins, a heterodimer of liver X receptors, elongin C, and an 18-kDa protein. This is the first report describing a novel and general strategy for suppressing this nonenzymatic modification by metabolic pathway engineering.

Senescence marker protein 30 functions as gluconolactonase in L-ascorbic acid biosynthesis, and its knockout mice are prone to scurvy

We originally identified senescence marker protein 30 (SMP30) as a distinctive protein whose expression decreases in an androgen-independent manner with aging. Here, we report its sequence homology found in two kinds of bacterial gluconolactonases (GNLs) by using the blast search. Then, through a biochemical study, we identify SMP30 as the lactone-hydrolyzing enzyme GNL of animal species. SMP30 purified from the rat liver had lactonase activity toward various aldonolactones, such as d- and l-glucono-delta-lactone, d- and l-gulono-gamma-lactone, and d- and l-galactono-gamma-lactone, with a requirement for Zn(2+) or Mn(2+) as a cofactor. Furthermore, in SMP30 knockout mice, no GNL activity was detectable in the liver. Thus, we conclude that SMP30 is a unique GNL in the liver. The lactonase reaction with l-gulono-gamma-lactone is the penultimate step in l-ascorbic acid (AA) biosynthesis, and the essential role of SMP30 in this synthetic process was verified here by a nutritional study using SMP30 knockout mice. These knockout mice (n = 6), fed a vitamin C-deficient diet, did not thrive; i.e., they displayed symptoms of scurvy such as bone fracture and rachitic rosary and then died by 135 days after the start of receiving the deficient diet. The AA levels in their livers and kidneys at the time of death were <1.6% of those in WT control mice. In addition, by using the SMP30 knockout mouse, we demonstrate that the alternative pathway of AA synthesis involving d-glucurono-gamma-lactone operates in vivo, although its flux is fairly small.

Increased protein glycation in non-diabetic pediatric nephrotic syndrome: possible role of lipid peroxidation

BACKGROUND

Malondialdehyde (MDA), ascorbic acid and reduced glutathione (GSH) have been reported to play a possible role in glycation of proteins. This study was performed to evaluate this correlation in nephrotic syndrome patients by comparing the levels of fructosamine with MDA, ascorbic acid and GSH.

METHODS

Fifteen children with nephrotic syndrome during relapse and 10 age- and sex-matched healthy controls were enrolled for this study. Whole blood GSH, plasma MDA, total ascorbic acid and fasting glucose were analyzed in both the groups. Partial correlation analysis was performed to predict the independent association of MDA, ascorbic acid and GSH on fructosamine.

RESULTS

Plasma MDA and fructosamine levels were found to be increased in nephrotic syndrome patients when compared with controls. Plasma ascorbic acid and whole blood GSH were decreased in nephrotic group vs. healthy controls. Partial correlation analysis showed a significant positive correlation between fructosamine and MDA.

CONCLUSIONS

Present data point to a possible involvement of MDA in the glycation of protein in non-diabetic nephrotic syndrome patients, and provide support for the potential use of an antioxidant therapy in these patients.

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.

Radicals and oxidative stress in diabetes

Recent evidence is reviewed indicating increased oxidative damage in Type 1 and Type 2 diabetes mellitus as well as deficits in antioxidant defence enzymes and vitamins. Mechanisms are considered whereby hyperglycaemia can increase oxidative stress, and change the redox potential of glutathione and whereby reactive oxygen species can cause hyperglycaemia. It is argued that oxygen, antioxidant defences, and cellular redox status should now be regarded as central players in diabetes and the metabolic syndrome.

A new rapid method to detect inhibition of Amadori product generated by delta-gluconolactone

Advanced glycation endproducts (AGEs) have been implicated as a major pathogenic process in leading to diabetic complications. An increasing number of drug candidates have recently been developed as potential inhibitors of AGEs. Aminoguanidine, a hydrazine-like molecule is the first drug that was extensively studied both in vitro and in vivo as an inhibitor of AGE formation. Several assay methods have been proposed to determine the inhibitory effect of glycation inhibitors, including assays based on inhibition of specific fluorescence generated in the course of glycation and AGE-formation; assays based on the inhibition of AGE-protein crosslinks, and dimer and trimer formation; and specific ELISA assays using anti-AGE antibodies for quantitative measurement of AGEs in the presence and absence of the inhibitor. However, none of these assays can accurately evaluate chemical intermediates and products generated during the early stages of glycation. We have devised a new rapid assay method for evaluation of the early stage of glycation (Amadori product). This assay is based on the interaction of delta-gluconolactone (delta-Glu), an oxidized (ketoaldehyde) analogue of glucose, with hemoglobin present in blood samples. The assay involves determination of the percentage of glycated hemoglobin (HbA1C) after incubation at 37 degrees for 16 h with delta-Glu (50 mmol/L) using a dedicated ion-exchange high-performance liquid chromatography (HPLC) system. The results using normal human red blood cells show HbA1C levels to be 180% higher than baseline controls. The effects of various inhibitors are determined by measuring the levels of HbA1C by the compound in question compared to delta-Glu-treated vehicle only blood samples. This new assay provides a relevant and physiological model to study glycation and potential inhibitors. Furthermore, it offers a means to differentiate between inhibitors of the early and late stages of glycation and provides a rapid method of screening large numbers of potential inhibitors of glycation. Contrary to the assay methods, which are based on the measurement of fluorescence of fluorophores generated during glycation, the proposed assay does not suffer from the possible problem of overlapping and interference of AGE-specific fluorescence with the intrinsic fluorescence of the inhibitor compound.

Reactivity of 6-phosphogluconolactone with hydroxylamine: the possible involvement of glucose-6-phosphate dehydrogenase in endogenous glycation reactions

The reactivity of 6-phosphogluconolactone and of delta-gluconolactone with hydroxylamine (a model compound in electrophilicity determination studies) was examined and compared with the reactivity of several other electrophiles, such as acid anhydrides and esters, some of which exhibit adverse biological effects (e.g. carcinogenicity). At pH 7.6 and 30 degrees C, and with an excess of hydroxylamine concentration, most of the compounds tested disappear from the medium in a monoexponential reaction. On the other hand, the reaction of 6-phosphogluconolactone with hydroxylamine is biexponential. This finding indicates the existence of 6-phosphogluconolactone in two interconvertible, isomeric forms. The reactivity, towards hydroxylamine, of 6-phosphogluconolactone and, to a lesser extent of delta-gluconolactone, is on the upper scale of reactivity of the electrophiles tested. It is concluded that 6-phosphogluconolactone (and in particular, one of its isomeric forms) is a highly electrophilic compound, and may possibly react with sundry intracellular nucleophiles, thereby exerting untoward metabolic effects. In this connection, it is of interest that a positive correlation has been found to exist between glucose-6-phosphate dehydrogenase activity and cell proliferation.

UVA irradiation of human lens proteins produces residual oxidation of ascorbic acid even in the presence of high levels of glutathione

The oxidation products of ascorbic acid (AscH-) can rapidly glycate and crosslink lens proteins in vitro, producing fluorophores and browning products similar to those present in cataractous lenses. The accumulation of AscH- oxidation products, however, would largely be prevented by the millimolar levels of glutathione (GSH) present in human lens. Here we investigate whether protein aggregation could allow the oxidation of AscH- by UVA-induced reactive oxygen species in the presence of physiological levels of GSH. The metal-catalyzed oxidation of 1.0 mM AscH- by 50 microM Cu(II) was almost complete after 1 h, but no oxidation was seen in the presence of GSH concentrations as low as 0.5 mM. UVA irradiation of protein aggregates from human lens, which accumulated more than 2.0 mM singlet oxygen after 1 h, caused a 50-60% oxidation of 1.0 mM AscH-. The addition of 204 mM GSH, however, decreased AscH- oxidation by less than half, and 30% of the AscH- was oxidized even in the presence of 15 mM GSH. This diminished protection may be due, in part, to the ability of AscH-, but not GSH, to penetrate to the sites of singlet oxygen generation located within the protein. Consistent with this hypothesis, greater GSH protection was seen when a proteolytic digest of the human proteins was subjected to the same irradiation or when singlet oxygen was chemically generated from 3-(4-methyl-1-naphthyl)propionic acid endoperoxide (MNPAE) at 37 degrees C in the medium. The addition of 50 microM Cu(II) had no effect on the rate of degradation of dehydroascorbic acid (DHA). Singlet oxygen, either UVA- or MNPAE-generated, increased the rate of DHA loss. This secondary oxidation of DHA by singlet oxygen would allow the accumulation of AscH- oxidation products was not reducible by GSH. Therefore, the data presented here argue that the protein aggregation seen in older human lenses may permit oxidized AscH--induced crosslinking to occur even at physiological GSH levels.