Arginine-directed glycation and decreased HDL plasma concentration and functionality

BACKGROUND/OBJECTIVES

Decreased plasma concentration of high-density lipoprotein cholesterol (HDL-C) is a risk factor linked to increased risk of cardiovascular disease (CVD). Decreased anti-atherogenic properties of HDL are also implicated in increased CVD risk. The cause is unknown but has been linked to impaired glucose tolerance. The aim of this study was to quantify the modification of HDL by methylglyoxal and related dicarbonyls in healthy people and patients with type 2 diabetes characterise structural, functional and physiological consequences of the modification and predict the importance in high CVD risk groups.

SUBJECTS/METHODS

Major fractions of HDL, HDL2 and HDL3 were isolated from healthy human subjects and patients with type 2 diabetes and fractions modified by methylglyoxal and related dicarbonyl metabolites quantified. HDL2 and HDL3 were glycated by methylglyoxal to minimum extent in vitro and molecular, functional and physiological characteristics were determined. A one-compartment model of HDL plasma clearance was produced including formation and clearance of dicarbonyl-modified HDL.

RESULTS

HDL modified by methylglyoxal and related dicarbonyl metabolites accounted for 2.6% HDL and increased to 4.5% in patients with type 2 diabetes mellitus (T2DM). HDL2 and HDL3 were modified by methylglyoxal to similar extents in vitro. Methylglyoxal modification induced re-structuring of the HDL particles, decreasing stability and plasma half-life in vivo. It occurred at sites of apolipoprotein A-1 in HDL linked to membrane fusion, intramolecular bonding and ligand binding. Kinetic modelling of methylglyoxal modification of HDL predicted a negative correlation of plasma HDL-C with methylglyoxal-modified HDL. This was validated clinically. It also predicted that dicarbonyl modification produces 2-6% decrease in total plasma HDL and 5-13% decrease in functional HDL clinically.

CONCLUSIONS

These results suggest that methylglyoxal modification of HDL accelerates its degradation and impairs its functionality in vivo, likely contributing to increased risk of CVD-particularly in high CVD risk groups.

Emerging role of thiamine therapy for prevention and treatment of early-stage diabetic nephropathy

Thiamine supplementation may prevent and reverse early-stage diabetic nephropathy. This probably occurs by correcting diabetes-linked increased clearance of thiamine, maintaining activity and expression of thiamine pyrophosphate-dependent enzymes that help counter the adverse effects of high glucose concentrations-particularly transketolase. Evidence from experimental and clinical studies suggests that metabolism and clearance of thiamine is disturbed in diabetes leading to tissue-specific thiamine deficiency in the kidney and other sites of development of vascular complications. Thiamine supplementation prevented the development of early-stage nephropathy in diabetic rats and reversed increased urinary albumin excretion in patients with type 2 diabetes and microalbuminuria in two recent clinical trials. The thiamine monophosphate prodrug, Benfotiamine, whilst preventing early-stage development of diabetic nephropathy experimentally, has failed to produce similar clinical effect. The probable explanations for this are discussed. Further definitive trials for prevention of progression of early-stage diabetic nephropathy by thiamine are now required.

Increased protein damage in renal glomeruli, retina, nerve, plasma and urine and its prevention by thiamine and benfotiamine therapy in a rat model of diabetes

AIMS/HYPOTHESIS

The aim of this study was to quantify protein damage by glycation, oxidation and nitration in a rat model of diabetes at the sites of development of microvascular complications, including the effects of thiamine and benfotiamine therapy.

METHODS

Diabetes was induced in male Sprague-Dawley rats by 55 mg/kg streptozotocin and moderated by insulin (2 U twice daily). Diabetic and control rats were given thiamine or benfotiamine (7 or 70 mg kg(-1) day(-1)) over 24 weeks. Plasma, urine and tissues were collected and analysed for protein damage by stable isotopic dilution analysis MS.

RESULTS

There were two- to fourfold increases in fructosyl-lysine and AGE content of glomerular, retinal, sciatic nerve and plasma protein in diabetes. Increases in AGEs were reversed by thiamine and benfotiamine therapy but increases in fructosyl-lysine were not. Methionine sulfoxide content of plasma protein and 3-nitrotyrosine content of sciatic nerve protein were increased in diabetes. Plasma glycation free adducts were increased up to twofold in diabetes; the increases were reversed by thiamine. Urinary excretion of glycation, oxidation and nitration free adducts was increased by seven- to 27-fold in diabetes. These increases were reversed by thiamine and benfotiamine therapy.

CONCLUSIONS/INTERPRETATION

AGEs, particularly arginine-derived hydroimidazolones, accumulate at sites of microvascular complication development and have markedly increased urinary excretion rates in experimental diabetes. Thiamine and benfotiamine supplementation prevented tissue accumulation and increased urinary excretion of protein glycation, oxidation and nitration adducts. Similar effects may contribute to the reversal of early-stage clinical diabetic nephropathy by thiamine.

High-dose thiamine therapy for patients with type 2 diabetes and microalbuminuria: a randomised, double-blind placebo-controlled pilot study

AIMS/HYPOTHESIS

High-dose supplements of thiamine prevent the development of microalbuminuria in experimental diabetes. The aim of this pilot study was to assess whether oral supplements of thiamine could reverse microalbuminuria in patients with type 2 diabetes.

METHODS

Type 2 diabetic patients (21 male, 19 female) with microalbuminuria were recruited at the Diabetes Clinic, Sheikh Zayed Hospital, Lahore, Pakistan, and randomised to placebo and treatment arms. Randomisation was by central office in sequentially numbered opaque, sealed envelopes. Participants, caregivers and those assessing the outcomes were blinded to group assignment. Patients were given 3 x 100 mg capsules of thiamine or placebo per day for 3 months with a 2 month follow-up washout period. The primary endpoint was change in urinary albumin excretion (UAE). Other markers of renal and vascular dysfunction and plasma concentrations of thiamine were determined.

RESULTS

UAE was decreased in patients receiving thiamine therapy for 3 months with respect to baseline (median -17.7 mg/24 h; p < 0.001, n = 20). There was no significant decrease in UAE in patients receiving placebo after 3 months of therapy (n = 20). UAE was significantly lower in patients who had received thiamine therapy compared with those who had received placebo (30.1 vs 35.5 mg/24 h, p < 0.01) but not at baseline. UAE continued to decrease in the 2 month washout period in both groups, but not significantly. There was no effect of thiamine treatment on glycaemic control, dyslipidaemia or BP. There were no adverse effects of therapy.

CONCLUSIONS/INTERPRETATION

In this pilot study, high-dose thiamine therapy produced a regression of UAE in type 2 diabetic patients with microalbuminuria. Thiamine supplements at high dose may provide improved therapy for early-stage diabetic nephropathy.

TRIAL REGISTRATION

CTRI (India) CTRI/2008/091/000112.

FUNDING

Pakistan Higher Education Commission.

High prevalence of low plasma thiamine concentration in diabetes linked to a marker of vascular disease

AIMS/HYPOTHESIS

To assess thiamine status by analysis of plasma, erythrocytes and urine in type 1 and type 2 diabetic patients and links to markers of vascular dysfunction.

METHODS

Diabetic patients (26 type 1 and 48 type 2) with and without microalbuminuria and 20 normal healthy control volunteers were recruited. Erythrocyte activity of transketolase, the concentrations of thiamine and related phosphorylated metabolites in plasma, erythrocytes and urine, and markers of metabolic control and vascular dysfunction were determined.

RESULTS

Plasma thiamine concentration was decreased 76% in type 1 diabetic patients and 75% in type 2 diabetic patients: normal volunteers 64.1 (95% CI 58.5-69.7) nmol/l, type 1 diabetes 15.3 (95% CI 11.5-19.1) nmol/l, p < 0.001, and type 2 diabetes 16.3 (95% CI 13.0-9.6) nmol/l, p < 0.001. Renal clearance of thiamine was increased 24-fold in type 1 diabetic patients and 16-fold in type 2 diabetic patients. Plasma thiamine concentration correlated negatively with renal clearance of thiamine (r = -0.531, p < 0.001) and fractional excretion of thiamine (r = -0.616, p < 0.001). Erythrocyte transketolase activity correlated negatively with urinary albumin excretion (r = -0.232, p < 0.05). Thiamine transporter protein contents of erythrocyte membranes of type 1 and type 2 diabetic patients were increased. Plasma thiamine concentration and urinary excretion of thiamine correlated negatively with soluble vascular adhesion molecule-1 (r = -0.246, p < 0.05, and -0.311, p < 0.01, respectively).

CONCLUSIONS/INTERPRETATION

Low plasma thiamine concentration is prevalent in patients with type 1 and type 2 diabetes, associated with increased thiamine clearance. The conventional assessment of thiamine status was masked by increased thiamine transporter content of erythrocytes.

Accumulation of free adduct glycation, oxidation, and nitration products follows acute loss of renal function

Glycation, oxidation, and nitration of endogenous proteins occur spontaneously and these modifications are also present in foods. Increased levels of these chemical changes are associated with chronic renal failure; however, little is known about acute kidney failure. We measured these modifications of plasma protein and related free adducts in plasma following bilateral nephrectomy and bilateral ureteral obstruction. Advanced glycation end-product (AGE) residues of plasma protein were increased 3 h post-surgery, and thereafter slowly decreased in all groups, reflecting changes in plasma protein synthesis and transcapillary flow post-surgery. Ureteral ligation increased oxidation and nitration adduct residues. There were, however, marked increases in AGE, dityrosine, or 3-nitrotyrosine free adducts in both nephrectomized and ureter-ligated rats compared to rats that had undergone sham operations. There were lower modified adduct concentrations in the ureter-ligated compared to the nephrectomized rats, reflecting residual glomerular filtration and tubular removal. There was no increase in glycated, oxidized, and nitrated proteins. Glyoxal and methylglyoxal were also increased in both renal failure models. Our study shows that the acute loss of renal function and urinary excretion leads to the accumulation of AGE, oxidation, and nitration free adducts in the plasma.

Advanced glycation endproducts: what is their relevance to diabetic complications?

Glycation is a major cause of spontaneous damage to proteins in physiological systems. This is exacerbated in diabetes as a consequence of the increase in glucose and other saccharides derivatives in plasma and at the sites of vascular complications. Protein damage by the formation of early glycation adducts is limited to lysine side chain and N-terminal amino groups whereas later stage adducts, advanced glycation endproducts (AGEs), modify these and also arginine and cysteine residues. Metabolic dysfunction in vascular cells leads to the increased formation of methylglyoxal which adds disproportionately to the glycation damage in hyperglycaemia. AGE-modified proteins undergo cellular proteolysis leading to the formation and urinary excretion of glycation free adducts. AGEs may potentiate the development of diabetic complications by activation of cell responses by AGE-modified proteins interacting with specific cell surface receptors, activation of cell responses by AGE free adducts, impairment of protein-protein and enzyme-substrate interactions by AGE residue formation, and increasing resistance to proteolysis of extracellular matrix proteins. The formation of AGEs is suppressed by intensive glycaemic control, and may in future be suppressed by thiamine and pyridoxamine supplementation, and several other pharmacological agents. Increasing expression of enzymes of the enzymatic defence against glycation provides a novel and potentially effective future therapeutic strategy to suppress protein glycation.

Advanced Glycation End Product Free Adducts Are Cleared by Dialysis

Plasma advanced glycation end product (AGE) free adducts are increased up to 50-fold among patients on dialysis. We examined the ability of hemodialysis (HD) and continuous ambulatory peritoneal dialysis (CAPD) to clear these compounds. The AGE free adducts Nepsilon-carboxymethyl-lysine (CML) and Nepsilon-(1-carboxyethyl)lysine (CEL) and the hydroimidazolones derived from glyoxal (G-H1), methylglyoxal (MG-H1), and 3-deoxyglucosone (3DG-H) were determined by LC-MS/MS and pentosidine by HPLC with fluorimetric detection in ultrafiltrates of plasma, urine, or PD effluent as appropriate from patients on HD (n = 8) or PD (n = 8), and from healthy controls (n = 8). Among patients on HD, all free AGEs predialysis were significantly higher than in controls and were decreased with dialysis. The removal of MG-H1 and 3DG-H was comparable to that of urea, whereas that of CML and pentosidine was some 20% higher; in contrast, the removal of CEL and G-H1 was 25% lower. Among patients on CAPD, free AGEs in PD effluent increased with increasing dwell time. The combined renal and peritoneal 24-h excretion rates of CML (4.7 micromol), CEL (6.5 micromol), 3DG-H (16.6 micromol), and pentosidine (0.08 micromol) were twofold higher than the amount excreted in healthy controls, whereas MG-H1 was ninefold higher (59 micromol); the combined clearances of all free AGEs except pentosidine were lower than in healthy controls. Impaired renal clearance contributes to increased plasma free AGEs in uremia, but the increased excretion rate among patients on PD demonstrates that there was also an increased synthesis of free AGEs. Both HD and PD are able to remove free AGEs.

Degradation products of proteins damaged by glycation, oxidation and nitration in clinical type 1 diabetes

AIMS/HYPOTHESIS

Hyperglycaemia in diabetes is associated with increased glycation, oxidative stress and nitrosative stress. Proteins modified consequently contain glycation, oxidation and nitration adduct residues, and undergo cellular proteolysis with release of corresponding free adducts. These free adducts leak into blood plasma for eventual renal excretion. The aim of this study was to perform a comprehensive quantitative analysis of protein glycation, oxidation and nitration adduct residues in plasma protein and haemoglobin as well as of free adducts in plasma and urine to quantify increased protein damage and flux of proteolytic degradation products in diabetes.

METHODS

Type 1 diabetic patients (n=21) and normal healthy control subjects (n=12) were studied. Venous blood samples, with heparin anticoagulant, and 24-h urine samples were taken. Samples were analysed for protein glycation, oxidation and nitration adducts by a quantitative comprehensive screening method using liquid chromatography with triple quadrupole mass spectrometric detection.

RESULTS

In type 1 diabetic patients, the concentrations of protein glycation, oxidation and nitration adduct residues increased up to three-fold in plasma protein and up to one-fold in haemoglobin, except for decreases in pentosidine and 3-nitrotyrosine residues in haemoglobin when compared with normal control subjects. In contrast, the concentrations of protein glycation and oxidation free adducts increased up to ten-fold in blood plasma, and urinary excretion increased up to 15-fold in diabetic patients.

CONCLUSIONS/INTERPRETATION

We conclude that there are profound increases in proteolytic products of glycated and oxidised proteins in diabetic patients, concurrent with much lower increases in protein glycation and oxidation adduct residues.

High-dose thiamine therapy counters dyslipidaemia in streptozotocin-induced diabetic rats

AIMS/HYPOTHESIS

Cardiovascular disease in diabetes is linked to increased risk of atherosclerosis, increased levels of triglyceride-rich lipoproteins and enhanced hepatic lipogenesis. The hepatic hexosamine pathway has been implicated in signalling for de novo lipogenesis by the liver. In this study, we assessed if decrease of flux through the hexosamine pathway induced by high-dose thiamine therapy counters diabetic dyslipidaemia.

METHODS

The model of diabetes used was the streptozotocin-induced diabetic rat with maintenance insulin therapy. Normal control and diabetic rats were studied for 24 weeks with and without oral high-dose therapy (7 and 70 mg/kg) with thiamine and benfotiamine. Plasma total cholesterol, HDL cholesterol and triglycerides were determined at 6-week intervals and hepatic metabolites and transketolase activity after death of the rats at 24 weeks.

RESULTS

We found that thiamine therapy (70 mg/kg) prevented diabetes-induced increases in plasma cholesterol and triglycerides in diabetic rats but did not reverse the diabetes-induced decrease of HDL. This was achieved by prevention of thiamine depletion and decreased transketolase activity in the liver of diabetic rats. There was a concomitant decrease in hepatic UDP-N-acetylglucosamine and fatty acid synthase activity. Thiamine also normalised food intake of diabetic rats. A lower dose of thiamine (7 mg/kg) and the thiamine monophosphate prodrug benfotiamine (7 and 70 mg/kg) were ineffective.

CONCLUSIONS/INTERPRETATION

High-dose thiamine therapy prevented diabetic dyslipidaemia in experimental diabetes probably by suppression of food intake and hexosamine pathway signalling but other factors may also be involved. Benfotiamine was ineffective.

Protecting the genome: defence against nucleotide glycation and emerging role of glyoxalase I overexpression in multidrug resistance in cancer chemotherapy

Glycation of nucleotides in DNA forms AGEs (advanced glycation end-products). Nucleotide AGEs are: the imidazopurinone derivative dG-G [3-(2'-deoxyribosyl)-6,7-dihydro-6,7-dihydroxyimidazo[2,3-b]purin-9(8)one], CMdG ( N (2)-carboxymethyldeoxyguanosine) and gdC (5-glycolyldeoxycytidine) derived from glyoxal, dG-MG [6,7-dihydro-6,7-dihydroxy-6-methylimidazo-[2,3-b]purine-9(8)one], dG-MG(2) [ N (2),7-bis-(1-hydroxy-2-oxopropyl)deoxyguanosine] and CEdG [ N (2)-(1-carboxyethyl)deoxyguanosine] derived from methylglyoxal, and dG-3DG [ N (2)-(1-oxo-2,4,5,6-tetrahydroxyhexyl)deoxyguanosine] derived from 3-deoxyglucosone and others. Glyoxal and methylglyoxal induce multi-base deletions, and base-pair substitutions - mostly occurring at G:C sites with G:C-->C:G and G:C-->T:A transversions. Suppression of nucleotide glycation by glyoxalase I and aldehyde reductases and dehydrogenases, and base excision repair, protects and recovers DNA from damaging glycation. The effects of DNA glycation may be most marked in diabetes and uraemia. Mutations arising from DNA glycation may explain the link of non-dietary carbohydrate intake to incidence of colorectal cancer. Overexpression of glyoxalase I was found in drug-resistant tumour cells and may be an example of an undesirable effect of the enzymatic protection against DNA glycation. Experimental overexpression of glyoxalase I conferred resistance to drug-induced apoptosis. Glyoxalase I-mediated drug resistance was found in human leukaemia and lung carcinoma cells. Methylglyoxal-mediated glycation of DNA may contribute to the cytotoxicity of some antitumour agents as a consequence of depletion of NAD(+) by poly(ADP-ribose) polymerase, marked increases in triosephosphate concentration and increased formation of methylglyoxal. S - p -Bromobenzylglutathione cyclopentyl diester is a cell-permeable glyoxalase I inhibitor. It countered drug resistance and was a potent antitumour agent against lung and prostate carcinoma. Glyoxalase I overexpression was also found in invasive ovarian cancer and breast cancer.

Quantitative screening of protein biomarkers of early glycation, advanced glycation, oxidation and nitrosation in cellular and extracellular proteins by tandem mass spectrometry multiple reaction monitoring

Glycation of proteins forms fructosamines and AGEs (advanced glycation end products). Oxidative and nitrosative stress leads to the formation of oxidative and nitrosative modifications. The modified amino acid residues formed in these processes are biomarkers of protein damage: some are risk markers and some may be risk factors for disease development. We developed a method for the concurrent quantitative measurement of 16 biomarkers indicative of protein glycation, oxidation and nitrosation damage using LC-MS/MS (LC with tandem MS detection). Underivatized analytes were detected free in physiological fluids and in enzymatic hydrolysates of cellular and extracellular proteins. Hydroimidazolones were the most important glycation biomarkers, and methionine sulphoxide was the most important oxidative biomarker quantitatively; 3-nitrotyrosine was the biomarker of nitrosation. Quantitative screening showed high levels of AGEs in cellular protein and moderate levels in protein of blood plasma. Levels of 3-nitrotyrosine were typically 100-fold lower than this. The major glycation adducts in blood plasma had high renal clearances in normal healthy human subjects, whereas methionine sulphoxide and 3-nitrotyrosine had low renal clearances due to further metabolism. Physiological AGEs in blood plasma were eliminated from the circulation in the kidney and not in the liver. LC-MS/MS peptide mapping was also used to locate the protein biomarkers. These studies reveal that advanced glycation is a significant modification of cellular and extracellular protein. The enzymatic defences against glycation, antioxidants and proteasomal protein degradation inside cells are probable factors regulating biomarker levels of cellular protein.

The enzymatic defence against glycation in health, disease and therapeutics: a symposium to examine the concept

Glycation of proteins, nucleotides and basic phospholipids by glucose, glyoxal, methylglyoxal, 3-deoxyglucosone and other saccharide derivatives is potentially damaging to the proteome and mutagenic. It is now recognized that there is an enzymatic defence against glycation--a group of enzymes that suppress the physiological levels of potent glycating agents and repair glycated proteins: glyoxalase I, aldehyde reductases and dehydrogenases, amadoriase and fructosamine 3-phosphokinase. The enzymatic defence against glycation influences morbidity and the efficiency of drug therapy in certain diseases. Improved understanding of the balance between glycation and the enzymatic anti-glycation defence will advance disease diagnosis and therapy.

Glyoxalase I – structure, function and a critical role in the enzymatic defence against glycation

Glyoxalase I is part of the glyoxalase system present in the cytosol of cells. The glyoxalase system catalyses the conversion of reactive, acyclic α-oxoaldehydes into the corresponding α-hydroxyacids. Glyoxalase I catalyses the isomerization of the hemithioacetal, formed spontaneously from α-oxoaldehyde and GSH, to S-2-hydroxyacylglutathione derivatives [RCOCH(OH)-SG→RCH(OH)CO-SG], and in so doing decreases the steady-state concentrations of physiological α-oxoaldehydes and associated glycation reactions. Physiological substrates of glyoxalase I are methylglyoxal, glyoxal and other acyclic α-oxoaldehydes. Human glyoxalase I is a dimeric Zn2+ metalloenzyme of molecular mass 42 kDa. Glyoxalase I from Escherichia coli is a Ni2+ metalloenzyme. The crystal structures of human and E. coli glyoxalase I have been determined to 1.7 and 1.5 Å resolution. The Zn2+ site comprises two structurally equivalent residues from each domain – Gln-33A, Glu-99A, His-126B, Glu-172B and two water molecules. The Ni2+ binding site comprises His-5A, Glu-56A, His-74B, Glu-122B and two water molecules. The catalytic reaction involves base-catalysed shielded-proton transfer from C-1 to C-2 of the hemithioacetal to form an ene-diol intermediate and rapid ketonization to the thioester product. R- and S-enantiomers of the hemithioacetal are bound in the active site, displacing the water molecules in the metal ion primary co-ordination shell. It has been proposed that Glu-172 is the catalytic base for the S-substrate enantiomer and Glu-99 the catalytic base for the R-substrate enantiomer; Glu-172 then reprotonates the ene-diol stereospecifically to form the R-2-hydroxyacylglutathione product. By analogy with the human enzyme, Glu-56 and Glu-122 may be the bases involved in the catalytic mechanism of E. coli glyoxalase I. The suppression of α-oxoaldehyde-mediated glycation by glyoxalase I is particularly important in diabetes and uraemia, where α-oxoaldehyde concentrations are increased. Decreased glyoxalase I activity in situ due to the aging process and oxidative stress results in increased glycation and tissue damage. Inhibition of glyoxalase I pharmacologically with specific inhibitors leads to the accumulation of α-oxoaldehydes to cytotoxic levels; cell-permeable glyoxalase I inhibitors are antitumour and antimalarial agents. Glyoxalase I has a critical role in the prevention of glycation reactions mediated by methylglyoxal, glyoxal and other α-oxoaldehydes in vivo.

Accumulation of fructosyl-lysine and advanced glycation end products in the kidney, retina and peripheral nerve of streptozotocin-induced diabetic rats

The accumulation of AGEs (advanced glycation end products) in diabetes mellitus has been implicated in the biochemical dysfunction associated with the chronic development of microvascular complications of diabetes--nephropathy, retinopathy and peripheral neuropathy. We investigated the concentrations of fructosyl-lysine and AGE residues in protein extracts of renal glomeruli, retina, peripheral nerve and plasma protein of streptozotocin-induced diabetic rats and normal healthy controls. Glycation adducts were determined by LC with tandem MS detection. In diabetic rats, the fructosyl-lysine concentration was increased markedly in glomeruli, retina, sciatic nerve and plasma protein. The concentrations of N (epsilon)-carboxymethyl-lysine and N (epsilon)-carboxyethyl-lysine were increased in glomeruli, sciatic nerve and plasma protein, and N(epsilon)-carboxymethyl-lysine also in the retina. Hydroimidazolone AGEs derived from glyoxal, methylglyoxal and 3-deoxylglucosone were major AGEs quantitatively. They were increased in the retina, nerve, glomeruli and plasma protein. AGE accumulation in renal glomeruli, retina, peripheral nerve and plasma proteins is consistent with a role for AGEs in the development of nephropathy, retinopathy and peripheral neuropathy in diabetes. High-dose therapy with thiamine and Benfotiamine suppressed the accumulation of AGEs, and is a novel approach to preventing the development of diabetic complications.

Removal of advanced glycation end products in clinical renal failure by peritoneal dialysis and haemodialysis

AGEs (advanced glycation end products) accumulate markedly in the plasma of human subjects with renal failure. We investigated the efficiency of removal of AGEs from the circulation by PD (peritoneal dialysis) and HD (haemodialysis) therapy. Free AGEs were measured by LC-MS/MS in blood plasma before dialysis, in dialysis fluid effusate after a 2–12 h dwell time in the peritoneal cavity of PD subjects, and in the HD dialysate before and after HD therapy. In clinical uraemia, the concentrations of free AGEs in blood plasma were increased up to 50-fold. For example, levels of MG-H1 (methylglyoxal-derived hydroimidazolone) were: normal controls, 110±46 nM; PD subjects, 1876±676 (P<0.01); HD subjects, 5496±1138 nM (P<0.001). In PD subjects, the AGE concentration in the effusate increased with increasing dwell time, reaching a maximum at a concentration higher than that in plasma for some AGEs at 4–12 h. This may reflect AGE formation in the peritoneal cavity. In HD, AGE concentrations in HD fluid were decreased markedly from the start to the end of a dialysis session, except that levels of the methylglyoxal-derived AGEs N∊-(1-carboxyethyl)lysine and MG-H1, and of pentosidine, remained 5-fold higher than control levels. Inadequate clearance of free AGEs may be linked to the increased risk of cardiovascular disease in patients with renal failure.

Increased serum levels of the specific AGE-compound methylglyoxal-derived hydroimidazolone in patients with type 2 diabetes

A time-delayed fluorescence immunoassay was developed for the determination of serum levels of methylglyoxal (MG)-derived hydroimidazolone using a monoclonal antiserum raised against Nalpha-acetyl-Ndelta-(5-hydro-5-methyl)-4-imidazolone, Europium-labeled anti-mouse IgG antiserum as indicator, and MG modified bovine serum albumin (BSA) as standard. Serum levels of hydroimidazolone were measured in 45 patients with type 2 diabetes aged 59.4 +/- 6.1 (mean +/- SD) years and with duration of diabetes of 7.3 +/- 3.1 years, and in 19 nondiabetic controls aged 56.3 +/- 4.3 years. The serum levels of hydroimidazolone were significantly higher in patients compared to controls: median, 3.0 (5-95 percentile, 1.6 to 5.4) U/mg protein versus 1.9 (1.2 to 2.8) U/mg protein (P =.0005). Significant positive correlations were observed between the serum levels of hydroimidazolone and serum levels of advanced glycation end products (AGEs), measured with a polyclonal anti-AGE antibody: r = 0.59 for patients (P <.0001), and r = 0.65 for controls (P =.002). Similarly, significant correlations were also found between serum levels of hydroimidazolone and N(epsilon)-(carboxymethyl)-lysine (CML): r = 0.36 in patients and r = 0.55 for controls (both P =.02). Serum hydroimidazolone levels did not correlate with fasting plasma glucose or hemoglobin A(1c) (HbA(1c)) levels. The observed differences between patients with diabetes and nondiabetic controls seem to be comparable to differences measured for other AGE compounds.

Suppression of the accumulation of triosephosphates and increased formation of methylglyoxal in human red blood cells during hyperglycaemia by thiamine in vitro

The accumulation of triosephosphates and the increased formation of the potent glycating agent methylglyoxal in intracellular hyperglycaemia are implicated in the development of diabetic complications. A strategy to counter this is to stimulate the anaerobic pentosephosphate pathway of glycolysis by maximizing transketolase activity by thiamine supplementation, with the consequent consumption of glyceraldehyde-3-phosphate and increased formation of ribose-5-phosphate. To assess the effect of thiamine supplementation on the accumulation of triosephosphates and methylglyoxal formation in cellular hyperglycaemia, we incubated human red blood cell suspensions (50% v/v) in short-term culture with 5 mM glucose and 50 mM glucose in Krebs-Ringer phosphate buffer at 37 degrees C as models of cellular metabolism under normoglycaemic and hyperglycaemic conditions. In hyperglycaemia, there is a characteristic increase in the concentration of the triosephosphate pool of glycolytic intermediates and a consequent increase in the concentration and metabolic flux of the formation of methylglyoxal. The addition of thiamine (50-500 microM) increased the activity of transketolase, decreased the concentration of the triosephosphate pool, decreased the concentration and metabolic flux of the formation of methylglyoxal, and increased the concentration of total sedoheptulose-7-phosphate and ribose-5-phosphate. Biochemical changes implicated in the development of diabetic complications were thereby prevented. This provides a biochemical basis for high dose thiamine therapy for the prevention of diabetic complications.

Signal transduction activated by the cancer chemopreventive isothiocyanates: cleavage of BID protein, tyrosine phosphorylation and activation of JNK

Phenethyl isothiocyanate and allyl isothiocyanate induce apoptosis of human leukaemia HL60 cells in vitro. Apoptosis was associated with cleavage of p22 BID protein to p15, p13 and p11 fragments and activation of JNK and tyrosine phosphorylation (18 kDa and 45 kDa proteins). All these effects and apoptosis were prevented by exogenous glutathione (15 mM). Protein tyrosine phosphatase activity was unchanged. The general caspase inhibitor Z-VAD-fmk prevented apoptosis but not JNK activation - excluding a role for caspases in JNK activation, whereas curcumin prevented JNK activation but only delayed apoptosis. This suggests that in isothiocyanate-induced apoptosis, the caspase pathway has an essential role, the JNK pathway a supporting role, and inhibition of protein tyrosine phosphatases is not involved.

Involvement of glutathione metabolism in the cytotoxicity of the phenethyl isothiocyanate and its cysteine conjugate to human leukaemia cells in vitro

The dietary isothiocyanate and cancer chemopreventive agent, phenethyl isothiocyanate, induced apoptosis of human leukaemia HL60 and human myeloblastic leukaemia ML-1 cells in vitro. Cytotoxicity was associated with an initial decrease in GSH and GSSG, with a concomitant formation of the GSH adduct S-(N-phenethylthiocarbamoyl)glutathione inside cells, which was then exported from cells. After 12 hr, the cellular concentration of GSH recovered and then declined after 24 hr. Buthionine sulphoximine prevented the recovery of cellular GSH concentration and potentiated the cytotoxicity of phenethyl isothiocyanate. S-(N-phenethylthiocarbamoyl)glutathione spontaneously fragmented to GSH and phenethyl isothiocyanate, GSH oxidized to GSSG and glutathionyl-protein disulphides, and phenethyl isothiocyanate hydrolyzed to phenylethylamine. GSH and GSSG depletion was more marked in ML-1 cells than in HL60 cells. Studies with [(14)C]-labelled phenethyl isothiocyanate gave evidence of phenethylthiocarbamoylation of cells that maximized after 2-3 hr. This occurred later than the maximum concentration of S-(N-phenethylthiocarbamoyl)glutathione, but coincided with the commitment to apoptosis and cytotoxicity which developed later. The cytotoxicity of phenethyl isothiocyanate was prevented by a high concentration of GSH (15 mM) and delayed by the antioxidant and c-Jun N-terminal kinase signalling pathway inhibitor curcumin. GSH prevented and curcumin partly prevented the decrease in cellular GSH. These studies show that the cysteinyl thiol group of GSH is an important site of thiocarbamoylation by phenethyl isothiocyanate during induction of apoptosis and that this may lead to depletion of cellular GSH by efflux of the GSH conjugate. Thiocarbamoylation also occurred at other sites. The recent demonstration of a critical role for activation of caspase-8 in phenethyl isothiocyanate-induced apoptosis suggests that this thiocarbamoylation directly or indirectly leads to functional activation of a cell death receptor/adaptor protein complex.

Mass spectrometric monitoring of albumin in uremia

BACKGROUND

Advanced glycation end products (AGEs) are a novel class of uremic toxins. In plasma, they are present in proteins and also in low molecular mass peptides. AGE-modified peptides are thought to bind and modify plasma proteins. Monitoring of the consequent increase in molecular mass of serum albumin may be used in surveillance of the clinical management of uremia.

METHODS

We investigated molecular mass changes of human serum albumin (HSA) glycated by glucose and methylglyoxal in vitro and of subjects with moderate renal impairment, end-stage renal disease (ESRD), ESRD on hemodialysis, and normal healthy controls by matrix-assisted laser desorption ionization mass spectrometry.

RESULTS

Fatty acid-free HSA had a molecular mass of 66,446 +/- 114 D. Mean (+/-SD) molecular mass increases were HSA minimally glycated by glucose 399 +/- 88 D (N = 5, P < 0.001), HSA highly glycated by glucose 6780 +/- 122 D (N = 5, P < 0.001), HSA minimally glycated by methylglyoxal 73 +/- 121 D (N = 5, P > 0.05), and HSA without fatty acid removal 535 +/- 90 D (N = 5, P < 0.001). For HSA of human subjects, mean (+/- SD) molecular mass increases were normal healthy controls 243 +/- 97 D (N = 5), moderate renal impairment 350 +/- 83 D (P > 0.05 with respect to controls, N = 5), ESRD 498 +/- 128 (P < 0.02 with respect to controls, N = 3), and ESRD on hemodialysis 438 +/- 85 D (P < 0.02 with respect to controls, N = 5). The mean molecular mass of albumin of all groups was increased significantly with respect to that of fatty acid free albumin (P < 0.001).

CONCLUSIONS

Only ESRD was associated with a significant increase in the molecular mass of HSA in vivo. Since this mass increase was very low and much lower than reported for AGE-modified peptides, it may reflect AGE formation on HSA by alpha-oxoaldehydes that accumulate in uremia, rather than modification of albumin by AGE-modified peptides. The molecular mass of HSA in vivo was indicative of a minimal and not a high extent of glycation.

Studies on the mechanism of the inhibition of human leukaemia cell growth by dietary isothiocyanates and their cysteine adducts in vitro

The dietary isothiocyanates and cancer chemopreventive agents phenethyl isothiocyanate and allyl isothiocyanate and their cysteine conjugates inhibited the growth and induced apoptosis of human leukaemia HL60 (p53-) and human myeloblastic leukaemia-1 cells (p53+) in vitro. The median growth inhibitory concentration (GC(50)) values were in the range 1.49-3.22 microM in cultures with 10% serum. Isothiocyanates and cysteine conjugates had increased potency against HL60 cells in serum-free medium, with GC(50) values of 0.8-0. 9 microM. The potency of the compounds decreased with increased serum content of the medium, but that of the cysteine conjugates decreased more markedly. Growth inhibition and toxicity was characterised by either a rapid interaction of the isothiocyanate with the cells in the first hour of culture or exposure to isothiocyanate liberated from the cysteine conjugate in the initial 3 hr of culture, inhibition of macromolecule synthesis, and a commitment to apoptosis which developed in the initial 24 hr. Activities of caspase-3 and caspase-8 were increased during isothiocyanate-induced apoptosis, but caspase-1 activity was not. The general caspase inhibitor N-benzyloxycarbonyl-Val-Ala-Asp(OMe)-fluoromethylketone and the specific caspase-8 inhibitor N-benzyloxycarbonyl-Ile-Glu(OMe)-Thr-Asp(OMe)-fluoromethylketone inhibited apoptosis, but specific caspase-1 and caspase-3 inhibitors did not. The antiproliferative activities were limited by hydrolysis of the isothiocyanate. This suggests that caspase-8 has a critical role, and caspase-3 a supporting role, in isothiocyanate-induced apoptosis in which p53 is not an obligatory participant. Isothiocyanate-induced apoptosis may suppress the growth of preclinical tumours and contribute to the well-established decreased cancer incidence associated with a vegetable-rich diet.

Kinetics and mechanism of the reaction of aminoguanidine with the alpha-oxoaldehydes glyoxal, methylglyoxal, and 3-deoxyglucosone under physiological conditions

Aminoguanidine (AG), a prototype agent for the preventive therapy of diabetic complications, reacts with the physiological alpha-oxoaldehydes glyoxal, methylglyoxal, and 3-deoxyglucosone (3-DG) to form 3-amino-1,2,4-triazine derivatives (T) and prevent glycation by these agents in vitro and in vivo. The reaction kinetics of these alpha-oxoaldehydes with AG under physiological conditions pH 7.4 and 37 degrees was investigated. The rate of reaction of AG with glyoxal was first order with respect to both reactants; the rate constant k(AG,G) was 0.892 +/- 0.037 M(-1) sec(-1). The kinetics of the reaction of AG with 3-DG were more complex: the rate equation was d[T](o)/dt (initial rate of T formation) = [3-DG](k(AG,3-DG)[AG] + k(3-DG)), where k(AG,3-DG) = (3. 23 +/- 0.25) x 10(-3) M(-1) sec(-1) and k(3-DG) = (1.73 +/- 0.08) x 10(-5) sec(-1). The kinetics of the reaction of AG with methylglyoxal were consistent with the reaction of both unhydrated (MG) and monohydrate (MG-H(2)O) forms. The rate equation was d[T](o)/dt = ¿k(1)k(AG,MG)/(k(-1) + k(AG,MG)[AG]) + k(AG, MG-H(2)O)¿[MG-H(2)O][AG], where the rate constant for the reaction of AG with MG, k(AG,MG), was 178 +/- 15 M(-1) sec(-1) and for the reaction of AG with MG-H(2)O, k(AG,MG-H(2)O), was 0.102 +/- 0.001 M(-1) sec(-1); k(1) and k(-1) are the forward and reverse rate constants for methylglyoxal dehydration MG-H(2)O right harpoon over left harpoon MG. The kinetics of these reactions were not influenced by ionic strength, but the reaction of AG with glyoxal and with methylglyoxal under MG-H(2)O dehydration rate-limited conditions increased with increasing phosphate buffer concentration. Kinetic modelling indicated that the rapid reaction of AG with the MG perturbed the MG/MG-H(2)O equilibrium, and the ratio of the isomeric triazine products varied with initial reactant concentration. AG is kinetically competent to scavenge the alpha-oxoaldehydes studied and decrease related advanced glycated endproduct (AGE) formation in vivo. This effect is limited, however, by the rapid renal elimination of AG. Decreased AGE formation is implicated in the prevention of microvascular complications of diabetes by AG.

Antitumour activity of sphingoid base adducts of phenethyl isothiocyanate

N-(N'-Phenethylthiocarbamoyl) derivatives of sphingosine and sphinganine were prepared. They had antitumour activity: GC50 values of 0.64+/-0.02 microM (N = 18) and 1.6+/-0.01 microM (N = 18). respectively, with human leukaemia 60 cells in vitro. This antitumour effect may contribute to the suppression of carcinogenesis associated with dietary phenethyl isothiocyanate and sphingolipid bases.

Formation of glyoxal, methylglyoxal and 3-deoxyglucosone in the glycation of proteins by glucose

The glycation of proteins by glucose has been linked to the development of diabetic complications and other diseases. Early glycation is thought to involve the reaction of glucose with N-terminal and lysyl side chain amino groups to form Schiff's base and fructosamine adducts. The formation of the alpha-oxoaldehydes, glyoxal, methylglyoxal and 3-deoxyglucosone, in early glycation was investigated. Glucose (50 mM) degraded slowly at pH 7.4 and 37 degrees C to form glyoxal, methylglyoxal and 3-deoxyglucosone throughout a 3-week incubation period. Addition of t-BOC-lysine and human serum albumin increased the rate of formation of alpha-oxoaldehydes - except glyoxal and methylglyoxal concentrations were low with albumin, as expected from the high reactivity of glyoxal and methylglyoxal with arginine residues. The degradation of fructosyl-lysine also formed glyoxal, methylglyoxal and 3-deoxyglucosone. alpha-Oxoaldehyde formation was dependent on the concentration of phosphate buffer and availability of trace metal ions. This suggests that alpha-oxoaldehydes were formed in early glycation from the degradation of glucose and Schiff's base adduct. Since alpha-oxoaldehydes are important precursors of advanced glycation adducts, these adducts may be formed from early and advanced glycation processes. Short periods of hyperglycaemia, as occur in impaired glucose tolerance, may be sufficient to increase the concentrations of alpha-oxoaldehydes in vivo.

Accumulation of alpha-oxoaldehydes during oxidative stress: a role in cytotoxicity

Glyoxal, methylglyoxal (MG), and 3-deoxyglucosone (3-DG) are physiological alpha-oxoaldehydes formed by lipid peroxidation, glycation, and degradation of glycolytic intermediates. They are enzymatically detoxified in cells by the cytosolic glutathione-dependent glyoxalase system (glyoxal and MG only) and by NADPH-dependent reductase and NAD(P)+-dependent dehydrogenase. In this study, the changes in the cellular and extracellular concentrations of these alpha-oxoaldehydes were investigated in murine P388D1 macrophages during necrotic cell death induced by median toxic concentrations of hydrogen peroxide and 1-chloro-2,4-dinitrobenzene (CDNB). Alpha-oxoaldehyde concentrations were determined by derivatization with 1,2-diamino-4,5-dimethoxybenzene. There were relatively small increases in cellular and extracellular glyoxal concentration, except that extracellular glyoxal was decreased with hydrogen peroxide. The cytosolic concentration of 3-DG and the cytosolic and extracellular concentrations of MG, however, increased markedly. Aminoguanidine inhibited alpha-oxoaldehyde accumulation and prevented cytotoxicity induced by hydrogen peroxide and CDNB. The accumulation of glyoxal and MG in toxicant-treated cells was a likely consequence of decreased in situ activity of glyoxalase 1. The effect was marked for MG but not for glyoxal, suggestive of a greater metabolic flux of MG formation than of glyoxal. The accumulation of 3-DG in toxicant-treated cells was probably due to the decreased availability of pyridine nucleotide cofactors for the detoxification of 3-DG. Impairment of alpha-oxoaldehyde detoxification is cytotoxic, and this may contribute to toxicity associated with GSH oxidation and S conjugation in oxidative stress and chemical toxicity, and to chronic pathogenesis associated with diabetes mellitus where there is oxidative stress and the formation of glyoxal, MG, and 3-DG is increased.

Trioses and related substances: tools for the study of pancreatic beta-cell function

Trioses, such as D-glyceraldehyde, have been used extensively for the study of stimulus-secretion coupling mechanisms in pancreatic beta-cells. It is generally assumed that trioses enter the glycolytic pathway at the triose phosphate level, and stimulate insulin release in a manner analogous to glucose. This review focuses on a number of triose effects that are not entirely consistent with this model. These effects are likely to result, at least in part, from the actions of alpha-ketoaldehydes. One such compound, methylglyoxal, appears to be a major contaminant of triose preparations, and exerts effects on the beta-cell identical to some of those evoked by glyceraldehyde. A related substance, hydroxypyruvaldehyde, is a product of triose autoxidation, which could exert similar effects. Study of the actions of trioses and alpha-ketoaldehydes could assist our understanding of cellular physiology, in general, and beta-cell function, in particular. These substances are also likely to be of pathophysiological importance, especially in the context of sugar toxicity and autoxidative cell damage.

Rapid hydrolysis and slow alpha,beta-dicarbonyl cleavage of an agent proposed to cleave glucose-derived protein cross-links

The putative protein glycation cross-link cleaving agent N-phenacylthiazolium bromide (PTB) underwent hydrolysis and cyclic hemithioacetal formation under physiological conditions to form two isomeric 2,3-dihydro-4-formyl-2-hydroxy-2-phenyl-1,4-thiazines: at pH 7.4 and 37 degrees, the rate constant k(Hydrolysis) was (2.6+/-0.1) x 10(-4) sec and the chemical half-life was ca. 44 min. The alpha,beta-dicarbonyl cleavage reaction only competed effectively with the hydrolysis when the alpha,beta-dicarbonyl substrate was at nonphysiological high levels. The high concentrations of PTB (10-30 mM) used previously to demonstrate chemical and biochemical activity also lead to acidification of incubation media. The mechanism of action of PTB now requires reappraisal.

Teratogenicity of 3-deoxyglucosone and diabetic embryopathy

The increased rate of embryonic dysmorphogenesis in diabetic pregnancy is correlated with the severity and duration of the concurrent hyperglycemia during early gestation. Whole embryo culture was used to investigate a possible association of hyperglycemia-induced disturbances of embryo development with tissue levels of the three alpha-oxoaldehydes: glyoxal, methylglyoxal, and 3-deoxyglucosone (3-DG). Rat embryos exposed to high glucose levels in vitro showed severe dysmorphogenesis and a 17-fold increased concentration of 3-DG compared with control embryos cultured in a low glucose concentration. Exogenous 3-DG (100 micromol/l) added to the medium of control cultures yielded an increased embryonic malformation rate and a 3-DG concentration similar to that of embryos cultured in high glucose. Addition of superoxide dismutase (SOD) to the culture medium decreased the malformation rates of embryos exposed to either high glucose or high 3-DG levels, but it did not decrease the high embryonic 3-DG concentrations caused by either agent. Our results implicate the potent glycating agent 3-DG as a teratogenic factor in diabetic embryopathy. In addition, the anti-teratogenic effect of SOD administration appears to occur downstream of 3-DG formation, suggesting that 3-DG accumulation leads to superoxide-mediated embryopathy.

Cell activation by glycated proteins. AGE receptors, receptor recognition factors and functional classification of AGEs

Proteins modified by advanced glycation endproducts (AGE) bind to cell surface receptors and other AGE binding proteins. AGE-binding receptors are: scavenger receptors types I and II, the receptor for advanced glycation endproducts (RAGE), oligosaccharyl transferase-48 (OST-48, AGE-R1), 80K-H phosphoprotein (AGE-R2) and galectin-3 (AGE-R3). AGE receptors are found in monocytes, macrophages, endothelial cells, pericytes, podocytes, astrocytes and microglia. AGE-modified proteins also bind to lysozyme and lactoferrin. A critical review of the evidence for receptors binding AGE-modified protein binding in vivo is presented. Scavenger receptors have only been shown to bind proteins modified by AGE to a much higher extent than found in vivo. 80K-H phosphoprotein is involved in FGFR3 signal transduction to MAP kinase, and may be involved in AGE-receptor signal transduction. Whether all of these proteins bind AGE-modified proteins in vivo is not yet clear. Cell activation in response to AGE-modified proteins is associated with increased expression of extracellular matrix proteins, vascular adhesion molecules, cytokines and growth factors. Depending on the cell type and concurrent signaling, this is associated with chemotaxis, angiogenesis, oxidative stress, cell proliferation or programmed cell death (PCD). Receptor recognition factors for agonism at the AGE receptor have been little studied but to date hydroimidazolones appear to be the most likely candidates. Pharmacologic inhibition of AGE receptor-mediated cell activation with specific antagonists may provide the basis for therapeutic intervention in diseases where AGE accumulation is a suspected etiological factor vascular complications of diabetes, macrovascular disease, renal insufficiency and Alzheimer's disease.

Effects of methylglyoxal on rat pancreatic beta-cells

The addition of the alpha-ketoaldehyde methylglyoxal (0.5 or 1 mmol/L) to single isolated rat pancreatic beta-cells caused a rapid, marked depolarization resulting in electrical activity. This effect of methylglyoxal on beta-cell was reversible upon removal of the alpha-ketoaldehyde, and could be inhibited by the anion channel blockers 4,4'-diisothiocyanatostilbene-2,2'-disulphonic acid (DIDS) and 5-nitro-2-(3-phenylpropylamino) benzoic acid (NPPB). Methylglyoxal also resulted in elevated cytosolic [Ca2+] and an intracellular acidification in intact rat islets. In perifused islets, methylglyoxal provoked a modest, transient stimulation of secretion but inhibited glucose-induced insulin release. Incubation of islets with methylglyoxal resulted in the formation of large quantities of D-lactate, indicating metabolism of the alpha-ketoaldehyde via the glyoxalase pathway. The effects of methylglyoxal on beta-cell membrane potential, cytosolic [Ca2+] and intracellular pH were also observed in response to phenylglyoxal which is also effectively metabolized via the glyoxalase pathway. However, t-butylglyoxal which is poorly metabolized via the glyoxalase pathway, caused neither depolarization of the membrane potential nor intracellular acidification, but did inhibit glucose-induced insulin release. These findings suggests that the depolarization and acidification evoked by methyl- and phenylglyoxal are dependent upon their metabolism via the glyoxalase pathway. The possible mechanisms coupling alpha-ketoaldehyde metabolism via the glyoxalase pathway with membrane depolarization are discussed.

Glutathione-dependent detoxification of alpha-oxoaldehydes by the glyoxalase system: involvement in disease mechanisms and antiproliferative activity of glyoxalase I inhibitors

The glyoxalase system is a metabolic pathway that catalyses the detoxification of alpha-oxoaldehydes RCOCHO to corresponding aldonic acids RCH(OH)CO2H. It thereby protects cells from alpha-oxoaldehyde-mediated formation of advanced glycation endproducts (AGEs). It is comprised of two enzymes, glyoxalase I and glyoxalase II, and a catalytic amount of reduced glutathione (GSH) as cofactor. It is present in the cytosol of cells of mammals and most micro-organisms. Physiological substrates of the glyoxalase system are: glyoxal--formed from lipid peroxidation and glycation reactions, methylglyoxal--formed from triosephosphates, ketone body metabolism and threonine catabolism, and 4,5-dioxovalerate--formed from 5-aminolevulinate and alpha-ketoglutarate. alpha-Oxoaldehydes react with guanyl residues in DNA and RNA, and with cysteine, lysine and arginine residues in proteins. The modification of DNA induces mutagenesis and apoptosis. The modification of proteins leads to protein degradation and activation of a cytokine-mediated immune response.

Overexpression of glyoxalase-I in bovine endothelial cells inhibits intracellular advanced glycation endproduct formation and prevents hyperglycemia-induced increases in macromolecular endocytosis

Methylglyoxal (MG), a dicarbonyl compound produced by the fragmentation of triose phosphates, forms advanced glycation endproducts (AGEs) in vitro. Glyoxalase-I catalyzes the conversion of MG to S-D-lactoylglutathione, which in turn is converted to D-lactate by glyoxalase-II. To evaluate directly the effect of glyoxalase-I activity on intracellular AGE formation, GM7373 endothelial cells that stably express human glyoxalase-I were generated. Glyoxalase-I activity in these cells was increased 28-fold compared to neo-transfected control cells (21.80+/-0.1 vs. 0. 76+/-0.02 micromol/min/mg protein, n = 3, P < 0.001). In neo-transfected cells, 30 mM glucose incubation increased MG and D-lactate concentration approximately twofold above 5 MM (35.5+/-5.8 vs. 19.6+/-1.6, P < 0.02, n = 3, and 21.0+/-1.3 vs. 10.0+/-1.2 pmol/ 10(6) cells, n = 3, P < 0.001, respectively). In contrast, in glyoxalase-I-transfected cells, 30 mM glucose incubation did not increase MG concentration at all, while increasing the enzymatic product D-lactate by > 10-fold (18.9+/-3.2 vs. 18.4+/- 5.8, n = 3, P = NS, and 107.1+/-9.0 vs. 9.4+/-0 pmol/10(6) cells, n = 3, P < 0.001, respectively). After exposure to 30 mM glucose, intracellular AGE formation in neo cells was increased 13.6-fold (2.58+/-0.15 vs. 0.19+/-0.03 total absorbance units, n = 3, P < 0.001). Concomitant with increased intracellular AGEs, macromolecular endocytosis by these cells was increased 2.2-fold. Overexpression of glyoxalase-I completely prevented both hyperglycemia-induced AGE formation and increased macromolecular endocytosis.

Synthesis and secretion of tumour necrosis factor-alpha by human monocytic THP-1 cells and chemotaxis induced by human serum albumin derivatives modified with methylglyoxal and glucose-derived advanced glycation endproducts

Human serum albumin minimally-modified by methylglyoxal (MGmin-HSA) stimulated the synthesis and secretion of tumour necrosis factor-alpha (TNF-alpha) from human monocytic THP-1 cells in vitro. Human serum albumin minimally-modified by glucose-derived advanced glycation endproducts (AGEmin-HSA) and human serum albumin highly-modified by glucose-derived advanced glycation endproducts (AGE-HSA) stimulated markedly lower synthesis and secretion of TNF-alpha from THP-1 cells than did MGmin-HSA. The median effective concentration EC50 value of MGmin-HSA for the secretion of TNF-alpha was 5.8 +/- 0.3 microM and the maximal secretion was 0.28 +/- 0.01 ng TNF-alpha/ml (n = 12) for incubations containing 5 x 10(5) cells/ml. MGmin-HSA (0.2-2.0 microM) also stimulated chemotaxis of THP-1 cells in vitro but AGE-HSA did not in this concentration range. The EC50 value of MGmin-HSA for the chemotactic response was 0.44 +/- 0.07 microM (n = 15). Similar induction of the synthesis and secretion of TNF-alpha and chemotaxis by monocytes in response to MGmin-HSA in vivo may contribute to atherosclerosis in macro- and micro-angiopathy, particularly in the development of chronic clinical complications of diabetes mellitus.

Methylglyoxal-modified arginine residues--a signal for receptor-mediated endocytosis and degradation of proteins by monocytic THP-1 cells

Non-enzymatic glycosylation or glycation of proteins to form advanced glycation endproducts (AGE) has been proposed as a process which provides a signal for the degradation of proteins. Despite this, the AGE which act a recognition factor for receptor-mediated endocytosis and degradation of glycated proteins by monocytes and macrophages has not been identified. Methylglyoxal, a reactive alpha-oxoaldehyde and physiological metabolite, reacted irreversibly with arginine residues in proteins to form Ndelta-(5-hydro-5-methyl-4-imidazolon-2-yl)ornithine and Ndelta-(5-methyl-4-imidazolon-2-yl)ornithine residues. Human serum albumin minimally-modified with methylglyoxal (MG(min)-HSA) was bound by cell surface receptors of human monocytic THP-1 cells in vitro at 4 degrees C: the binding constant K(d) value was 377 +/- 35 nM and the number of receptors per cell was 5.9 +/- 0.2 X 10(5) (n = 12). N alpha-Acetyl-Ndelta-(5-hydro-5-methyl-4-imidazolon-2-yl)orni thine displaced MG(min)-HSA from THP-1 cells, suggesting that the Ndelta-(5-hydro-5-methyl-4-imidazolon-2-yl)ornithine residue was the receptor recognition factor. At 37 degrees C, MG(min)-HSA was internalised by THP-1 cells and degraded. Similar binding and degradation of human serum albumin modified by glucose-derived AGE was found but only when highly modified. MG(min)-HSA, therefore, is the first example of a protein minimally-modified by AGE-like compounds that binds specifically to monocyte receptors. The irreversible modification of proteins by methylglyoxal is a potent signal for the degradation of proteins by monocytic cells in which the arginine derivative, Ndelta-(5-hydro-5-methyl-4-imidazolon-2-yl)ornithine, is the receptor recognition factor. This factor is not present in glucose-modified proteins.

Negative association between erythrocyte reduced glutathione concentration and diabetic complications

1. Multiple logistic regression analysis of biochemical and clinical variables in diabetic patients was performed to identify those associated with the presence of diabetic complications (retinopathy, neuropathy and nephropathy). 2. The presence of diabetic complications correlated positively with duration of diabetes and patients age and negatively with the concentration of reduced glutathione in erythrocytes. Individually, retinopathy, neuropathy and nephropathy correlated with duration of diabetes, but retinopathy also correlated positively with haemoglobin A1C in diabetic patients. In insulin-dependent patients, the concentration of methylglyoxal was also in the logistic model for retinopathy and diabetic complications, but the logistic regression coefficient was not significant. 3. Multiple linear regression analysis indicated that erythrocyte reduced glutathione concentration correlated negatively with D-lactate concentration and positively with duration of diabetes in insulin-dependent patients and correlated negatively with glucose concentration in non-insulin-dependent diabetic patients. 4. In non-diabetic subjects, erythrocyte glyoxalase I activity correlated positively with methylglyoxal concentration. There was no similar correlation in diabetic patients. In insulin-dependent patients, methylglyoxal concentration correlated positively with duration of diabetes. 5. Glyoxal and methylglyoxal are detoxified by the glyoxalase system with reduced glutathione as co-factor. The concentration of reduced glutathione may be decreased by oxidative stress and by decreased in situ glutathione reductase activity in diabetes mellitus. A reduced concentration of reduced glutathione may predispose diabetic patients to oxidative damage and to alpha-oxoaldehydemediated glycation by decreasing the in situ glyoxalase I activity. Recent studies of vascular endothelial cells in vitro have suggested that alpha-oxoaldehydes detoxified by glyoxalase I are the major precursors of advanced glycation end products implicated in the development of diabetic complications. The role of these factors in the development of diabetic complications and the prospective prevention of diabetic complications by supplementation of reduced glutathione and/or alpha-oxoaldehyde-scavenging agents now deserve investigation.

Synthesis and secretion of macrophage colony stimulating factor by mature human monocytes and human monocytic THP-1 cells induced by human serum albumin derivatives modified with methylglyoxal and glucose-derived advanced glycation endproducts

Human serum albumin minimally-modified by methylglyoxal (MGmin-HSA) stimulated the synthesis and secretion of macrophage-colony stimulating factor (M-CSF) by mature human monocytes in vitro. Human serum albumin minimally-modified by glucose-derived advanced glycation endproducts (AGEmin-HSA) and human serum albumin highly-modified by glucose-derived advanced glycation endproducts (AGE-HSA) stimulated much lower secretion of M-CSF from human monocytes than did MGmin-HSA. MGmin-HSA and AGE-HSA but not AGEmin-HSA also stimulated the growth of human monocytic THP-1 cells in vitro which was inhibited by polyclonal antibodies to human M-CSF. For MGmin-HSA, the median growth stimulatory concentration EC50 value was 0.24 +/- 0.07 microM and the maximal increase in cell growth was 36% of control cell growth (n = 24). Similar induction of secretion of M-CSF from monocytes in vivo may contribute to atherosclerosis in macro- and micro-angiopathy, particularly in the development of diabetic complications.

Antitumor activity of S-(p-bromobenzyl)glutathione diesters in vitro: a structure-activity study

S-(p-Bromobenzyl)glutathione is a competitive inhibitor of human glyoxalase I which is part of the cytosolic glyoxalase system. It may be delivered into the cystosol of cells by diesterification wherein it is deesterified by cytosolic nonspecific esterases. S-(p-Bromobenzyl)glutathione diesters had antitumor activity in vitro and in vivo. The inhibition of human leukemia 60 cell growth in vitro by a series of alkyl and cycloalkyl diesters of S-(p-bromobenzyl)glutathione was investigated. For n-alkyl diesters, the n-propyl diester was the most potent derivative with a median growth inhibitory concentration GC50 value of 7.77 +/- 0.01 microM (N = 18). The most potent derivative was S-(p-bromobenzyl)glutathione cyclopentyl diester which had a GC50 value of 4.23 +/- 0.01 microM (N = 21) and also had antitumor activity in vivo.

Pharmacology of methylglyoxal: formation, modification of proteins and nucleic acids, and enzymatic detoxification--a role in pathogenesis and antiproliferative chemotherapy

1. Methylglyoxal is a reactive alpha-oxoaldehyde and physiological metabolite formed by the fragmentation of triose-phosphates, and by the metabolism of acetone and aminoacetone. 2. Methylglyoxal modifies guanylate residues to form 6,7-dihydro-6,7-dihydroxy-6-methyl-imidazo[2,3-b]purine-9(8)one and N2-(1-carboxyethyl)guanylate residues and induces apoptosis. 3. Methylglyoxal modifies arginine residues in proteins to form N(delta)-(4,5-dihydroxy-4-methylimidazolidin-2-yl) ornithine, N(delta)-(5-hydro-5-methylimidazol-4-on-2-yl)ornithine and N(delta)-(5)methylimidazol-4-on-2-yl)ornithine residues. 4. Methylglyoxal-modified proteins undergo receptor-mediated endocytosis and lysosomal degradation in monocytes and macrophages, and induce cytokine synthesis and secretion. 5. Methylglyoxal is detoxified by the glyoxalase system. Decreased detoxification of methylglyoxal may be induced pharmacologically by glyoxalase I inhibitors which have anti-tumor and anti-malarial activities. 6. The modification of nucleic acids and protein by methylglyoxal is a signal for their degradation and may have a role in the development of diabetic complications, atherosclerosis, the immune response in starvation, aging and oxidative stress.

Antitumour activity of S-p-bromobenzylglutathione cyclopentyl diester in vitro and in vivo. Inhibition of glyoxalase I and induction of apoptosis

The glyoxalase I inhibitor diester, S-p-bromobenzyl-glutathione cyclopentyl diester (BrBzGSHCp2), inhibited the growth of human leukaemia 60 (HL60) cells in vitro. The median growth inhibitory concentration GC50 value of BrBzGSHCp2 was 4.23 +/- 0.001 microM (n = 21), and the median toxic concentration TC50 value was 8.86 +/- 0.01 microM (n = 21). BrBzGSHCp2 inhibited DNA synthesis in the third hr of incubation: the median inhibitory concentration IC50 value was 6.11 +/- 0.02 microM (n = 8). Incubation of HL60 cells with 10 microM BrBzGSHCp2 delivered the diester into cells: de-esterification of the diester there in lead to formation of the S-p-bromobenzylglutathione, inhibition of glyoxalase I activity in situ, increase in the methylglyoxal concentration after 1 hr, and induction of apoptosis after 6 hr. BrBzGSHCp2 (50-200 mg/kg) also inhibited the growth of murine adenocarcinoma 15A in vivo. Glyoxalase I inhibitor diesters may, therefore, inhibit tumour growth by inducing the accumulation of methylglyoxal in tumour cells, and induction of apoptosis.

Effect of methylglyoxal on human leukaemia 60 cell growth: modification of DNA G1 growth arrest and induction of apoptosis

Methylglyoxal induced growth arrest in the G1 phase of the cell cycle and toxicity in human leukaemia 60 cells in vitro. Inhibition of DNA synthesis but not inhibition of RNA synthesis, protein synthesis or inhibition of glyceraldehyde-3-phosphate dehydrogenase activity correlated with cytotoxicity. Incubation of human leukaemia 60 cells with methylglyoxal led to the rapid accumulation of adducts of methylglyoxal with DNA, and a lower accumulation of methylglyoxal adducts with RNA and protein in the initial hour of culture; fragmentation of nuclear DNA characteristic of apoptosis developed in the second hour of culture. Methylglyoxal induced apoptosis in human leukaemia 60 cells but did not affect the growth and viability of concanavalin A-stimulated human peripheral lymphocytes in vitro. These effects confirm and further substantiate the anti-proliferative anti-tumour activity of methylglyoxal in vitro, which may mediate the anti-tumour activity of glyoxalase I inhibitors in vivo.

Induction of synthesis and secretion of interleukin 1 beta in the human monocytic THP-1 cells by human serum albumins modified with methylglyoxal and advanced glycation endproducts

Human serum albumin modified with 1-2 methylglyoxal residues per molecule of protein (MGmin-HSA) stimulated the synthesis and secretion of interleukin 1 beta (IL-1 beta) from human monocytic THP-1 cells in vitro. It was a more potent inducer of IL-1 beta synthesis than human serum albumin highly-modified with glucose-derived advanced glycation endproducts (AGE-HSA). With 20 microM ligand. IL-1 beta synthesis was (pg/10(6) cells): MGmin-HSA 484.5 +/- 50.3; AGE-HSA 30.6 +/- 2.0 (n = 3). IL-1 beta synthesis increased markedly with MGmin-HSA concentrations > 5 microM. IL-1 beta synthesis and secretion from monocytes in response to methylglyoxal-modified proteins in vivo may contribute to the development of macro- and micro-angiopathy, particularly in diabetes mellitus.

Inhibition of human leukaemia 60 cell growth by mercapturic acid metabolites of phenylethyl isothiocyanate

Mercapturic acid pathway metabolites of phenylethyl isothiocyanate inhibited the growth of human leukaemia 60 (HL60) cells in vitro. The adduct with L-cysteine, S-(N-phenylethylthiocarbamoyl)cysteine, was the most potent with strong antileukaemic activity: the median growth inhibitory concentration (GC50) value was 336 +/- 1 nM (N = 18) compared with GC50 values of the precursor formed from dietary glucosinolates, phenylethyl isothiocyanate, 1.49 +/- 0.01 microM (N = 8), and the initial mercapturic acid pathway metabolite S-(N-phenylethylthiocarbamoyl)glutathione 5.46 +/- 0.36 microM (N = 18). S-(N-Benzylthiocarbamoyl)cysteine and S-(N-phenylpropylthiocarbamoyl)cysteine also had antiproliferative activity but S-(N-phenylethylthiocarbamoyl)cysteine was the most potent compound studied. The latter induced DNA fragmentation in HL60 cells but DNA laddering characteristic of apoptosis was not observed. It had low toxicity to corresponding differentiated cells, neutrophils, in culture, and therefore the cytotoxicity had selectivity for leukaemia cells. The antiproliferative activity of S-(N-phenylethylthiocarbamoyl)cysteine was lost during preincubation with culture medium, attributed to s-thiocarbamoyl transfer to serum proteins, which may decrease its effectiveness in vivo. The antiproliferative activity of S-(N-phenylalkylthiocarbamoyl)cysteine derivatives, by inhibiting tumour growth in pre-clinical development, may contribute to the association of decreased cancer incidence with dietary glucosinolate consumption.

Inhibition of human leukaemia 60 cell growth by S-D-lactoylglutathione in vitro. Mediation by metabolism to N-D-lactoylcysteine and induction of apoptosis

The inhibition of human leukaemia 60 cell growth by S-D-lactoylglutathione in vitro is mediated by the inhibtion of de novo pyridimine synthesis. When S-D-lactoylglutathione was added to human leukaemia 60 cells in culture, it was hydrolysed by thiolesterase activity to reduced glutathione and D-lactate but also converted to N-D-lactoylcysteinylglycine and N-D-lactoylcysteine by gamma-glutamyl transferase and dipeptidase. The N-D-lactoylcysteine inhibited human leukaemia 60 cell growth: the median growth inhibitory concentration IC(50) value was 46.7 +/ -0.9 (N=30) and the median toxic concentration TC(50) value was 103 +/- 1 microM. Other N-(R)2-hydroxyacylcysteine derivatives, N-D-mandelylcysteine and N-L-glyceroylcysteine, were less effective inhibitors of human leukaemia 60 cell growth, whereas N-D-lactoylcysteine ethyl ester was more effective: the IC(50) value was 16.5 +/- 1.5 microM(N=8). Cytotoxic concentrations of S-D-lactoylglutathione-induced apoptosis in human leukaemia 60 cells. The S-D-lactoylglutathione was not toxic to peripheral human lymphocytes at the same concentrations but rather induced growth arrest. The expected mechanism of action of N-D-lactoylcysteine is inhibition of dihydro-orotase, which is particularly susceptible to inhibition by cysteine derivatives.

Reduced glutathione esters--antidotes to toxicity. Cytotoxicity induced by hydrogen peroxide, 1-chloro-2,4-dinitrobenzene, and menadione in murine P388D1 macrophages in vitro

Repletion of depleted cellular reduced glutathione (GSH) levels in oxidative stress and exposure to arylating agents is a strategy for the development of antidotes to chemical toxicity. The effect of GSH, reduced glutathione ethyl monoester (GSHEt), and reduced glutathione ethyl diester (GSHEt2) on the cytotoxicity of hydrogen peroxide, 1-chloro-2,4-dinitrobenzene (CDNB), and menadione to P388D1 macrophages in vitro was investigated. The median toxic concentration TC50 values of the toxicants were hydrogen peroxide 24 +/- 2 mM (N = 19), CDNB 63 +/- 6 microM (N = 18), and menadione 30 +/- 4 microM (N = 22). Reduced glutathione, GSHEt, and GSHEt2 were poor antidotes to hydrogen peroxide toxicity. Indeed, the observed antidote effects were attributed to the nonenzymatic reaction of the GSH derivatives with hydrogen peroxide in the extracellular medium. Reduced glutathione ethyl diester was a more potent antidote of CDNB- and menadione-mediated toxicity than GSHEt and GSH. For cell incubations with the approximate median toxic concentration TC50 values of hydrogen peroxide, CDNB, and menadione, the respective median effective antidote concentration EC50 values were GSHEt 23.8 +/- 4.1 mM (N = 9), 3.6 +/- 0.6 mM (N = 11), and 226 +/- 93 microM (N = 12); and GSHEt2 20.4 +/- 1.9 mM (N = 6), 603 +/- 2 microM (N = 9), and 7.6 +/- 2.3 microM (N = 12). Reduced glutathione ethyl diester was a potent antidote to CDNB- and menadione-induced toxicities but not to hydrogen peroxide-induced toxicity under acute intoxication conditions.

Molecular characteristics of methylglyoxal-modified bovine and human serum albumins. Comparison with glucose-derived advanced glycation endproduct-modified serum albumins

The amino acid modification, gel filtration chromatographic, and electrophoretic characteristics of bovine and human serum albumins irreversibly modified by methylglyoxal (MG-SA) and by glucose-derived advanced glycation endproducts (AGE-SA) were investigated. Methylglyoxal selectively modified arginine residues at low concentration (1 mM); at high methylglyoxal concentration (100 mM), the extent of arginine modification increased and lysine residues were also modified. Both arginine and lysine residues were modified in AGE-SA. Analytical gel filtration HPLC of serum albumin derivatives suggested that the proportion of dimers and oligomers increased with modification in both low and highly modified MG-SA and AGE-SA derivatives relative to unmodified serum albumins. In SDS-PAGE analysis, dimers and oligomers of low-modified MG-SA were dissociated into monomers, but not in highly modified MG-SA. MG-SA had increased anodic electrophoretic mobility under nondenaturing conditions at pH 8.6, indicating an increased net negative charge, which increased with extent of modification; highly modified MG-SA and AGE-SA had similar high electrophoretic mobilities. MG-SA derivatives were fluorescent: the fluorescence was characteristic of the arginine-derived imidazolone N delta-(5-methyl-4-imidazolon-2-yl)ornithine, but other fluorophores were also present. AGE-SA had similar fluorescence, attributed, in part, to glucose-derived imidazolones. AGE formed from glucose-modified proteins and AGE-like compounds formed from methylglyoxal-modified proteins may both be signals for recognition and degradation of senescent macromolecules.