The Human Glyoxalase Gene Family in Health and Disease

The glyoxalase gene family consists of six structurally and functionally diverse enzymes with broad roles in metabolism. The common feature that defines this family is based on structural motifs that coordinate divalent cations which are required for activity. These family members have been implicated in a variety of physiological processes, including amino-acid metabolism (4-hydroxyphenylpyruvate dioxygenase; HPD), primary metabolism (methylmalonyl-CoA epimerase; MCEE), and aldehyde detoxication (glyoxalase 1; GLO1) and therefore have significant associations with disease. A central function of this family is the detoxification of reactive dicarbonyls (e.g., methylglyoxal), which react with cellular nucleophiles, resulting in the modification of lipids, proteins, and DNA. These damaging modifications activate canonical stress responses such as heat shock, unfolded protein, antioxidant, and DNA damage responses. Thus, glyoxalases serve an important role in homeostasis, preventing the pathogenesis of metabolic disease states, including obesity, diabetes, cardiovascular disease, renal failure, and aging. This review presents a thorough overview of the literature surrounding this diverse enzyme class. Although extensive literature exists for some members of this family (e.g., GLO1), little is known about the physiological role of glyoxalase domain-containing protein 4 (GLOD4) and 5 (GLOD5), paving the way for exciting avenues for future research.

Glyoxalase System as a Therapeutic Target against Diabetic Retinopathy

Hyperglycemia, a defining characteristic of diabetes, combined with oxidative stress, results in the formation of advanced glycation end products (AGEs). AGEs are toxic compounds that have adverse effects on many tissues including the retina and lens. AGEs promote the formation of reactive oxygen species (ROS), which, in turn, boost the production of AGEs, resulting in positive feedback loops, a vicious cycle that compromises tissue fitness. Oxidative stress and the accumulation of AGEs are etiologically associated with the pathogenesis of multiple diseases including diabetic retinopathy (DR). DR is a devastating microvascular complication of diabetes mellitus and the leading cause of blindness in working-age adults. The onset and development of DR is multifactorial. Lowering AGEs accumulation may represent a potential therapeutic approach to slow this sight-threatening diabetic complication. To set DR in a physiological context, in this review we first describe relations between oxidative stress, formation of AGEs, and aging in several tissues of the eye, each of which is associated with a major age-related eye pathology. We summarize mechanisms of AGEs generation and anti-AGEs detoxifying systems. We specifically feature the potential of the glyoxalase system in the retina in the prevention of AGEs-associated damage linked to DR. We provide a comparative analysis of glyoxalase activity in different tissues from wild-type mice, supporting a major role for the glyoxalase system in the detoxification of AGEs in the retina, and present the manipulation of this system as a therapeutic strategy to prevent the onset of DR.

Generation and characterization of mouse knockout for glyoxalase 1

Glyoxalase 1 (Glo1) is the first enzyme involved in glutathione-dependent detoxification of methylglyoxal, eventually generating d-lactate by the second enzyme glyoxalase 2 (Glo2). An accumulation of intracellular glyoxal and methylglyoxal leads to protein malfunction and mutation via formation of the advanced glycation end products (AGEs). Studies on mouse behavior suggest that methylglyoxal has anxiolytic properties. In this report, we generated and characterized a mouse knockout for Glo1. The knockout mice were viable without a pronounced phenotypic defect. Increased level of AGEs in Glo1 knockout mice was detected by immunoblotting with anti-MGH1 in liver homogenate, but not in brain. Alterations in behavior were observed in open field, light-dark transition, and tail suspension test. Open field data indicate increased exploration for novel environment and entry/stay in center zone in Glo1 knockout mice. In addition, increased light-dark transition and immobility was observed in the knockout mice. These data indicate that Glo1 knockout reduces anxiety-like behavior, but increases depression-like behavior.

Dicarbonyls and Advanced Glycation End-Products in the Development of Diabetic Complications and Targets for Intervention

Advanced glycation end-products (AGEs) are non-enzymatic protein and amino acid adducts as well as DNA adducts which form from dicarbonyls and glucose. AGE formation is enhanced in diabetes and is associated with the development of diabetic complications. In the current review, we discuss mechanisms that lead to enhanced AGE levels in the context of diabetes and diabetic complications. The methylglyoxal-detoxifying glyoxalase system as well as alternative pathways of AGE detoxification are summarized. Therapeutic approaches to interfere with different pathways of AGE formation are presented.

Measurement of glyoxalase activities

Glyoxalase I catalyses the isomerization of the hemithioacetal formed non-enzymatically from methylglyoxal and glutathione to S-D-lactoylglutathione. The activity of glyoxalase I is conventionally measured spectrophotometrically by following the increase in A240 for which the change in molar absorption coefficient Δε240=2.86 mM⁻¹·cm⁻¹. The hemithioacetal is pre-formed in situ by incubation of methylglyoxal and glutathione in 50 mM sodium phosphate buffer (pH 6.6) at 37°C for 10 min. The cell extract is then added, the A240 is monitored over 5 min, and the initial rate of increase in A240 and hence glyoxalase I activity deduced with correction for blank. Glyoxalase I activity is given in units per mg of protein or cell number where one unit is the amount of enzyme that catalyses the formation of 1 μmol of S-D-lactoylglutathione per min under assay conditions. Glyoxalase II catalyses the hydrolysis of S-D-lactoylglutathione to D-lactate and glutathione. Glyoxalase II activity is also measured spectrophotometrically by following the decrease in A240 for which the change in molar absorption coefficient Δε240=-3.10 mM⁻¹·cm⁻¹. It is given in units per mg of protein or cell number where one unit is the amount of enzyme that catalyses the hydrolysis of 1 μmol of S-D-lactoylglutathione per min under assay conditions. Glyoxalase I and glyoxalase II activity measurements have been modified for use with a UV-transparent microplate for higher sample throughput.

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.

Inhibition of radiation-induced changes of glyoxalase I activity in mouse spleen and liver by phenothiazines

Swiss albino mice (male) were irradiated with gamma-rays at a dose-rate of 0.05 Gy s-1, and the activities of glyoxalase I (GI) and glyoxalase II (GII) were determined after 24 h in the spleen and liver. Radiation up to 4 Gy increased the activity of GI and decreased that of GII. It was possible that the radiation-induced changes in the activity of the glyoxalase system, particularly that of GI, were suggestive of the regeneration status of the tissue. Phenothiazines such as chlorpromazine (CPZ), promethazine (PMZ) and trimeprazine (TMZ) inhibited the radiation-enhanced activity of GI in a concentration-dependent manner. On the other hand, almost no change in the activity of GII was observed using phenothiazines. The effect of phenothiazines on radiation-induced changes of glyoxalase activity were reversed in the presence of ferrous (Fe2+) ions. However, phenothiazines inhibited the radiation effect in the presence of ferric (Fe3+) ions. This combined effect was predominant in the liver. A possible mechanism for the modifying effect of phenothiazines is suggested.

Prevention of S-D-lactoylglutathione-induced inhibition of human leukaemia 60 cell growth by uridine

The anti-proliferative activity of S-D-lactoylglutathione is of interest since it has a low toxicity to differentiated and non-malignant proliferating tissues, and its mechanism of action appears to be dissimilar to other anti-proliferative agents. Addition of uridine completely and addition of cytidine partially prevented S-D-lactoylglutathione-induced growth inhibition of human leukaemia 60 (HL60) cells in vitro. Other nucleosides had no significant effect. The concentrations of UTP, CTP, UDP and also ATP, ADP, GTP and GDP decreased in S-D-lactoylglutathione-treated HL60 cells, whereas the concentration of UDP-N-acetylhexosamine (UDP-N-acetyl-glucosamine + N-acetyl-galactosamine) increased, prior to cell death. This suggests that the anti-proliferative effects of S-D-lactoylglutathione are mediated by inhibition of uridylate synthesis.

Purification and characterisation of glyoxalase II from human red blood cells

Glyoxalase II was purified from human red blood cells. The purification factor was 83,300 and the yield was 24% or 1.7 micrograms/ml red blood cells. The purified protein was a monomer with a molecular mass of 29,200 Da and an isoelectric point of 8.3. The rate of hydrolysis of S-D-lactoylglutathione to reduced glutathione and D-lactate, catalysed by glyoxalase II, followed Michaelis-Menten kinetics where the Km and kcat values were 146 +/- 9 microM and 727 +/- 16 s-1, respectively in 50 mM Tris/HCl, pH 7.4 at 37 degrees C. Other S-2-hydroxyacylglutathione derivatives were also acceptable substrates. S-p-Nitrobenzoxycarbonylglutathione was a potent competitive inhibitor of glyoxalase II with a Ki value of 1.20 +/- 0.21 microM, and the hemithioacetal formed non-enzymically from the reaction of methylglyoxal with reduced glutathione was a weak competitive inhibitor with a Ki value of 834 +/- 98 microM.

Characteristics of the inhibition of human promyelocytic leukaemia HL60 cell growth by S-D-lactoylglutathione in vitro

The mechanism of the inhibition of proliferation of human leukaemia 60 (HL60) cells by S-D-lactoylglutathione in vitro was investigated. The median inhibitory concentration IC50 value was 66 microM (95% C.I. 50-87 microM; n = 18). The inhibition of leukaemia cell growth required exposure of HL60 cells to S-D-lactoylglutathione (and metabolites) for 12 h, with maximum growth inhibition achieved after 24 h. Removal and replacement of culture medium within the initial 12 h of culture prevented inhibition of growth and toxicity. S-D-lactoylglutathione was consumed within the initial 3 h of culture. Pretreatment of culture medium containing 10% foetal calf serum for 3 h produced no subsequent inhibition of HL60 cell growth. Incubation of HL60 cells in culture medium with low serum content (5% v/v) produced a decreased rate of cell proliferation and a decreased response to S-D-lactoylglutathione. S-D-lactoylglutathione inhibited uptake of 3H-thymidine into DNA in the third hour of culture where the median inhibitory concentration IC50 value was 74 microM (95% C.I. 51-102; n = 10). The mechanism of inhibition of HL60 cell growth by S-D-lactoylglutathione is unknown but may be cell cycle related, mediated by inhibition of DNA synthesis and involve an active metabolite which may be removed and/or inactivated by a change in culture medium.

A simplified method for the purification of human red blood cell glyoxalase. I. Characteristics, immunoblotting, and inhibitor studies

Glyoxalase I (EC 4.4.1.5) was purified from human red blood cells by a simplified method using S-hexylglutathione affinity chromatography with a modified concentration gradient of S-hexylglutathione for elution. The pure protein had a specific activity of 1830 U/mg of protein, where the overall yield was 9%. The pure protein had a molecular mass of 46,000 D, comprised of two subunits of 23,000 D each, and an isoelectric point value of 5.1. The KM value for methylglyoxal-glutathione hemithioacetal was 192 +/- 8 microM and the kcat value was 10.9 +/- 0.2 x 10(4) min-1 (N = 15). The glyoxalase I inhibitor S-p-bromobenzylglutathione had a Ki value of 0.16 +/- 0.04 microM and S-p-nitrobenzoxycarbonylglutathione, previously thought to inhibit only glyoxalase II, also inhibited glyoxalase I with a Ki value of 3.12 +/- 0.88 microM. Reduced glutathione was a weak competitive inhibitor of glyoxalase I with a Ki value of 18 +/- 8 mM. The polyclonal antibodies were raised to the purified enzyme and were found to react specifically with glyoxalase I antigen by immunoblotting. This procedure gave a protein of high purity with simple low pressure chromatographic techniques with a moderate but adequate yield for small-scale preparations.

Effect of radiation on glyoxalase I and glyoxalase II activities in spleen and liver of mice

Swiss albino mice (7-8 weeks old) were irradiated with different doses (0-25 Gy) of gamma-radiation at a dose-rate of 0.05 Gy/s. The specific activities of glyoxalase I (GI) and glyoxalase II (GII) were determined in the spleen and liver immediately and on the 3rd and 6th day postirradiation. The results indicate that the glyoxalase system is radiosensitive, particularly glyoxalase I whose activity was enhanced even at low doses (0.5 Gy). The magnitude and mode of the radiation effect depends on dose and tissue. The patterns of the GI/GII ratio in the liver and spleen was very similar when measured immediately after irradiation. The radiation effect on the glyoxalase system persists even in the postirradiation period and was inversely related to the dose-rate. GSH and caffeine increased and chlorpromazine decreased the radiation-induced activity of GI, but all three modifiers enhanced radiation-induced inactivation of GII. Since the glyoxalase system may play an important role in the regulation of cell division and differentiation, radiation effects on this system may have some biochemical consequences.

Inhibition of growth of human leukaemia 60 cells by S-2-hydroxyacylglutathiones and monoethyl ester derivatives

S-2-Hydroxyacylglutathione derivatives were found to induce growth arrest and toxicity in human leukaemia 60 cells in culture. S-D-Lactoylglutathione was the most effective with a median inhibitory concentration IC50 of 82 microM (95% C.I. 65-105 microM). No similar toxicity was induced by reduced glutathione and/or the corresponding aldonic acid (500 microM) in human leukaemia 60 cells, nor by S-D-lactoylglutathione (500 microM) in mature human neutrophils under the same culture conditions. Monoethyl ester derivatives of the S-2-hydroxyacylglutathiones were prepared and also induced growth arrest and toxicity but were less effective than the corresponding unesterified compounds. S-2-Hydroxyacylglutathione derivatives also inhibited the incorporation of [3H]thymidine into DNA early in the development of toxicity: for S-D-lactoylglutathione, the median inhibitory concentration was 74 microM (95% C.I. 47-116 microM). The mechanism of the inhibition of human leukaemia cell growth by S-D-lactoylglutathione and other S-2-hydroxyacylglutathione derivatives is unknown but appears to be mediated by the inhibition of DNA synthesis.

Fluorimetric assay of D-lactate

A fluorimetric assay for D-lactate in human blood samples was developed using an endpoint enzymatic assay with D-lactate dehydrogenase from Staphylococcus epidermidis. The intrabatch and interbatch coefficients of variance were 8.7% (n = 4) and 16.6% (n = 4), respectively. The limit of detection in blood was 3.73 nmol/ml. The assay suffers minor interference from S-D-lactoylglutathione, which was also present in the blood samples. The concentration of D-lactate in blood was (mean +/- SE, nmol/ml) normal healthy individuals, 11.0 +/- 1.2 (n = 7); and diabetic patients, 20.0 +/- 1.3 (n = 55) (a significant increase in diabetes mellitus; P < 0.01, Mann-Whitney U test).