Fructosamine 3-kinase is involved in an intracellular deglycation pathway in human erythrocytes

Evolution of Escherichia coli strains under competent or compromised adaptive immunity

Escherichia coli is a commensal of the intestine of most mammals, but also an important human pathogen. Within a healthy human its population structure is highly dynamic, where typically a dominant E. coli strain is accompanied by several low abundance satellite strains. However, the factors underlying E. coli strain dynamics and evolution within hosts are still poorly understood. Here, we colonised germ-free immune-competent (wild-type) or immune-compromised (Rag2KO) mice, with two phylogenetically distinct strains of E. coli, to determine if strain co-existence and within-strain evolution are shaped by the adaptive immune system. Irrespectively of the immune status of the mice one strain reaches a 100-fold larger abundance than the other. However, the abundance of the dominant strain is significantly higher in Rag2KO mice. Strains co-exist for thousands of generations and accumulate beneficial mutations in genes coding for different resource preferences. A higher rate of mutation accumulation in immune-compromised vs. immune-competent mice is observed and adaptative mutations specific to immune-competent mice are identified. Importantly, the presence of the adaptive immune system selects for mutations that increase stress resistance and the dynamics of such evolutionary events associates with the onset of an antibody response.

Presumptive Identification of Clinically Significant Hemoglobin Variants Hb E, Hb S, Hb D in Hb A1c Capillary Electrophoresis

BACKGROUND

The quantitation of glycated hemoglobin (Hb A1c) represents an average blood glucose level for a period of 2 to 3 months for diagnosing, monitoring, and managing diabetes mellitus. Unreliable results are reported when hemoglobin (Hb) variants are present in the sample. Patients are advised to use an alternate method due to the presence of the variant Hb and a reflex test to Hb electrophoresis to obtain precise information about the Hb variant. The present study utilizes x axis values from Hb A1c capillary electrophoresis (CE) to identify clinically significant Hb variants Hb E, S, and D.

METHODS

Patient samples (n = 60) that showed a variant peak in the Hb A1c test with an x axis value of 190 to 240 were selected for the study. The migration position of the Hb variant (x axis value) and variant percent of the Hb A1c test were compared with the x axis value and variant percent in the Hb electrophoresis test to presumptively identify the variants. The identity of the variants was confirmed using mass spectrometry (MS).

RESULTS

Out of 60 samples, 20 samples were identified as Hb E (x axis 225-227), 20 samples were identified as Hb S (x axis 210-214), and 18 samples were identified as Hb D-Punjab (x axis 200-201). Two variants with an x axis value of 194 were identified as an α variant Hb Q India using MS. There is an overall negative shift of the x axis (-1 to -13 units) and a lower variant percent (-0.2% to -8.7%) in Hb A1c CE when compared with Hb electrophoresis.

CONCLUSIONS

The present study highlights the significance of the x axis value and variant percent to identify clinically significant Hb variants in the Hb A1c CE test.

Structural basis for FN3K-mediated protein deglycation

Protein glycation is a universal, non-enzymatic modification that occurs when a sugar covalently attaches to a primary amine. These spontaneous modifications may have deleterious or regulatory effects on protein function, and their removal is mediated by the conserved metabolic kinase fructosamine-3-kinase (FN3K). Despite its crucial role in protein repair, we currently have a poor understanding of how FN3K engages or phosphorylates its substrates. By integrating structural biology and biochemistry, we elucidated the catalytic mechanism for FN3K-mediated protein deglycation. Our work identifies key amino acids required for binding and phosphorylating glycated substrates and reveals the molecular basis of an evolutionarily conserved protein repair pathway. Additional structural-functional studies revealed unique structural features of human FN3K as well as differences in the dimerization behavior and regulation of FN3K family members. Our findings improve our understanding of the structure of FN3K and its catalytic mechanism, which opens new avenues for therapeutically targeting FN3K.

Assessing Glycosylated Hemoglobin Thresholds for Development of Cardiovascular Disease by Racial and Ethnic Groups

BACKGROUND

Diabetes is the strongest risk factor for cardiovascular disease, and although glycosylated hemoglobin (HbA1c) levels are known to vary by race, no racial and ethnic-specific diagnostic thresholds exist for diabetes in prediction of cardiovascular disease events. The purpose of this study is to determine whether HbA1c thresholds for predicting major adverse cardiovascular events (MACEs) differ among racial and ethnic groups.

METHODS AND RESULTS

This is a retrospective cohort study of Kaiser Permanente Northern California adult members (n=309 636) with no history of cardiovascular disease who had HbA1c values and race and ethnicity data available between 2014 and 2019. Multivariable logistic regression was used to evaluate the odds of MACEs by the following racial and ethnic groups: Filipino, South Asian, East Asian, Black, White, and Hispanic. A Youden index was used to calculate HbA1c thresholds for MACE prediction by each racial and ethnic group, stratified by sex. Among studied racial and ethnic groups, South Asian race was associated with the greatest odds of MACEs (1.641 [95% CI, 1.456-1.843]; P<0.0001). HbA1c was a positive predictor for MACEs, with an odds ratio of 1.024 (95% CI, 1.022-1.025) for each 0.1% increment increase in HbA1c. HbA1c values varied between 6.0% and 7.6% in MACE prediction by race and ethnicity and sex. White individuals, South Asian individuals, East Asian women, and Black men had HbA1c thresholds for MACE prediction in the prediabetic range, between 6.0% and 6.2%. Black women, Hispanic men, and East Asian men had HbA1c thresholds of 6.2% to 6.6%, less than the typical threshold of 7.0% that is used as a treatment goal.

CONCLUSIONS

Findings suggest that the use of race and ethnic- and sex-specific HbA1c thresholds may need to be considered in treatment goals and cardiovascular disease risk estimation.

Deglycation activity of the Escherichia coli glycolytic enzyme phosphoglucose isomerase

Glycation is a spontaneous chemical reaction, which affects the structure and function of proteins under normal physiological conditions. Therefore, organisms have evolved diverse mechanisms to combat glycation. In this study, we show that the Escherichia coli glycolytic enzyme phosphoglucose isomerase (Pgi) exhibits deglycation activity. We found that E. coli Pgi catalyzes the breakdown of glucose 6-phosphate (G6P)-derived Amadori products (APs) in chicken lysozyme. The affinity of Pgi to the glycated lysozyme (Km, 1.1 mM) was ten times lower than the affinity to its native substrate, fructose 6-phosphate (Km, 0.1 mM). However, the high kinetic constants of the enzyme with the glycated lysozyme (kcat, 396 s-1 and kcat/Km, 3.6 × 105 M-1 s-1) indicated that the Pgi amadoriase activity may have physiological implications. Indeed, when using total E. coli protein (20 mg/mL) as a substrate in the deglycation reaction, we observed a release of G6P from the bacterial protein at a Pgi specific activity of 33 μmol/min/mg. Further, we detected 11.4 % lower APs concentration in protein extracts from Pgi-proficient vs. deficient cells (p = 0.0006) under conditions where the G6P concentration in Pgi-proficient cells was four times higher than in Pgi-deficient cells (p = 0.0001). Altogether, these data point to physiological relevance of the Pgi deglycation activity.

New Horizons: Testosterone or Exercise for Cardiometabolic Health in Older Men

Middle-aged and older men have typically accumulated comorbidities, are increasingly sedentary, and have lower testosterone concentrations (T) compared to younger men. Reduced physical activity (PA) and lower T both are associated with, and may predispose to, metabolically adverse changes in body composition, which contribute to higher risks of cardiometabolic disease. Exercise improves cardiometabolic health, but sustained participation is problematic. By contrast, rates of T prescription have increased, particularly in middle-aged and older men without organic diseases of the hypothalamus, pituitary, or testes, reflecting the unproven concept of a restorative hormone that preserves health. Two recent large randomized trials of T, and meta-analyses of randomized trials, did not show a signal for adverse cardiovascular (CV) events, and T treatment on a background of lifestyle intervention reduced type 2 diabetes by 40% in men at high risk. Men with both higher endogenous T and higher PA levels have lower CV risk, but causality remains unproven. Exercise training interventions improve blood pressure and endothelial function in middle-aged and older men, without comparable benefits or additive effects of T treatment. Therefore, exercise training improves cardiometabolic health in middle-aged and older men when effectively applied as a supervised regimen incorporating aerobic and resistance modalities. Treatment with T may have indirect cardiometabolic benefits, mediated via favorable changes in body composition. Further evaluation of T as a pharmacological intervention to improve cardiometabolic health in aging men could consider longer treatment durations and combination with targeted exercise programs.

Michaelis-Menten kinetic modeling of hemoglobin A1c status facilitates personalized glycemic control

INTRODUCTION

Discrepancy between measured HbA1c and HbA1c calculated from plasma glucose is associated with higher risk for diabetic complications. However, quantification of this difference is inaccurate due to the imperfect linear conversion models. We propose to introduce a mathematical formula that correlates with the observational data and supports individualized glycemic control.

METHODS

We analysed 175,437 simultaneous plasma glucose and HbA1c records stored in our laboratory database. Employing the Michaelis-Menten (MM) equation, we compared the calculated HbA1c levels to the measured HbA1c levels. Data from patients with multiple records were used to establish the patients' glycemic status and to assess the predictive power of our MM model.

RESULTS

HbA1c levels calculated with the MM equation closely matched the population's average HbA1c levels. The Michaelis constant (Km) had a negative correlation with HbA1c (r2 = 0.403). Using personalized Km values in the MM equation, 85.1% of HbA1c predictions were within 20% error (ADAG calculation: 78.4%). MM prediction also performed better in predicting pathologic HbA1c levels (0.904 AUC vs. 0.849 AUC for ADAG).

CONCLUSION

MM equation is an improvement over linear models and could be readily employed in routine diabetes management. Km is a reliable and quantifiable marker to characterize variations in glucose tolerance.

Inactivation of Minar2 in mice hyperactivates mTOR signaling and results in obesity

OBJECTIVE

Obesity is a complex disorder and is linked to chronic diseases such as type 2 diabetes. Major intrinsically disordered NOTCH2-associated receptor2 (MINAR2) is an understudied protein with an unknown role in obesity and metabolism. The purpose of this study was to determine the impact of Minar2 on adipose tissues and obesity.

METHOD

We generated Minar2 knockout (KO) mice and used various molecular, proteomic, biochemical, histopathology, and cell culture studies to determine the pathophysiological role of Minar2 in adipocytes.

RESULTS

We demonstrated that the inactivation of Minar2 results in increased body fat with hypertrophic adipocytes. Minar2 KO mice on a high-fat diet develop obesity and impaired glucose tolerance and metabolism. Mechanistically, Minar2 interacts with Raptor, a specific and essential component of mammalian TOR complex 1 (mTORC1) and inhibits mTOR activation. mTOR is hyperactivated in the adipocytes deficient for Minar2 and over-expression of Minar2 in HEK-293 cells inhibited mTOR activation and phosphorylation of mTORC1 substrates, including S6 kinase, and 4E-BP1.

CONCLUSION

Our findings identified Minar2 as a novel physiological negative regulator of mTORC1 with a key role in obesity and metabolic disorders. Impaired expression or activation of MINAR2 could lead to obesity and obesity-associated diseases.

Enzymatic Deglycation of Damaged Skin by Means of Combined Treatment of Fructosamine-3-Kinase and Fructosyl-Amino Acid Oxidase

The consensus in aging is that inflammation, cellular senescence, free radicals, and epigenetics are contributing factors. Skin glycation through advanced glycation end products (AGEs) has a crucial role in aging. Additionally, it has been suggested that their presence in scars leads to elasticity loss. This manuscript reports fructosamine-3-kinase (FN3K) and fructosyl-amino acid oxidase (FAOD) in counteracting skin glycation by AGEs. Skin specimens were obtained (n = 19) and incubated with glycolaldehyde (GA) for AGE induction. FN3K and FAOD were used as monotherapy or combination therapy. Negative and positive controls were treated with phosphate-buffered saline and aminoguanidine, respectively. Autofluorescence (AF) was used to measure deglycation. An excised hypertrophic scar tissue (HTS) (n = 1) was treated. Changes in chemical bonds and elasticity were evaluated using mid-infrared spectroscopy (MIR) and skin elongation, respectively. Specimens treated with FN3K and FAOD in monotherapy achieved an average decrease of 31% and 33% in AF values, respectively. When treatments were combined, a decrease of 43% was achieved. The positive control decreased by 28%, whilst the negative control showed no difference. Elongation testing of HTS showed a significant elasticity improvement after FN3K treatment. ATR-IR spectra demonstrated differences in chemical bounds pre- versus post-treatment. FN3K and FAOD can achieve deglycation and the effects are most optimal when combined in one treatment.

Comparison of the loci associated with HbA1c and blood glucose levels identified by a genome-wide association study in the Japanese population

Aims

Hemoglobin A1c (HbA1c) levels are widely employed to diagnose diabetes. However, estimates of the heritability of HbA1c and glucose levels are different. Therefore, we explored HbA1c- and blood glucose-associated loci in a non-diabetic Japanese population.

Methods

We conducted a two-stage genome-wide association study (GWAS) on variants associated with HbA1c and blood glucose levels in a Japanese population. In the initial stage, data of 4911 participants of the Japan Multi-Institutional Collaborative Cohort (J-MICC) were subjected to discovery analysis. In the second stage, two datasets from the Tohoku Medical Megabank project, with 8175 and 40,519 participants, were used for the replication study. Association of the imputed variants with HbA1c and blood glucose levels was determined via linear regression analyses adjusted for age, sex, body mass index (BMI), smoking, and genetic principal components (PC1-PC10). Moreover, we performed a BMI-stratified GWAS on HbA1c levels in the J-MICC. The discovery analysis and BMI-stratified GWAS results were validated with re-analyses of normalized HbA1c levels adjusted for site in addition to the above, and blood glucose adjusted for fasting time as an additional covariate.

Results

Genetic variants associated with HbA1c levels were identified in KCNQ1 and TMC6. None of the genetic variants associated with blood glucose levels in the discovery analysis were replicated. Association of rs2299620 in KCNQ1 with HbA1c levels showed heterogeneity between individuals with BMI ≥ 25 kg/m2 and BMI < 25 kg/m2.

Conclusions

The variant rs2299620 in KCNQ1 might affect HbA1c levels differentially based on BMI grouping in the Japanese population.

Supplementary Information

The online version contains supplementary material available at 10.1007/s13340-023-00618-0.

Impact of haemoglobin variants on the use of haemoglobin A1c for the diagnosis and monitoring of diabetes: a contextualised review

HbA1c is the established test for monitoring glycaemic control in diabetes, and intervention trials studying the impact of treatment on glycaemic control and risk of complications focus predominantly on this parameter in terms of evaluating the glycaemic outcomes. It is also the main parameter used when targets for control are being individualised, and more recently, it has been used for the diagnosis of type 2 diabetes. For laboratories performing this test and clinicians utilising it in their decision-making process, a thorough understanding of factors that can impact on the accuracy, and appropriate interpretation of the test is essential. The changing demographic in the Irish population over the last two decades has brought this issue sharply into focus. It is therefore timely to review the utility, performance and interpretation of the HbA1c test to highlight factors impacting on the results, specifically the impact of haemoglobin variants, and the impact of these factors on its utilisation in clinical practice.

Yeast-produced fructosamine-3-kinase retains mobility after ex vivo intravitreal injection in human and bovine eyes as determined by Fluorescence Correlation Spectroscopy

Globally, over 2 billion people suffer from vision impairment. Despite complex multifactorial etiology, advanced glycation end products are involved in the pathogenesis of many causative age- and diabetes-related eye diseases. Deglycating enzyme fructosamine-3-kinase (FN3K) was recently proposed as a potential therapeutic, but for further biopharmaceutical development, knowledge on its manufacturability and stability and mobility in the vitreous fluid of the eye is indispensable. We evaluated recombinant production of FN3K in two host systems, and its diffusion behavior in both bovine and human vitreous. Compared to Escherichia coli, intracellular production in Pichia pastoris yielded more and higher purity FN3K. The yeast-produced enzyme was used in a first attempt to use fluorescence correlation spectroscopy to study protein mobility in non-sonicated bovine vitreous, human vitreous, and intact bovine eyes. It was demonstrated that FN3K retained mobility upon intravitreal injection, although a certain delay in diffusion was observed. Alkylation of free cysteines was tolerated both in terms of enzymatic activity and vitreous diffusion. Ex vivo diffusion data gathered and the availability of yeast-produced high purity enzyme now clear the path for in vivo pharmacokinetics studies of FN3K.

Resistance to glycation in the zebra finch: Mass spectrometry-based analysis and its perspectives for evolutionary studies of aging

In humans, hyperglycemia is associated with protein glycation, which may contribute to aging. Strikingly, birds usually outlive mammals of the same body mass, while exhibiting high plasma glucose levels. However, how birds succeed in escaping pro-aging effects of glycation remains unknown. Using a specific mass spectrometry-based approach in captive zebra finches of known age, we recorded high glycaemia values but no glycated hemoglobin form was found. Still, we showed that zebra finch hemoglobin can be glycated in vitro, albeit only to a limited extent compared to its human homologue. This may be due to peculiar structural features, as supported by the unusual presence of three different tetramer populations with balanced proportions and a still bound cofactor that could be inositol pentaphosphate. High levels of the glycated forms of zebra finch plasma serotransferrin, carbonic anhydrase 2, and albumin were measured. Glucose, age or body mass correlations with either plasma glycated proteins or hemoglobin isoforms suggest that those variables may be future molecular tools of choice to monitor glycation and its link with individual fitness. Our molecular advance may help determine how evolution succeeded in associating flying ability, high blood glucose and long lifespan in birds.

Role of Atypical Chemokines and Chemokine Receptors Pathways in the Pathogenesis of COPD

Chronic obstructive pulmonary disease (COPD) represents a heightened inflammatory response in the lung generally resulting from tobacco smoking-induced recruitment and activation of inflammatory cells and/or activation of lower airway structural cells. Several mediators can modulate activation and recruitment of these cells, particularly those belonging to the chemokines (conventional and atypical) family. There is emerging evidence for complex roles of atypical chemokines and their receptors (such as high mobility group box 1 (HMGB1), antimicrobial peptides, receptor for advanced glycosylation end products (RAGE) or toll-like receptors (TLRs)) in the pathogenesis of COPD, both in the stable disease and during exacerbations. Modulators of these pathways represent potential novel therapies for COPD and many are now in preclinical development. Inhibition of only a single atypical chemokine or receptor may not block inflammatory processes because there is redundancy in this network. However, there are many animal studies that encourage studies for modulating the atypical chemokine network in COPD. Thus, few pharmaceutical companies maintain a significant interest in developing agents that target these molecules as potential antiinflammatory drugs. Antibody-based (biological) and small molecule drug (SMD)-based therapies targeting atypical chemokines and/or their receptors are mostly at the preclinical stage and their progression to clinical trials is eagerly awaited. These agents will most likely enhance our knowledge about the role of atypical chemokines in COPD pathophysiology and thereby improve COPD management.

Protein glycation - biomarkers of metabolic dysfunction and early-stage decline in health in the era of precision medicine

Protein glycation provides a biomarker in widespread clinical use, glycated hemoglobin HbA1c (A1C). It is a biomarker for diagnosis of diabetes and prediabetes and of medium-term glycemic control in patients with established diabetes. A1C is an early-stage glycation adduct of hemoglobin with glucose; a fructosamine derivative. Glucose is an amino group-directed glycating agent, modifying N-terminal and lysine sidechain amino groups. A similar fructosamine derivative of serum albumin, glycated albumin (GA), finds use as a biomarker of glycemic control, particularly where there is interference in use of A1C. Later stage adducts, advanced glycation endproducts (AGEs), are formed by the degradation of fructosamines and by the reaction of reactive dicarbonyl metabolites, such as methylglyoxal. Dicarbonyls are arginine-directed glycating agents forming mainly hydroimidazolone AGEs. Glucosepane and pentosidine, an intense fluorophore, are AGE covalent crosslinks. Cellular proteolysis of glycated proteins forms glycated amino acids, which are released into plasma and excreted in urine. Development of diagnostic algorithms by artificial intelligence machine learning is enhancing the applications of glycation biomarkers. Investigational glycation biomarkers are in development for: (i) healthy aging; (ii) risk prediction of vascular complications of diabetes; (iii) diagnosis of autism; and (iv) diagnosis and classification of early-stage arthritis. Protein glycation biomarkers are influenced by heritability, aging, decline in metabolic, vascular, renal and skeletal health, and other factors. They are applicable to populations of differing ethnicities, bridging the gap between genotype and phenotype. They are thereby likely to find continued and expanding clinical use, including in the current era of developing precision medicine, reporting on multiple pathogenic processes and supporting a precision medicine approach.

A Potential Role for Fructosamine-3-Kinase in Cataract Treatment

Cataracts are the major cause of blindness worldwide, largely resulting from aging and diabetes mellitus. Advanced glycation end products (AGEs) have been identified as major contributors in cataract formation because they alter lens protein structure and stability and induce covalent cross-linking, aggregation, and insolubilization of lens crystallins. We investigated the potential of the deglycating enzyme fructosamine-3-kinase (FN3K) in the disruption of AGEs in cataractous lenses. Macroscopic changes of equine lenses were evaluated after ex vivo intravitreal FN3K injection. The mechanical properties of an equine lens pair were evaluated after treatment with saline and FN3K. AGE-type autofluorescence (AF) was measured to assess the time-dependent effects of FN3K on glycolaldehyde-induced AGE-modified porcine lens fragments and to evaluate its actions on intact lenses after in vivo intravitreal FN3K injection of murine eyes. A potential immune response after injection was evaluated by analysis of IL-2, TNFα, and IFNγ using an ELISA kit. Dose- and time-dependent AF kinetics were analyzed on pooled human lens fragments. Furthermore, AF measurements and a time-lapse of macroscopic changes were performed on intact cataractous human eye lenses after incubation with an FN3K solution. At last, AF measurements were performed on cataractous human eyes after crossover topical treatment with either saline- or FN3K-containing drops. While the lenses of the equine FN3K-treated eyes appeared to be clear, the saline-treated lenses had a yellowish-brown color. Following FN3K treatment, color restoration could be observed within 30 min. The extension rate of the equine FN3K-treated lens was more than twice the extension rate of the saline-treated lens. FN3K treatment induced significant time-dependent decreases in AGE-related AF values in the AGE-modified porcine lens fragments. Furthermore, in vivo intravitreal FN3K injection of murine eyes significantly reduced AF values of the lenses. Treatment did not provoke a systemic immune response in mice. AF kinetics of FN3K-treated cataractous human lens suspensions revealed dose- and time-dependent decreases. Incubation of cataractous human eye lenses with FN3K resulted in a macroscopic lighter color of the cortex and a decrease in AF values. At last, crossover topical treatment of intact human eyes revealed a decrease in AF values during FN3K treatment, while showing no notable changes with saline. Our study suggests, for the first time, a potential additional role of FN3K as an alternative treatment for AGE-related cataracts.

FN3K expression in COPD: a potential comorbidity factor for cardiovascular disease

INTRODUCTION

Cigarette smoking and oxidative stress are common risk factors for the multi-morbidities associated with chronic obstructive pulmonary disease (COPD). Elevated levels of advanced glycation endproducts (AGE) increase the risk of cardiovascular disease (CVD) comorbidity and mortality. The enzyme fructosamine-3-kinase (FN3K) reduces this risk by lowering AGE levels.

METHODS

The distribution and expression of FN3K protein in lung tissues from stable COPD and control subjects, as well as an animal model of COPD, was assessed by immunohistochemistry. Serum FN3K protein and AGE levels were assessed by ELISA in patients with COPD exacerbations receiving metformin. Genetic variants within the FN3K and FN3K-RP genes were evaluated for associations with cardiorespiratory function in the Subpopulations and Intermediate Outcome Measures in COPD Study cohort.

RESULTS

This pilot study demonstrates that FN3K expression in the blood and human lung epithelium is distributed at either high or low levels irrespective of disease status. The percentage of lung epithelial cells expressing FN3K was higher in control smokers with normal lung function, but this induction was not observed in COPD patients nor in a smoking model of COPD. The top five nominal FN3K polymorphisms with possible association to decreased cardiorespiratory function (p<0.008-0.02), all failed to reach the threshold (p<0.0028) to be considered highly significant following multi-comparison analysis. Metformin enhanced systemic levels of FN3K in COPD subjects independent of their high-expression or low-expression status.

DISCUSSION

The data highlight that low and high FN3K expressors exist within our study cohort and metformin induces FN3K levels, highlighting a potential mechanism to reduce the risk of CVD comorbidity and mortality.

Fructosamine-3-Kinase as a Potential Treatment Option for Age-Related Macular Degeneration

Age-related macular degeneration is the leading cause of blindness in the developed world. Since advanced glycation end products (AGEs) are implicated in the pathogenesis of AMD through various lines of evidence, we investigated the potential of fructosamine-3-kinase (FN3K) in the disruption of retinal AGEs, drusenoid material and drusenoid lesions in patients with AMD. AGE-type autofluorescence was measured to evaluate the effects of FN3K on glycolaldehyde-induced AGE-modified neural porcine retinas and unmodified human neural retinas. Eye pairs from cigarette-smoke- and air-exposed mice were treated and evaluated histologically. Automated optical image analysis of human tissue sections was performed to compare control- and FN3K-treated drusen and near-infrared (NIR) microspectroscopy was performed to examine biochemical differences. Optical coherence tomography (OCT) was used to evaluate the effect of FN3K on drusenoid deposits after treatment of post-mortem human eyes. FN3K treatment provoked a significant decrease (41%) of AGE-related autofluorescence in the AGE-modified porcine retinas. Furthermore, treatment of human neural retinas resulted in significant decreases of autofluorescence (−24%). FN3K-treated murine eyes showed less drusenoid material. Pairwise comparison of drusen on tissue sections revealed significant changes in color intensity after FN3K treatment. NIR microspectroscopy uncovered clear spectral differences in drusenoid material (Bruch’s membrane) and drusen after FN3K treatment. Ex vivo treatment strongly reduced size of subretinal drusenoid lesions on OCT imaging (up to 83%). In conclusion, our study demonstrated for the first time a potential role of FN3K in the disruption of AGE-related retinal autofluorescence, drusenoid material and drusenoid lesions in patients with AMD.

Bioremediation of a Common Product of Food Processing by a Human Gut Bacterium

Dramatic increases in processed food consumption represent a global health threat. Maillard reaction products (MRPs), which are common in processed foods, form upon heat-induced reaction of amino acids with reducing sugars and include advanced glycation end products with deleterious health effects. To examine how processed foods affect the microbiota, we fed gnotobiotic mice, colonized with 54 phylogenetically diverse human gut bacterial strains, defined sugar-rich diets containing whey as the protein source or a matched amino acid mixture. Whey or ϵ-fructoselysine, an MRP in whey and many processed foods, selectively increases Collinsella intestinalis absolute abundance and induces Collinsella expression of genomic loci directing import and metabolism of ϵ-fructoselysine to innocuous products. This locus is repressed by glucose in C. aerofaciens, whose abundance decreases with whey, but is not repressed in C. intestinalis. Identifying gut organisms responding to and degrading potentially harmful processed food components has implications for food science, microbiome science, and public health.

Potential Clinical Error Arising From Use of HbA1c in Diabetes: Effects of the Glycation Gap

The glycation gap (GGap) and the similar hemoglobin glycation index (HGI) define consistent differences between glycated hemoglobin and actual glycemia derived from fructosamine or mean blood glucose, respectively. Such a disparity may be found in a substantial proportion of people with diabetes, being >1 U of glycated HbA1c% or 7.2 mmol/mol in almost 40% of estimations. In this review we define these indices and explain how they can be calculated and that they are not spurious, being consistent in individuals over time. We evaluate the evidence that GGap and HGI are associated with variation in risk of complications and mortality and demonstrate the potential for clinical error in the unquestioning use of HbA1c. We explore the underlying etiology of the variation of HbA1c from mean glucose in blood plasma, including the potential role of enzymatic deglycation of hemoglobin by fructosamine-3-kinase. We conclude that measurement of GGap and HGI are important to diabetes clinicians and their patients in individualization of therapy and the avoidance of harm arising from consequent inappropriate assessment of glycemia and use of therapies.

Studies of advanced glycation end products and oxidation biomarkers for type 2 diabetes

Advanced glycation end products (AGEs) are formed upon nonenzymatic reactions of sugars or their metabolites with proteins and other cellular constituents. Many AGEs are long lived. Recent findings suggest that AGEs may predict diabetes and its complications and thus may warrant further study. The objective of this study was to assess the validity of our experimental procedures for measuring AGEs in stored blood sample and to conduct a pilot study for developing AGE biomarkers for diabetes and/or age-related changes of glucose metabolism. We conducted a reliability study of the samples and methods using liquid chromatography-tandem mass spectrometry (LC-MS)/MS assays for 10 AGEs (including methylglyoxal-derived hydroimidazolone (MG-H1), glucosepane (GSP) and two oxidation measures, in stored repository blood samples from the Nurses' Health Study and the Health Professionals Follow-up Study. We also analyzed data relating blood GSP levels to type 2 diabetes status in a case-control study (25 cases and 15 controls). Among the AGEs, GSP, and MG-H1 showed the highest reliability across the various measures: reliability in duplicate samples and stability with delayed processing and storage over 1-2 year period. Furthermore, plasma GSP was associated with older age (P = 0.04) and type 2 diabetes status (age-adjusted P = 0.0475). Our findings suggest that analysis of these AGEs may be developed as biomarkers for diabetes and/or age-related changes of glucose metabolism. © 2018 BioFactors, 44(3):281-288, 2018.

Kidney, heart and brain: three organs targeted by ageing and glycation

Advanced glycation end-product (AGE) is the generic term for a heterogeneous group of derivatives arising from a non-enzymatic reaction between reducing sugars and proteins. In recent years, evidence has accumulated that incriminates AGEs in pathogenic processes associated with both chronic hyperglycaemia and age-related diseases. Regardless of their exogenous or endogenous origin, the accumulation of AGEs and their derivatives could promote accelerated ageing by leading to protein modifications and activating several inflammatory signalling pathways via AGE-specific receptors. However, it remains to be demonstrated whether preventing the accumulation of AGEs and their effects is an important therapeutic option for successful ageing. The present review gives an overview of the current knowledge on the pathogenic role of AGEs by focusing on three AGE target organs: kidney, heart and brain. For each of these organs we concentrate on an age-related disease, each of which is a major public health issue: chronic kidney disease, heart dysfunction and neurodegenerative diseases. Even though strong connections have been highlighted between glycation and age-related pathogenesis, causal links still need to be validated. In each case, we report evidence and uncertainties suggested by animal or epidemiological studies on the possible link between pathogenesis and glycation in a chronic hyperglycaemic state, in the absence of diabetes, and with exogenous AGEs alone. Finally, we present some promising anti-AGE strategies that are currently being studied.

Schiff bases of putrescine with methylglyoxal protect from cellular damage caused by accumulation of methylglyoxal and reactive oxygen species in Dictyostelium discoideum

Polyamines protect protein glycation in cells against the advanced glycation end product precursor methylglyoxal, which is inevitably produced during glycolysis, and the enzymes that detoxify this α-ketoaldehyde have been widely studied. Nonetheless, nonenzymatic methylglyoxal-scavenging molecules have not been sufficiently studied either in vitro or in vivo. Here, we hypothesized reciprocal regulation between polyamines and methylglyoxal modeled in Dictyostelium grown in a high-glucose medium. We based our hypothesis on the reaction between putrescine and methylglyoxal in putrescine-deficient (odc^-) or putrescine-overexpressing (odc^oe) cells. In these strains, growth and cell cycle were found to be dependent on cellular methylglyoxal and putrescine contents. The odc^- cells showed growth defects and underwent G1 phase cell cycle arrest, which was efficiently reversed by exogenous putrescine. Cellular methylglyoxal, reactive oxygen species (ROS), and glutathione levels were remarkably changed in odc^oe cells and odc̄ cells. These results revealed that putrescine may act as an intracellular scavenger of methylglyoxal and ROS. Herein, we observed interactions of putrescine and methylglyoxal via formation of a Schiff base complex, by UV-vis spectroscopy, and confirmed this adduct by liquid chromatography with mass spectrometry via electrospray ionization. Schiff bases were isolated, analyzed, and predicted to have molecular masses ranging from 124 to 130. We showed that cellular putrescine-methylglyoxal Schiff bases were downregulated in proportion to the levels of endogenous or exogenous putrescine and glutathione in the odc mutants. The putrescine-methylglyoxal Schiff base affected endogenous metabolite levels. This is the first report showing that cellular methylglyoxal functions as a signaling molecule through reciprocal interactions with polyamines by forming Schiff bases.

A simple colorimetric assay for measuring fructosamine 3 kinase activity

BACKGROUND

Fructosamine 3 kinase (FN3K) is a deglycating enzyme, which may play a key role in reducing diabetes-induced organ damage by removing bound glucose from glycated proteins. We wanted to develop a simple colorimetric method for assaying FN3K activity in human body fluids.

METHODS

Glycated bovine serum albumin (BSA) was obtained by glycation with a 10% glucose solution at 37 °C. After 72 h, glycated BSA was dialyzed against phosphate buffered saline (0.1 mol/L, pH 7.4). The dialyzed solution (containing ±1000 µmol/L fructosamine) was used as an FN3K substrate. In the assay, 300 µL of substrate was incubated with 50 µL of serum and 100 µL of MgCl2 (0.7 mmol/L)/ATP (3.2 mmol/L). The fructosamine concentration was determined at the start and after incubation (120 min, 25 °C). The decrease in fructosamine concentration over time is a measure for the FN3K activity (1 U corresponding to 1 µmol/min). Concomitantly, the FN3K SNP rs1056534 and the ferroportin SNP rs1156350 were genotyped.

RESULTS

Within-assay CV was 6.0%. Reference values for FN3K activity in serum were 14.2±1.6 U/L (n=143). Reference values for FN3K were neither age- nor sex-dependent. The various FN3K SNP rs1056534 genotypes showed no significant differences in serum FN3K activity. In diabetics (n=191), values (14.0±2.2 U/L) were comparable to those of the controls. FN3K activity in erythrocytes was significantly higher (170.3±7.6 U/L). The intra-erythrocytic FN3K activity makes the results prone to hemolysis. FN3K activity depended on the ferroportin Q248H genotypes, with the highest value for the wild type genotype. Neither transferrin saturation nor ferritin were confounders for the FN3K activity. FN3K activity was significantly (p<0.0001) correlated with HbA1c values, although the correlation between FN3K and HbA1c was weak.

CONCLUSIONS

The simple colorimetric method allows determining FN3K activity in human serum. The assay may be useful for studying the impact of deglycation processes in diabetes mellitus.

A Mannose Family Phosphotransferase System Permease and Associated Enzymes Are Required for Utilization of Fructoselysine and Glucoselysine in Salmonella enterica Serovar Typhimurium

Salmonella enteric serovar Typhimurium, a major cause of food-borne illness, is capable of using a variety of carbon and nitrogen sources. Fructoselysine and glucoselysine are Maillard reaction products formed by the reaction of glucose or fructose, respectively, with the ε-amine group of lysine. We report here that S. Typhimurium utilizes fructoselysine and glucoselysine as carbon and nitrogen sources via a mannose family phosphotransferase (PTS) encoded by gfrABCD (glucoselysine/fructoselysine PTS components EIIA, EIIB, EIIC, and EIID; locus numbers STM14_5449 to STM14_5454 in S. Typhimurium 14028s). Genes coding for two predicted deglycases within the gfr operon, gfrE and gfrF, were required for growth with glucoselysine and fructoselysine, respectively. GfrF demonstrated fructoselysine-6-phosphate deglycase activity in a coupled enzyme assay. The biochemical and genetic analyses were consistent with a pathway in which fructoselysine and glucoselysine are phosphorylated at the C-6 position of the sugar by the GfrABCD PTS as they are transported across the membrane. The resulting fructoselysine-6-phosphate and glucoselysine-6-phosphate subsequently are cleaved by GfrF and GfrE to form lysine and glucose-6-phosphate or fructose-6-phosphate. Interestingly, although S. Typhimurium can use lysine derived from fructoselysine or glucoselysine as a sole nitrogen source, it cannot use exogenous lysine as a nitrogen source to support growth. Expression of gfrABCDEF was dependent on the alternative sigma factor RpoN (σ(54)) and an RpoN-dependent LevR-like activator, which we designated GfrR.

IMPORTANCE

Salmonella physiology has been studied intensively, but there is much we do not know regarding the repertoire of nutrients these bacteria are able to use for growth. This study shows that a previously uncharacterized PTS and associated enzymes function together to transport and catabolize fructoselysine and glucoselysine. Knowledge of the range of nutrients that Salmonella utilizes is important, as it could lead to the development of new strategies for reducing the load of Salmonella in food animals, thereby mitigating its entry into the human food supply.

The role of advanced glycation end products in various types of neurodegenerative disease: a therapeutic approach

Protein glycation is initiated by a nucleophilic addition reaction between the free amino group from a protein, lipid or nucleic acid and the carbonyl group of a reducing sugar. This reaction forms a reversible Schiff base, which rearranges over a period of days to produce ketoamine or Amadori products. The Amadori products undergo dehydration and rearrangements and develop a cross-link between adjacent proteins, giving rise to protein aggregation or advanced glycation end products (AGEs). A number of studies have shown that glycation induces the formation of the β-sheet structure in β-amyloid protein, α-synuclein, transthyretin (TTR), copper-zinc superoxide dismutase 1 (Cu, Zn-SOD-1), and prion protein. Aggregation of the β-sheet structure in each case creates fibrillar structures, respectively causing Alzheimer's disease, Parkinson's disease, amyotrophic lateral sclerosis, familial amyloid polyneuropathy, and prion disease. It has been suggested that oligomeric species of glycated α-synuclein and prion are more toxic than fibrils. This review focuses on the pathway of AGE formation, the synthesis of different types of AGE, and the molecular mechanisms by which glycation causes various types of neurodegenerative disease. It discusses several new therapeutic approaches that have been applied to treat these devastating disorders, including the use of various synthetic and naturally occurring inhibitors. Modulation of the AGE-RAGE axis is now considered promising in the prevention of neurodegenerative diseases. Additionally, the review covers several defense enzymes and proteins in the human body that are important anti-glycating systems acting to prevent the development of neurodegenerative diseases.

Multiple nonglycemic genomic loci are newly associated with blood level of glycated hemoglobin in East Asians

Glycated hemoglobin A1c (HbA1c) is used as a measure of glycemic control and also as a diagnostic criterion for diabetes. To discover novel loci harboring common variants associated with HbA1c in East Asians, we conducted a meta-analysis of 13 genome-wide association studies (GWAS; N = 21,026). We replicated our findings in three additional studies comprising 11,576 individuals of East Asian ancestry. Ten variants showed associations that reached genome-wide significance in the discovery data set, of which nine (four novel variants at TMEM79 [P value = 1.3 × 10(-23)], HBS1L/MYB [8.5 × 10(-15)], MYO9B [9.0 × 10(-12)], and CYBA [1.1 × 10(-8)] as well as five variants at loci that had been previously identified [CDKAL1, G6PC2/ABCB11, GCK, ANK1, and FN3KI]) showed consistent evidence of association in replication data sets. These variants explained 1.76% of the variance in HbA1c. Several of these variants (TMEM79, HBS1L/MYB, CYBA, MYO9B, ANK1, and FN3K) showed no association with either blood glucose or type 2 diabetes. Among individuals with nondiabetic levels of fasting glucose (<7.0 mmol/L) but elevated HbA1c (≥6.5%), 36.1% had HbA1c <6.5% after adjustment for these six variants. Our East Asian GWAS meta-analysis has identified novel variants associated with HbA1c as well as demonstrated that the effects of known variants are largely transferable across ethnic groups. Variants affecting erythrocyte parameters rather than glucose metabolism may be relevant to the use of HbA1c for diagnosing diabetes in these populations.

Dietary influence on urinary excretion of 3-deoxyglucosone and its metabolite 3-deoxyfructose

3-Deoxyglucosone (3-DG), a reactive 1,2-dicarbonyl compound derived from d-glucose in food and in vivo, is an important precursor for advanced glycation endproducts (AGEs). At present, virtually no information about the metabolic transit of dietary 3-DG is available. One possible metabolic pathway of 3-DG during digestion is enzymatic transformation to less reactive compounds such as 3-deoxyfructose (3-DF). To study the handling of dietary 1,2-dicarbonyl compounds by the human body, 24 h urinary excretion of 3-DG and its metabolite, 3-deoxyfructose, was investigated. Urinary 3-DG and 3-DF excretion was monitored for nine healthy volunteers following either a diet with no dietary restrictions or a diet avoiding the ingestion of 3-DG and other Maillard reaction products ("raw food" diet). During the "raw food" diet, the urinary 3-DG and 3-DF excretion decreased approximately to 50% compared to the excretions during the diet with no restrictions. When subjects received a single dose of wild honey (50 g) naturally containing a defined amount of 3-DG (505 μmol), median excretion of 3-DG and 3-DF increased significantly from 4.6 and 77 to 7.5 and 147 μmol/day, respectively. The obtained experimental data for the first time demonstrate a dietary influence on urinary 3-DG and 3-DF levels in healthy human subjects.

The association of hypomagnesaemia, high normal uricaemia and dyslipidaemia in the patients with diabetic retinopathy

CONTEXT

Diabetic retinopathy is fast becoming an important cause of a visual disability. The visual disability which results from diabetes is a significant public health problem; however, this morbidity is largely preventable and treatable. If it is managed with a timely intervention, the quality of life can be preserved.

AIMS

The objective of this study was to investigate the association of serum uric acid, magnesium and the lipid profile in diabetic retinopathy with Normal subjects and Diabetes mellitus without retinopathy, among the south Indian population.

SETTINGS AND DESIGN

The diabetic retinopathy patients were identified from the diabetic health camps which were held in rural areas, and they were compared with those with diabetes without complications and the normal subjects.

MATERIAL AND METHODS

The diabetic retinopathy patients were compared with the healthy subjects and with diabetes without retinopathy. Furthermore, the Diabetic retinopathy patients were grouped as proliferative and non-proliferative, based on the fundoscopic findings. Magnesium, uric acid, FPG, fructosamine and the lipid profile were measured in the above groups and they were analyzed.

STATISTICAL ANALYSIS

The statistical analysis was done by using the SPSS software, by applying the Student 't' test.

RESULTS

The mean serum magnesium concentration was observed to be low in the diabetic retinopathy group (1.43mg/dl) as compared to those in the controls and the diabetic subjects. The serum Uric acid concentration was high normal (4.84mg/dl), which was associated with the dyslipidaemia in diabetic retinopathy.

CONCLUSION

The poor glycaemic control in diabetes is associated with hypomagnesaemia, and increased uric acid concentration with dyslipidaemia, which can be an initial picture of the ongoing biochemical changes in the complication of diabetes, which can help in predicting the onset of diabetic retinopathy in diabetes.

Advanced glycoxidation and lipoxidation end products (AGEs and ALEs): an overview of their mechanisms of formation

Advanced lipoxidation end products (ALEs) and advanced glycation end products (AGEs) have a pathogenetic role in the development and progression of different oxidative-based diseases including diabetes, atherosclerosis, and neurological disorders. AGEs and ALEs represent a quite complex class of compounds that are formed by different mechanisms, by heterogeneous precursors and that can be formed either exogenously or endogenously. There is a wide interest in AGEs and ALEs involving different aspects of research which are essentially focused on set-up and application of analytical strategies (1) to identify, characterize, and quantify AGEs and ALEs in different pathophysiological conditions; (2) to elucidate the molecular basis of their biological effects; and (3) to discover compounds able to inhibit AGEs/ALEs damaging effects not only as biological tools aimed at validating AGEs/ALEs as drug target, but also as promising drugs. All the above-mentioned research stages require a clear picture of the chemical formation of AGEs/ALEs but this is not simple, due to the complex and heterogeneous pathways, involving different precursors and mechanisms. In view of this intricate scenario, the aim of the present review is to group the main AGEs and ALEs and to describe, for each of them, the precursors and mechanisms of formation.

Influence of smoking and diet on glycated haemoglobin and 'pre-diabetes' categorisation: a cross-sectional analysis

BACKGROUND

The new HbA1c criteria for diagnosis of pre-diabetes have been criticised for misdiagnosis. It is possible that some elevation of HbA1c is not driven by hyperglycaemia. This study assesses associations of HbA1c, commonly assumed to relate solely to glucose concentration, with (i) smoking, a major source of reactive oxygen species (ROS) and (ii) fruit & vegetables consumption associated with improved redox status.

METHODS

One-way ANOVA, Chi-squared and multivariate linear regressions, adjusted for all known confounders were used to explore associations of HbA1c with self-reported smoking status and fruit & vegetables consumptions in the Scottish Health Surveys 2003-2010, among individuals without known diabetes and HbA1c < 6.5%.

RESULTS

Compared to non-smokers (n = 2831), smokers (n = 1457) were younger, consumed less fruit & vegetables, had lower physical activity levels, lower BMI, higher HbA1c and CRP (p < 0.05). HbA1c was higher in smokers by 0.25 SDs (0.08%), and 0.38 SDs higher (0.14%) in heavy smokers (>20 cigarettes/day) than non-smokers (p < 0.001 both). Smokers were twice as likely to have HbA1c in the 'pre-diabetic' range (5.7-6.4%) (p < 0.001, adj.model). Pre-diabetes and low grade inflammation did not affect the associations. For every extra 80 g vegetable portion consumed, HbA1c was 0.03 SDs (0.01%) lower (p = 0.02), but fruit consumption did not impact on HbA1c, within the low range of consumptions in this population.

CONCLUSION

This study adds evidence to relate smoking (an oxidative stress proxy) with protein glycation in normoglycaemic subjects, with implications for individuals exposed to ROS and for epidemiological interpretation of HbA1c.

Molecular strategies to prevent, inhibit, and degrade advanced glycoxidation and advanced lipoxidation end products

The advanced glycoxidation end products (AGEs) and lipoxidation end products (ALEs) contribute to the development of diabetic complications and of other pathologies. The review discusses the possibilities of counteracting the formation and stimulating the degradation of these species by pharmaceuticals and natural compounds. The review discusses inhibitors of ALE and AGE formation, cross-link breakers, ALE/AGE elimination by enzymes and proteolytic systems, receptors for advanced glycation end products (RAGEs) and blockade of the ligand-RAGE axis.

Detection of oxidized and glycated proteins in clinical samples using mass spectrometry--a user's perspective

BACKGROUND

Proteins in human tissues and body fluids continually undergo spontaneous oxidation and glycation reactions forming low levels of oxidation and glycation adduct residues. Proteolysis of oxidised and glycated proteins releases oxidised and glycated amino acids which, if they cannot be repaired, are excreted in urine.

SCOPE OF REVIEW

In this review we give a brief background to the classification, formation and processing of oxidised and glycated proteins in the clinical setting. We then describe the application of stable isotopic dilution analysis liquid chromatography-tandem mass spectrometry (LC-MS/MS) for measurement of oxidative and glycation damage to proteins in clinical studies, sources of error in pre-analytic processing, corroboration with other techniques - including how this may be improved - and a systems approach to protein damage analysis for improved surety of analyte estimations.

MAJOR CONCLUSIONS

Stable isotopic dilution analysis LC-MS/MS provides a robust reference method for measurement of protein oxidation and glycation adducts. Optimised pre-analytic processing of samples and LC-MS/MS analysis procedures are required to achieve this.

GENERAL SIGNIFICANCE

Quantitative measurement of protein oxidation and glycation adducts provides information on level of exposure to potentially damaging protein modifications, protein inactivation in ageing and disease, metabolic control, protein turnover, renal function and other aspects of body function. Reliable and clinically assessable analysis is required for translation of measurement to clinical diagnostic use. Stable isotopic dilution analysis LC-MS/MS provides a "gold standard" approach and reference methodology to which other higher throughput methods such as immunoassay and indirect methods are preferably corroborated by researchers and those commercialising diagnostic kits and reagents. This article is part of a Special Issue entitled Current methods to study reactive oxygen species - pros and cons and biophysics of membrane proteins. Guest Editor: Christine Winterbourn.

Applicability of a combination of hemoglobin A(1c) and fasting plasma glucose in population-based prediabetes screening

BACKGROUND

The purpose of this study is to determine: (i) the concordance between a combination of hemoglobin A(1c) (Hb(A1c)) and fasting plasma glucose (FPG) (Hb(A1c) + FPG) and a combination of FPG and 2-h plasma glucose (2hPG) (FPG + 2hPG); and (ii) whether substituting FPG + 2hPG with Hb(A1c) + FPG can enhance the detection of prediabetes in diabetes-free non-Hispanic Whites, non-Hispanic Blacks, and Mexican-Americans adults.

METHODS

Data (n = 1376) from the 2007 to 2008 U.S. National Health and Nutrition Examination Surveys were used for this investigation. Prediabetes cut points were determined using 5.7-6.4%, 100-125, and 140-199 mg/dL for Hb(A1c), FPG, and 2hPG, respectively. Concordances between Hb(A1c) and FPG, Hb(A1c) and 2hPG, Hb(A1c) + FPG and FPG + 2hPG in screening for undiagnosed prediabetes were determined using sensitivity, specificity, and positive and negative likelihood ratios.

RESULTS

The overall concordance between Hb(A1c) + FPG and FPG + 2hPG in screening for prediabetes was high, as indicated by a sensitivity of 92.4% (95% CI = 90.5-94.5) and specificity of 84.1% (81.2-87.0). The application of Hb(A1c) + FPG was associated with a higher prevalence of prediabetes compared to FPG + 2hPG. Compared with FPG + 2hPG, screening with Hb(A1c) + FPG was associated with 3.2%, 24.3%, and 4.2% relative increases in the identification of prediabetes in nondiabetic non-Hispanic Whites, non-Hispanic Blacks and Mexican-Americans, respectively.

CONCLUSIONS

The enhanced prevalence of prediabetes using Hb(A1c) + FPG compared with FPG + 2hPG calls for the need to redefine at a more basic and practical level how to apply Hb(A1c) in screening for prediabetes. A redefined Hb(A1c) that incorporates FPG, age, race/ethnicity, and body mass index may be a better way to use Hb(A1c) in population-based and clinical settings.

Ethnic disparity in hemoglobin A1c levels among normoglycemic offspring of parents with type 2 diabetes mellitus

OBJECTIVE

To investigate the racial/ethnic disparities in hemoglobin A1c levels among nondiabetic persons with similar parental history of type 2 diabetes mellitus.

METHODS

We studied a community-based sample of adult offspring of parents with type 2 diabetes mellitus. Measurements included anthropometry, hematology assessments, serial fasting plasma glucose, oral glucose tolerance testing, plasma insulin, hemoglobin A1c, insulin sensitivity, and β-cell function, using a homeostasis model assessment.

RESULTS

The study included 302 participants (135 white, 167 black). Compared with white participants, black participants had lower fasting plasma glucose levels (91.9 ± 0.51 mg/dL vs 93.6 ± 0.50 mg/dL, P = .015), lower area under the curve of plasma glucose during oral glucose tolerance testing (P = <.001), higher body mass index (31.1 ± 0.61 kg/m² vs 28.5 ± 0.57 kg/m², P = <.001), and similar insulin sensitivity and β-cell function. Hemoglobin A1c was higher in black participants than in white participants (5.68 ± 0.033% vs 5.45 ± 0.028%, P<.001). The absolute black-white difference in hemoglobin A1c level of approximately 0.22% persisted after adjusting for age, hemoglobin, hematocrit, body mass index, waist circumference, fasting plasma glucose, glucose area under the curve, and other covariates.

CONCLUSIONS

Among healthy offspring of parents with type 2 diabetes mellitus in this study, African American participants had higher hemoglobin A1c levels than white participants after adjusting for age, adiposity, blood glucose, and known variables. Thus, plasma glucose level is more valid than hemoglobin A1c for diagnosing prediabetes or diabetes in black persons.

The physiological substrates of fructosamine-3-kinase-related-protein (FN3KRP) are intermediates of nonenzymatic reactions between biological amines and ketose sugars (fructation products)

The physiological function of fructosamine-3-kinase (FN3K) is relatively well understood. As shown in several studies, most conclusively by data on the FN3K-KO mouse, this enzyme breaks down compounds produced by the non-enzymatic glycation of proteins by D-glucose. In contrast with FN3K, very little is known about the function of the fructosamine-3-kinase-related-protein (FN3KRP) even though it has a 65% amino-acid sequence identity with FN3K. We do know that this enzyme is a kinase as evidenced by its ability to phosphorylate non-physiological compounds such a psicosamines, ribulosamines, erythrulosamines, and glucitolamines. However, FN3KRP does not phosphorylate any of the numerous Amadori products that are the physiological substrates of FN3K. The fact that FN3KRP is highly conserved in all vertebrates and present throughout nature suggests that it plays an important role in cellular metabolism and makes identification of its physiological substrates an important objective. In this paper, we propose that FN3KRP phosphorylates products resulting from a non-enzymatic glycation of amines by ketoses (fructation) that involves a 2,3-enolization and produces the stable Amadori intermediate, 2-amino-2-deoxy-D-ribo-hex-3-ulose (ADRH). This ketosamine is then phosphorylated to 2-amino-2-deoxy-D-ribo-hex-3-ulose-4-phosphate (ADRH-4-P). Since phosphates are much better leaving groups than hydroxyls, this destabilizes the C-2 amine bond and results in a spontaneous β-elimination of the phosphate to regenerate an unmodified amine with the concomitant production of 4-deoxy-2,3-diulose. Consequently, we postulate that the principal physiological function of FN3KRP is the breakdown of nonenzymatic fructation products. If confirmed in future studies, this hypothesis opens up new perspectives for an improved understanding of biological Maillard reactions and mechanisms for their control and/or reversal.

Genetic variability of the fructosamine 3-kinase gene in diabetic patients

BACKGROUND

Nonenzymatic glycation appears to be an important factor in the pathogenesis of diabetic complications. Fructosamine 3-kinase (FN3K), initially identified in erythrocytes, appears to be responsible for the removal of fructosamine from proteins, suggesting a protective role in nonenzymatic glycation. Recently, genetic variants in the FN3K gene have been studied in diabetic patients. The aim of our study was the molecular characterization of the FN3K gene in a representative group of Italian patients with type 1 (T1DM) and 2 (T2DM) diabetes mellitus and in a cohort of healthy controls.

METHODS

Seventy diabetic subjects (35 type 1 and 35 type 2) with stable glycemic control and 33 healthy control subjects were evaluated using PCR and direct sequencing of the FN3K gene. Denaturing high performance liquid chromatography (DHPLC) was used in controls for screening for the presence of the genetic variants previously found in diabetic patients.

RESULTS

Seven different genetic variants were identified, five of them already reported and two new: the p.R187X and p.Y239C mutations identified in two females affected by T2DM. No significant association was found between certain polymorphisms and diabetes conditions. Preliminary haplotype studies are also reported. With respect to genotypes, we noted that some were not present in all the investigated cohort, and some were found related to higher glycated hemoglobin compared to others, although not at a significant level, probably because of the small number of subjects investigated.

CONCLUSIONS

In conclusion, this study identified two new mutations and additional variants within the FN3K gene. This is the first study on FN3K in Italy. Future work is needed to achieve a better understanding of the FN3K enzyme and its possible clinical utility in the management of diabetic patients.

Progression of nephropathy in type 2 diabetes: the glycation gap is a significant predictor after adjustment for glycohemoglobin (Hb A1c)

BACKGROUND

The glycation gap has been proposed as an index of nonglycemic determinants of glycated hemoglobin (Hb A(1c)). We investigated whether it predicts progression of nephropathy in type 2 diabetic patients.

METHODS

We recorded albumin excretion rate, Hb A(1c), and serum fructosamine in 2314 patients over an average of 6.5 years. Hb A(1c) was regressed on fructosamine by using a repeated-measures longitudinal regression model and data for all visits of all patients; the raw glycation gap gg was calculated at each visit, as measured by Hb A(1c) minus the value predicted by the regression; and the mean glycation gap (GG) was defined for each patient as the mean of the values for the raw glycation gap (gg) calculated at each visit. The study group was divided into high-, medium- and low-GG groups of equal sizes, which were compared for progression of nephropathy by Cox regression analyses controlling for age, sex, duration of diabetes, initial nephropathy status, therapy, baseline Hb A(1c), mean Hb A(1c), and mean fructosamine. The design of the study was a retrospective cohort study with follow-up for 6.5 (SD 4.2) years.

RESULTS

The gg exhibited considerable stability over time. In the high- and medium-GG groups, the risk of progression of nephropathy was respectively 2.5 and 1.6 times that of the low-GG group (P < 0.0001 and P = 0.001, respectively) after adjustment as described above.

CONCLUSIONS

GG predicts the progression of nephropathy in type 2 diabetic patients independently of fructosamine and even after adjustment for Hb A(1c). The joint use of the glycation gap and fructosamine as measures of nonglycemic and glycemic determinants of glycation, respectively, may improve evaluation of the risk of nephropathy and of the glycemic control desirable for the individual patient.

Enzymatic repair of Amadori products

Protein deglycation, a new form of protein repair, involves several enzymes. Fructosamine-3-kinase (FN3K), an enzyme found in mammals and birds, phosphorylates fructosamines on the third carbon of their sugar moiety, making them unstable and causing them to detach from proteins. This enzyme acts particularly well on fructose-epsilon-lysine, both in free form and in the accessible regions of proteins. Mice deficient in FN3K accumulate protein-bound fructosamines and free fructoselysine, indicating that the deglycation mechanism initiated by FN3K is operative in vivo. Mammals and birds also have an enzyme designated 'FN3K-related protein' (FN3KRP), which shares ≈ 65% sequence identity with FN3K. Unlike FN3K, FN3KRP does not phosphorylate fructosamines, but acts on ribulosamines and erythrulosamines. As with FN3K, the third carbon is phosphorylated and this leads to destabilization of the ketoamines. Experiments with intact erythrocytes indicate that FN3KRP is also a protein-repair enzyme. Its physiological substrates are most likely formed from ribose 5-phosphate and erythrose 4-phosphate, which give rise to ketoamine 5- or 4-phosphates. The latter are dephosphorylated by 'low-molecular-weight protein-tyrosine-phosphatase-A' (LMW-PTP-A) before FN3KRP transfers a phosphate on the third carbon. The specificity of FN3K homologues present in plants and bacteria is similar to that of mammalian FN3KRP, suggesting that deglycation of ribulosamines and/or erythrulosamines is an ancient mechanism. Mammalian cells contain also a phosphatase acting on fructosamine 6-phosphates, which result from the reaction of proteins with glucose 6-phosphate.

Racial disparity in A1C independent of mean blood glucose in children with type 1 diabetes

OBJECTIVE

Mean blood glucose (MBG) and MBG-independent factors both influence A1C levels. Race was related to A1C independent of MBG in adults. The goal of this study was to determine if racial disparity exists in A1C independent of MBG in children with diabetes.

RESEARCH DESIGN AND METHODS

Participants included 276 children with type 1 diabetes. A1C and MBG were obtained from multiple clinic visits, and a hemoglobin glycation index (HGI) (an assessment of A1C levels independent of MBG) was calculated. A1C and HGI were analyzed controlling for age, diabetes duration, and MBG. RESULTS African Americans had statistically significantly higher A1C (9.1 +/- 0.1) and HGI (0.64 +/- 0.11) than Caucasians (A1C 8.3 +/- 0.1, HGI -0.15 +/- 0.07) independent of covariates.

CONCLUSIONS

Because of racial disparity in A1C, which is independent of MBG, we recommend that A1C and MBG be used together to make therapeutic decisions for children with diabetes.

Binding of mannose-binding lectin to fructosamines: a potential link between hyperglycaemia and complement activation in diabetes

BACKGROUND

Complement activation via the MBL pathway has been proposed to play a role in the pathogenesis of diabetic complications. As protein glycation is increased in diabetes, we tested the possibility that the glycation product fructoselysine is a ligand for MBL and that its interaction with this protein may initiate complement activation.

METHODS

We investigated the binding of MBL to fructoselysine by chromatography of human serum on fructoselysine-Sepharose, followed by Western blot and mass spectrometry analysis. We also performed enzyme-linked immunosorbent assays using purified MBL and fructoselysine-derivatized (binding assay) or mannan-coated plates (inhibition assay). Complement activation was determined by the fixation of C3d following incubation of fructoselysine-derivatized plates with serum from subjects with different levels of MBL.

RESULTS

MBL and its associated proteases were selectively purified from serum by chromatography on fructoselysine-Sepharose. Competition experiments indicated that MBL had a similar affinity for mannose, fructose and fructoselysine. MBL bound, in a highly cooperative manner, to fructoselysine-derivatized plates. This binding was associated with complement activation and was much lower with serum from subjects with low-MBL genotypes.

CONCLUSIONS

MBL binding to fructoselysine and the ensuing complement activation may provide a physiopathological link between enhanced glycation and complement activation in diabetes. The cooperative character of this binding may explain the high sensitivity of diabetic complications to hyperglycaemia.

Effects of fructosamine-3-kinase deficiency on function and survival of mouse pancreatic islets after prolonged culture in high glucose or ribose concentrations

Due to their high glucose permeability, insulin-secreting pancreatic beta-cells likely undergo strong intracellular protein glycation at high glucose concentrations. They may, however, be partly protected from the glucotoxic alterations of their survival and function by fructosamine-3-kinase (FN3K), a ubiquitous enzyme that initiates deglycation of intracellular proteins. To test that hypothesis, we cultured pancreatic islets from Fn3k-knockout (Fn3k(-/-)) mice and their wild-type (WT) littermates for 1-3 wk in the presence of 10 or 30 mmol/l glucose (G10 or G30, respectively) and measured protein glycation, apoptosis, preproinsulin gene expression, and Ca(2+) and insulin secretory responses to acute glucose stimulation. The more potent glycating agent d-ribose (25 mmol/l) was used as positive control for protein glycation. In WT islets, a 1-wk culture in G30 significantly increased the amount of soluble intracellular protein-bound fructose-epsilon-lysines and the glucose sensitivity of beta-cells for changes in Ca(2+) and insulin secretion, whereas it decreased the islet insulin content. After 3 wk, culture in G30 also strongly decreased beta-cell glucose responsiveness and preproinsulin mRNA levels, whereas it increased islet cell apoptosis. Although protein-bound fructose-epsilon-lysines were more abundant in Fn3k(-/-) vs. WT islets, islet cell survival and function and their glucotoxic alterations were almost identical in both types of islets, except for a lower level of apoptosis in Fn3k(-/-) islets cultured for 3 wk in G30. In comparison, d-ribose (1 wk) similarly decreased preproinsulin expression and beta-cell glucose responsiveness in both types of islets, whereas it increased apoptosis to a larger extent in Fn3k(-/-) vs. WT islets. We conclude that, despite its ability to reduce the glycation of intracellular islet proteins, FN3K is neither required for the maintenance of beta-cell survival and function under control conditions nor involved in protection against beta-cell glucotoxicity. The latter, therefore, occurs independently from the associated increase in the level of intracellular fructose-epsilon-lysines.

Do race and ethnicity impact hemoglobin A1c independent of glycemia?

Hemoglobin A1c (HbA1c) is widely used as an index of mean glycemia, a measure of risk for the development of diabetes complications, and a measure of the quality of diabetes care. Emerging literature suggests that, although HbA1c levels change little over time within persons without diabetes, they vary considerably among individuals, suggesting that factors other than glycemia may impact HbA1c. Racial and ethnic differences in HbA1c have been described that do not appear to be explained by differences in glycemia. It is imperative that the nonglycemic factors that affect HbA1c be more clearly defined. Even more important, it must be determined whether differences among individuals or groups correlate with susceptibility to complications or merely reflect variation in hemoglobin glycation.