Determination of urinary pyrraline by solid-phase extraction and high performance liquid chromatography

Effect of transferrin glycation induced by high glucose on HK-2 cells in vitro

BACKGROUND AND OBJECTIVE

Glycation is a common post-transcriptional modification of proteins. Previous studies have shown that advanced glycation end modified transferrin (AGE-Tf) levels in diabetic rat kidney tissues were increased; however, its role in diabetic nephropathy remains unclear. In this study, differences in glycation degree and Tf sites induced by differing high glucose concentrations in vitro and the effect on total iron binding capacity (TIBC) were observed. Moreover, the effect of AGE-Tf on human renal tubular epithelial cells (HK-2) was investigated.

METHODS

In vitro Tf was incubated with increasing glucose concentrations (0 mM, 5.6 mM, 11.1 mM, 33.3 mM, 100 mM, 500 mM, and 1,000 mM) for AGE-Tf. Differences in AGE-Tf glycation degree and TIBC level were analyzed via colorimetric method. The AGE-Tf glycation sites were identified with LC-MS/MS. HK-2 cells were treated with AGE-Tf prepared with different glucose concentrations (33.3 mM and 500 mM) in vitro. The effects of AGE-Tf on HK-2 cell viability, proliferation, oxidative stress index, and Tf receptor expression levels were then observed.

RESULTS

With increasing glucose concentrations (100 mM, 500 mM, and 1,000 mM) in vitro, Tf glycation degree was significantly increased. The TIBC levels of AGE-Tf were decreased significantly with increasing glucose concentrations (33.3 mM, 100 mM, 500 mM, and 1,000 mM). Four glycated modification sites in Tf and 17 glycated modification sites were detected in AGE-Tf (500 mM) by LC-MS/MS. The structural types of AGEs were CML, G-H1, FL-1H2O, FL, and MG-H1. No significant differences were found in the survival rate of HK-2 cells among the AGE-Tf (500 mM), AGE-Tf (33.3 mM), and Tf groups (all p > 0.05). The apoptosis rate of HK-2 cells in the AGE-Tf (500 mM) group was significantly higher than that in the AGE-Tf (33.3 mM) group. Additionally, both of them were significantly higher than that in the Tf group (both p < 0.05). The MDA levels of HK-2 cells in the AGE-Tf (500 mM) and AGE-Tf (33.3 mM) groups were higher than that in the Tf group, but not significantly (both p > 0.05). The T-AOC level of HK-2 in the AGE-Tf (500 mM) group was significantly lower than that in the AGE-Tf (33.3 mM) and Tf groups (both p < 0.001). The GSH level of HK-2 cells in the AGE-Tf (500 mM) group was significantly lower than that in the Tf group (p < 0.05). The expression level of TfR in the AGE-Tf (500 mM) group was also significantly lower than that in the Tf group (p < 0.05).

CONCLUSION

The degree and sites of Tf glycation were increased in vitro secondary to high-glucose exposure; however, the binding ability of Tf to iron decreased gradually. After HK-2 was stimulated by AGE-Tf in vitro, the apoptosis of cells was increased, antioxidant capacity was decreased, and TfR expression levels were downregulated.

Core-Shell Metal-Organic Frameworks/Molecularly Imprinted Nanoparticles as Absorbents for the Detection of Pyrraline in Milk and Milk Powder

A novel core-shell metal-organic framework coated with a dummy template molecularly imprinted polymer (MOF@DMIP) was synthesized by one-pot bulk polymerization for the detection of pyrraline in food samples. The pyrraline analogue pyrrolidine-3-carboxylic acid was used as the template because of its lower cost, and MIL-101 was used as the MOF core owing to its numerous inherent advantages, including high chemical and hydrothermal stabilities. MIL-101@DMIP was used to detect trace pyrraline in foods by solid-phase extraction combined with high-performance liquid chromatography. It exhibited the advantages of faster mass transport, excellent sensitivity, and selectivity. Under optimum conditions, the detection limit of this system was 40.7 μg L^-1, and a linear range was from 5 × 10^-7 to 2 × 10^-3 mol L^-1, within relative standard deviations of 4.46-6.87%. The recoveries ranged from 92.23 to 103.87%, indicating the excellent ability of the prepared MIL-101@DMIP to recognize pyrraline in complex food matrices and its potential for application in pyrraline detection.

Glycated Serum Albumin and AGE Receptors

In vivo modification of proteins by molecules with reactive carbonyl groups leads to intermediate and advanced glycation end products (AGE). Glucose is a significant glycation reagent due to its high physiological concentration and poorly controlled diabetics show increased albumin glycation. Increased levels of glycated and AGE-modified albumin have been linked to diabetic complications, neurodegeneration, and vascular disease. This review discusses glycated albumin formation, structural consequences of albumin glycation on drug binding, removal of circulating AGE by several scavenger receptors, as well as AGE-induced proinflammatory signaling through activation of the receptor for AGE. Analytical methods for quantitative detection of protein glycation and AGE formation are compared. Finally, the use of glycated albumin as a novel clinical marker to monitor glycemic control is discussed and compared to glycated hemoglobin (HbA1c) as long-term indicator of glycemic status.

Studies on absorption and elimination of dietary maillard reaction products

A nine-day dietary study involving 18 healthy volunteers was performed in order to investigate the influence of nutrition on the urinary excretion of the Maillard reaction products (MRPs) fructoselysine, pyrraline, and pentosidine. From day two through day eight, most types of Maillard product-containing food had to be avoided. On day five, participants were divided into four groups, three of them receiving a test meal (pretzel sticks, brewed coffee, or custard) containing defined amounts of MRPs. The fourth group served as a control. Urine samples taken over a 24-h period were analyzed for MRPs using chromatographic means. As a result of the MRP-free diet, urinary excretion of free pyrraline and fructoselysine, which was calculated from furosine analysis, were lowered about 90%. Excretion of pentosidine decreased about 40%. Consumption of pretzel sticks and coffee on day five resulted in increased amounts of pyrraline and pentosidine in urine samples on days five to seven. Related to the supplied amounts of pyrraline, about 50% were recovered in the urine samples after ingestion of the pretzel sticks. For pentosidine, 60% of the ingested free derivative from coffee brew and 2% of the peptide-bound amino acid ingested with the bakery product were recovered in the urine samples, indicating a better bioavailability for free pentosidine compared to the protein-bound form. For peptide-bound Amadori products, no influence on the excretion was observed after ingestion of the test foods, indicating degradation in the intestine or plasma to yet-unknown metabolites. In conclusion, differences concerning the excretion rate of individual MRPs point to individual resorption and metabolic pathways. These results are of importance for the discussion of a possible (patho)physiological role of dietary advanced glycation end products (AGEs).

Dissociation between urinary pyrraline and pentosidine concentrations in diabetic patients with advanced nephropathy

It has been reported that the concentrations of both pyrraline and pentosidine, well-characterized advanced glycation end products, are increased in the urine of diabetic patients. To determine factors that influence the urinary excretion of pyrraline or pentosidine, we compared pyrraline or pentosidine concentrations with glycemic-control indexes, urinary albumin excretion, and urinary beta2-microglobulin in patients with type 2 diabetes. The study was conducted in 39 age-matched healthy control subjects and 50 diabetic patients, including 22 patients with normoalbuminuria, 15 with microalbuminuria, and 13 with macroalbuminuria. Both urinary pyrraline and pentosidine were measured in early-morning urine specimens with the use of high-pressure liquid chromatography. The urinary pentosidine concentration was significantly higher in diabetic patients than in control subjects (P <.01). In contrast, the urinary pyrraline concentration was significantly lower in diabetic patients than in control subjects (P <.001). Urinary pentosidine concentrations were greater in diabetic patients with macroalbuminuria and microalbuminuria than in those with normoalbuminuria. However, urinary pyrraline concentrations were significantly lower in diabetic patients with advanced nephropathy. Both the hemoglobin A(1c) (HbA(1c)) and the preceding year's mean HbA(1c) were lower in patients with macroalbuminuria than in those with normoalbuminuria or microalbuminuria. Urinary pyrraline, but not pentosidine, showed a significantly positive correlation with the preceding year's mean HbA(1c) (P<0.01). Multivariate analysis disclosed that urinary beta-2-microglobulin was independently correlated with the urinary concentrations of pentosidine and pyrraline (P <.05 for both). We conclude that the urinary concentration of pentosidine is greater in diabetic patients with overt nephropathy, whereas the urinary pyrraline concentration is significantly lower in diabetic patients with overt nephropathy. Because urinary pyrraline is more directly influenced by glycemia than by pentosidine, the difference in glycemic control among diabetic patients with various grades of nephropathy may be responsible for a dissociation between urinary pyrraline and pentosidine concentrations in patients with overt diabetic nephropathy.

High levels of urinary pentosidine, an advanced glycation end product, in children with acute exacerbation of atopic dermatitis: relationship with oxidative stress

Pentosidine is an advanced glycation end product formed by sequential glycation and oxidation. The formation of pentosidine is increased in diseases associated with oxidative stress, such as inflammatory conditions. The aim of the present study was to determine the urinary concentration of pentosidine in atopic dermatitis (AD) and its relationship to the inflammatory status of AD. Urine samples of 32 children with AD and 30 age-matched healthy control subjects were assayed for pentosidine, pyrraline (another advanced glycation end product formed by nonoxidative glycation), and 8-hydroxy-2'-deoxyguanosine (8-OHdG) (an established marker of oxidative stress). Of these 3 markers, urinary concentrations of pentosidine were significantly higher in patients with acute exacerbation of AD than in healthy controls and patients with stable AD. Urinary concentrations of 8-OHdG were significantly higher in AD patients with and without acute exacerbation than in healthy controls. Urinary pentosidine levels correlated significantly with those of 8-OHdG when all data of healthy controls and AD patients were plotted. In patients with acute exacerbation of AD, both urinary pentosidine and 8-OHdG significantly decreased after 7 to 9 days of treatment. Our findings in patients with acute exacerbation of AD suggest that pentosidine levels are partly determined by the prevailing oxidative stress in these patients.

Formation of advanced glycosylation end products and oxidative stress in young patients with type 1 diabetes

Increased production of advanced glycosylation end products (AGEs) and augmented oxidative stress may contribute to vascular complications in diabetes. Little is known about the formation and accumulation of AGEs in young patients with type 1 diabetes. The aim of the present study was to investigate whether AGE production and oxidative stress are augmented in young patients with type 1 diabetes at early clinical stages of the disease. Urine samples of 38 patients with type 1 diabetes [mean age (+/-SD), 12.8 +/- 4.5 y; diabetes duration, 5.7 +/- 4.3 y; HbA1c, 8.0 +/- 1.6%; urinary albumin excretion, 12.6 +/- 14.4 mg/g creatinine (Cr)] and those of 60 age-matched healthy control subjects were assayed for AGEs, pentosidine and pyrraline, and markers of oxidative stress, 8-hydroxy-2'-deoxyguanosine (8-OHdG) and acrolein-lysine. Of these four markers, urinary concentrations of pentosidine, 8-OHdG, and acrolein-lysine were significantly higher in the patients with diabetes than in the healthy control subjects. For the patient group, pentosidine correlated significantly with 8-OHdG and acrolein-lysine, and pyrraline correlated significantly with acrolein-lysine. Urinary pentosidine, 8-OHdG, and acrolein-lysine but not pyrraline correlated significantly with urinary albumin excretion. Patients with microalbuminuria (> or =15 mg/g Cr) showed significantly higher levels of all four markers than did normoalbuminuric patients and control subjects. The present study indicates that accumulation of AGEs, whose formation is closely linked to oxidative stress, and resultant endothelial dysfunction may start early in the course of type 1 diabetes. This means that the risk of vascular complications may be present at an early age and that the best possible glycemic control should be emphasized from the diagnosis of diabetes.

Increased urinary levels of pentosidine, pyrraline and acrolein adduct in type 2 diabetes

This study investigates whether urinary levels of pentosidine, pyrraline and acrolein adduct are increased in type 2 diabetes (DM), and whether these levels are correlated with glycemic control and clinical traits. Urinary levels of pentosidine, pyrraline and acrolein adduct in DM patients (n = 100) recruited from the outpatient clinic of our university hospital were compared with those of age- and sex-matched non-diabetic subjects (n = 50). The correlation of these urinary levels with the glycemic control and the clinical traits were examined. Furthermore, the influence of smoking habit on the levels of acrolein adduct was examined. Urinary levels of pentosidine, pyrraline and acrolein adduct were all significantly (p<0.001) higher in the DM group than in the non-DM group (pentosidine (log(pmol/mgCr)), 1.579 +/- 0.147 vs 1.427 +/- 0.142; pyrraline (log(nmol/mgCr)), 0.888 +/- 0.402 vs 0.581 +/- 0.336; acrolein adduct (log(nmol/mgCr)), 2.316 +/- 0.221 vs 2.051 +/- 0.201). Glycemic control parameters, such as fasting plasma glucose (FPG) and HbA1c, were significantly correlated with these urinary levels. Age was correlated with the urinary levels of pentosidine but not with those of pyrraline and acrolein adduct. The urinary albumin excretion rate did not correlate with any of these urinary levels. The levels of acrolein adduct were higher in the subjects with smoking habit than in those without the habit in the DM group as well as in the non-DM group (DM, 2.391 +/- 0.230 and 2.212 +/- 0.190, p=0.0004; Non-DM, 2.120 +/- 0.171 and 1.993 +/- 0.206, p=0.0503). The urinary levels of pentosidine, pyrraline and acrolein adduct were increased in DM and were significantly correlated with glycemic control levels. In addition, smoking habit seems to increase the urinary levels of acrolein adduct.