Spectrophotometric determination of browning products of glycation of protein amino groups based on their reactivity with nitro blue tetrazolium salts

Relationship between the reduction of tetrazolium salt XTT and DNA strand breakage with aminosugars

Dihydropyrazine derivatives formed by the self-condensation reaction of D-glucosamine have the DNA breaking activity. To establish the monitoring method of the biological active dihydropyrazines, we investigated the relationship between the XTT (3'-[1-[(phenylamino)-carbonyl]-3, 4-tetrazolium]bis(4-methoxy-6-nitro)benzensulfonic acid hydrate) reducibility and the DNA breaking activity of aminosugars. Aminosugar in 50 mM sodium phosphate buffer (pH 7.4) was incubated at 37 degrees C. At a given time, the XTT reducibility and the DNA breaking activity of the incubated aminosugar were measured. Both XTT reducibility and DNA breaking activity showed a maximum value within 1-4 h after the incubation and then gradually decreased with the incubation time. Superoxide anion was suggested to involve in both of the DNA breaking activity and the XTT reducibility by the addition of the radical scavengers into those assay mixtures. The quantity of remaining covalently closed circular DNA and the XTT reducibility of all aminosugars showed a good correlation (r = 0.825, n = 26). This means that the XTT assay is applicable for the monitoring of those biologically active products derived from aminosugars when the participation of superoxide anion in DNA scission is recognized.

Allergenicity of Maillard reaction products from peanut proteins

It is known that peanut allergy is caused by peanut proteins. However, little is known about the impact of roasting on the allergenicity of peanuts. During roasting, proteins react with sugars to form Maillard reaction products, which could affect allergenicity. To determine if the Maillard reaction could convert a nonallergenic peanut protein into a potentially allergenic product, nonallergenic lectin was reacted with glucose or fructose at 50 degrees C for 28 days. Browning products from heat-treated peanuts were also examined. The products were analyzed in immunoblot and competitive assays, using a pooled serum (i.e., IgE antibodies) from patients with peanut anaphylaxis. Results showed that the products were recognized by IgE and had an inhibitory effect on IgE binding to a peanut allergen. Thus, the findings suggest that these Maillard reaction products are potentially allergenic and indicate the need to verify whether the Maillard reaction products formed in peanuts during roasting increase their allergenicity.

Monitoring the progress of non-enzymatic glycation in vitro

The progress of in vitro non-enzymatic glycation of bovine serum albumin was followed by using 14C-glucose and a nitroblue tetrazolium assay, absorption and fluorescence spectroscopy, SDS gel electrophoresis and protease digestion. The number of adducts detectable using both 14C-tracers and a fructosamine assay remained low at physiological glucose concentrations, fewer than five adducts being detectable. When glucose concentrations > 1.0 M were used the number of adducts was found to greatly exceed the number of lysyl residues available in BSA, indicative of cross-linking between Maillard products. Incubation of BSA with glucose concentrations of up to 160 mM for one month produced no observable increase in molecular weight by SDS gel electrophoresis, showing that at physiological glucose concentrations, increases in molecular weight were minimal for short incubation periods, any marked changes (indicated by non-penetration of the 7.5% SDS gel) requiring nine months incubation with > or = 20 mM glucose. Increases in absorption were proportional to both the glucose concentration and the incubation time. Several absorption peaks, at 370, 488 and 554 nm, were consistent in appearance throughout the course of each incubation. Fluorescence spectroscopy of the modified proteins showed a disappearance of the fluorescence associated with peptide bonds and aromatic residues and the appearance of a broad peak at longer wavelengths due to the wide range of absorptive/fluorescent wavelengths of the developing Maillard products. Protease digestion gave similar patterns with non-glycated and glycated protein, suggesting that glycation did not block digestion sites, and that partial digestion did not cause significant further exposure of susceptible sites.(ABSTRACT TRUNCATED AT 250 WORDS)

Non-specific binding of advanced-glycosylation end-products to macrophages outweighs specific receptor-mediated interactions

On binding to murine peritoneal macrophages, maleylated BSA exhibited saturable-binding kinetics, with about 24000 sites/cell. Prolonged incubation of BSA with > 20 mM glucose or 2 months incubation with > or = 0.5 M glucose induced the modified protein to readily bind non-specifically to both cell and tube surfaces. Kinetic studies on the binding of advanced glycated end-products (AGEs) and other modified proteins to macrophages and hepatocytes showed no evidence for specific receptor binding, as neither binding saturation nor cross-competition (homologous or heterologous) was detected. Although there was evidence for uptake of BSA which had been incubated with 0.5 M glucose for 2 months, there was no uptake or degradation of AGEs which had been produced at physiological concentrations of glucose. This has implications for the role of macrophages in the recognition of AGEs, and suggests that the non-specific binding may be important in adhesion of AGEs, particularly in poorly controlled diabetics, and might act as a 'damage limitation' mechanism in the potential development of diabetic complications, while low macrophage levels in the blood could seriously potentiate the long-term effects of non-enzymic post-translational protein modifications.

Glycation of albumin: reaction with glucose, fructose, galactose, ribose or glyceraldehyde measured using four methods

Albumin was glycated (nonenzymatically glycosylated) with glucose, fructose, galactose, ribose or glyceraldehyde for 5, 9, 15 and 19 days. The extent of glycation was determined (a) by the thiobarbituric acid method, (b) by fructosamine assay, (c) by method based on the reaction with hydrazine, and (d) by measurement of fluorescence. Results show that the three colorimetric methods used differ in the sensitivity and in addition with the use of each method not the same extent of glycation with various sugars was found.