[Maillard reaction of bovine serum albumin with glucose. Determination of 2-formyl-5-(hydroxymethyl)pyrrole-1-norleucine by high-performance liquid chromatography after alkaline hydrolysis]

Kilning invokes oxidative changes in malt proteins

Beneath glycation, oxidation reactions may take place at cereal proteins during production of malt. The extent of oxidative chemical changes at malt proteins has not yet been studied. In the present short communication, malt protein was characterized by the determination of free thiol groups and degree of methionine oxidation as well as the sites that are reactive to covalent modification by 2,4-dinitrophenylhydrazine (DNPH, “protein carbonylation”). Protein carbonylation in pale malts was around 1.5 nmol/mg protein and increased with increasing malt colour. Investigations on the protein pellet isolated for determination of carbonylation revealed that solubility and colour may disturb the quantification of carbonyl sites in roasted malts. Free thiols decreased with increasing malt colour already in pale malts (EBC < 10). The formation of methionine sulfoxide (MetSO) was intensified with increasing malt colour. An amount of 7–20% of methionine was converted to MetSO in pale and dark malt, whereas nearly 60% of methionine was oxidized to MetSO in roasted malts. The formation of methionine sulfone was negligible. This study shows that malt proteins suffer from oxidation during kilning, and future studies will have to show whether this supports the pro- or antioxidant activity of malt.

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.

Determination of Free-Form and Peptide Bound Pyrraline in the Commercial Drinks Enriched with Different Protein Hydrolysates

Pyrraline, a causative factor for the recent epidemics of diabetes and cardiovascular disease, is also employed as an indicator to evaluate heat damage and formation of advanced glycation end-products (AGEs) in foods. Peptide-enriched drinks (PEDs) are broadly consumed worldwide due to rapid rate of absorption and perceived health effects. It can be hypothesized that PED is an important source of pyrraline, especially peptide bound pyrraline (Pep-Pyr). In this study we determined free-form pyrraline (Free-Pyr) and Pep-Pyr in drinks enriched with whey protein hydrolysate (WPH), soy protein hydrolysate (SPH) and collagen protein hydrolysate (CPH). A detection method was developed using ultrahigh-performance liquid chromatography with UV-visible detector coupled with tandem mass spectrometry after solid-phase extraction (SPE). The SPE led to excellent recovery rates ranging between 93.2% and 98.5% and a high reproducibility with relative standard deviations (RSD) of <5%. The limits of detection and quantification obtained were 30.4 and 70.3 ng/mL, respectively. Pep-Pyr was identified as the most abundant form (above 96 percent) of total pyrraline, whereas Free-Pyr was present in a small proportion (less than four percent) of total pyrraline. The results indicate that PED is an important extrinsic source of pyrraline, especially Pep-Pyr. As compared with CPH- and SPH-enriched drinks, WPH-enriched drinks contained high content of Pep-Pyr. The Pep-Pyr content is associated with the distribution of peptide lengths and the amino acid compositions of protein in PEDs.

Analysis and biological properties of amino acid derivates formed by Maillard reaction in foods

Maillard reaction products (MRPs), especially early stage MRPs and melanoidins, are currently gaining a lot of attention due to their reported health-promoting properties and their potential to be used as functional food ingredients. It is often not clear which specific biological function is assigned to which MRP, due to the large amount of MRPs formed during the reaction and difficulties in their purification and identification. This paper provides an overview of amino acid derivatives such as Amadori compounds, carboxymethyllysine, pyrraline, cross-linking products and melanoidins, which can be formed by Maillard reaction in foods, their biological properties and the analytical tools commonly employed for their determination.

Maillard reaction of D-glucose: identification of a colored product with conjugated pyrrole and furanone rings

Formation of colored compounds during the Maillard reaction of D-glucose with butylammonium acetate in aqueous solution has been investigated. Butylamine was used as a model compound analogous to the lysine side chains of proteins. The previously unknown, yellow product, 4-hydroxy-5-methyl-2-(N-butyl-3-hydroxy-5-(2-hydroxyethyl)pyrrolyl-2-methylidene)-2H-furan-3-one (1a), was isolated from the reaction mixtures and identified by spectroscopic data.

Synthesis of pyrraline reference material

A simplified and improved method is described for the preparation of pyrraline, a lysine derivative from the advanced Maillard reaction and potential indicator for heat treatment of foods. The compound was obtained in a high degree of purity and with a yield of 31% from N alpha-t-butyloxycarbonyl-L-lysine after heating with 3-deoxy-D-erythro-hexos-2-ulose for 2 h at 70 degrees C in the dry state, preparative fractionation of the resulting N alpha-t-butyloxycarbonyl pyrraline with reverse-phase liquid chromatography and final deprotection of the intermediate compound with acetic acid.

Chromatographic evidence for pyrraline formation during protein glycation in vitro and in vivo

Pyrraline (epsilon-2-(formyl-5-hydroxymethyl-pyrrol-1-yl)-L-norleucine) is an advanced Maillard reaction product formed from 3-deoxyglucosone in the non-enzymatic reaction between glucose and the epsilon-amino group of lysine residues on proteins. Although its presence in vivo as well as in in vitro incubations of proteins with sugars has been documented by immunochemical methods using polyclonal and monoclonal antibodies, its formation in proteins has recently been questioned by similar methodology. To clarify this issue, we investigated pyrraline formation in proteins following alkaline hydrolysis and quantitation by high-performance liquid chromatography on a C18 reverse-phase column. Time- and sugar concentration-dependent increase in pyrraline formation was noted in serum albumin incubated with either 100 mM glucose or 50 mM 3-deoxyglucosone. Formation of pyrraline from 3-deoxyglucosone was rapid at slightly acidic pH, confirming its synthetic pathway through this Maillard reaction intermediate. Low levels of pyrraline (< 10 pmol/mg protein) were also detected in a pool of human skin collagen by this method, but no age effect was apparent. Using a slightly different approach, pyrraline-like material was detected in human plasma proteins following enzyme digestion and analysis by high performance liquid chromatography. Plasma from diabetic patients showed a significant increase in pyrraline-like material compared to controls. The levels in diabetic and normal individuals were 21.6 +/- 9.56 and 12.8 +/- 5.6 pmol per mg protein, respectively (P = 0.005), reflecting thereby the elevated levels of the immediate precursor of pyrraline, 3-deoxyglucosone, in diabetic plasma.

[The decomposition of reducing sugars and amines in the Maillard reaction]

Through the Maillard reaction, occurring in food and in the human body, reducing sugars, amino acids and proteins are transformed. Some general and also specific degradation pathways for pentoses, mono- and disaccharides can be defined. The structures of the products are influenced by the type of amine (primary or secondary). During the degradation reactions C-C fissions of the sugars and amino acids (Strecker degradation) can be observed. Some Maillard products are suited as indicator substances to determine the extent of the Maillard reaction.

Separation and partial characterization of Maillard reaction products by capillary zone electrophoresis

Capillary zone electrophoresis proved useful for separating small amounts of both charged and uncharged solutes that are otherwise difficult to analyse. A typical complex mixture that had previously resisted all analytical approaches, including reversed-phase separations, is the products arising from the reaction of free amino acids with aldehydic sugars (Maillard reaction products). By using capillary zone electrophoresis [untreated capillary 50 cm x 75 microns I.D., 18 kV, 0.02 mol/l phosphate buffer (pH 7.5)], a number of products resulting from the reaction of glucose or ribose with glycine, alanine and isoleucine were separated and partially characterized. They were separated (1) without derivatization (and profiles of compounds absorbing at 220 nm were obtained), (2) as phenylthiocarbamyl derivatives in a search for reactive amino groups and (3) after derivatzation with 2,4-dinitrophenylhydrazine in a search for a method for compounds with a free aldehydic group. Phenylthiocarbamyl derivatives were separated in 0.005 mol/l borate buffer (pH 9.6) at 20 kV and 25 microA. Separation of 2,4-dinitrophenylhydrazones was effected by electrokinetic micellar chromatography in the same apparatus using a 50 cm x 75 microns I.D. capillary at 10 kV in 0.01 mol/l Na2HPO4-0.006 mol/l tetraborate, 0.050 mol/l with respect to sodium dodecyl sulphate. The results are compared with those given by high-performance liquid and thin-layer chromatography.