Proteomic approaches for qualitative and quantitative characterisation of food allergens.

Food allergy is an IgE-mediated hypersensitive reaction estimated to affect up to 4% of infants and adults in developed countries. Proteins termed allergens are mostly responsible for food allergic reactions, consisting of mild to severe systemic reactions. Proteomics include multi-dimensional separation and protein identification by mass spectrometry, followed by data analysis by bioinformatic tools. Proteomics have increasingly been used in the allergy field to (i) identify the genetic and phenotypic variability of allergens in crops, (ii) obtain well-characterised allergens as reported within the EC-funded Integrated Project EuroPrevall, (iii) detect and quantify allergens, either in their native form or in forms resulting from food processing, in complex foods such as bread, cookies, etc., as considered by the EC-funded MoniQA project. These approaches are helping to improve food allergy diagnosis, therapy, and allergenic risk assessment. In the future, the development of more cost effective and sensitive technologies will further enhance the value of proteomics to the allergy field allowing routine use of this approach. We review the applications of proteomics in the field of food allergy.

Authentication of food allergen quality by physicochemical and immunological methods.

Purified allergens are required to detect cross-contamination with other allergenic foods and to understand allergen interaction with other components of the food matrix. Pure allergens are also used for the diagnosis and treatment of food allergies. For example, serological methods are being developed to improve the quality of diagnosis, and to reduce the need for food challenge tests. In addition, recombinant allergens are being evaluated as candidate vaccines for safe and efficacious specific immunotherapy. Pure allergens are indispensable as reference materials for the calibration and standardization of methods between different laboratories and operators for risk assessment in the food industry. Therefore, there is a need for well-defined purified food allergens. In this context, a panel of 46 food allergens from plant and animal sources has been purified, from either the food sources or as recombinant forms, within the EU-funded EuroPrevall project. These allergens have been characterized by a battery of diagnostic tests demonstrating that they constitute an authentic, well-defined library of comparable quality. The review summarizes the applications, potentials and limitations of key techniques used for the characterization and authentication of these allergen preparations, with a special emphasis on protein purity and identity, folding, post-translational modifications and immunochemical properties. One key area identified is the development of powerful analytical techniques, such as mass spectrometry and nuclear magnetic resonance, to improve the authentication of allergens for routine applications in allergy management.

Allergenicity assessment of apple cultivars: hurdles in quantifying labile fruit allergens.

BACKGROUND: Assessment of allergenicity of foods is important for allergic consumers and regulators. Immunoassays to measure major food allergens are widely applied, often giving variable results. Using the major apple allergen Mal d 1 as a model, we aimed to establish at the molecular level why different immunoassays for assessing allergenicity of apple cultivars produce conflicting outcomes. METHODS: Mal d 1 was measured in 53 cultivars from Italy and 35 from The Netherlands, using four different immunoassays. Purified Mal d 1 standards were molecularly characterized by size-exclusion chromatography (SEC) and mass spectrometry (MS). RESULTS: Three immunoassays using an identical standard gave similar results. Minor differences in sample preparation already resulted in significant loss of allergenicity. The fourth assay, using a different Mal d 1 standard, gave 10- to 100-fold lower outcomes. By SEC, this standard was shown to be almost fully aggregated. This aggregation was accompanied by a decrease of the mass of the Mal d 1 molecule by approximately 1 kDa as analyzed by MS. The deviating immunoassay was shown to selectively recognize this aggregated form of Mal d 1, whereas the other three assays, including the one based on IgE antibody recognition, preferentially bound non-aggregated allergen. CONCLUSIONS: Variable and poorly controllable major allergen modification in both extracts and standards hamper accurate allergenicity assessments of fruits.

The effect of thermal processing on the IgE reactivity of the non-specific lipid transfer protein from apple, Mal d 3.

BACKGROUND: Non-specific lipid transfer proteins (LTPs) are involved in allergy to fresh and processed fruits. We have investigated the effect of thermal treatment and glycation on the physico-chemical and IgE-binding properties of the LTP from apple (Mal d 3). METHODS: Mal d 3 was purified from apple peel and the effect of heating in the absence and presence of glucose investigated by CD spectroscopy, electrospray and MALDI-TOF mass spectrometry. IgE reactivity was determined by RAST and immunoblot inhibition, SPT and basophil histamine release test. RESULTS: The identity and IgE reactivity of purified Mal d 3 was confirmed. Mild heat treatment (90 degrees C, 20 min) in the absence or presence of glucose did not alter its IgE reactivity. More severe heat treatment (100 degrees C, 2 h) induced minor changes in protein structure, but a significant decrease in IgE-binding (30-fold) and biological activity (100- to 1000-fold). Addition of glucose resulted in up to four glucose residues attached to Mal d 3 and only a 2- and 10-fold decrease of IgE-binding and biological activity, respectively. CONCLUSIONS: Only severe heat treatment caused a significant decrease in the allergenicity of Mal d 3 but glycation had a protective effect. The presence of sugars in fruits may contribute to the thermostability of the allergenic activity of LTP in heat-processed foods.

Mono- and dimeric ferulic acid release from brewer's spent grain by fungal feruloyl esterases.

Ultraflo L, a beta-glucanase preparation from Humicola insolens sold for reducing viscosity problems in the brewing industry, exhibited activity against the methyl esters of ferulic, caffeic, p-coumaric and sinapic acids, displaying mainly type-B feruloyl esterase activity. Ultraflo also contained the ability to release 65% of the available ferulic acid (FA) together with three forms of diferulate from brewer's spent grain (BSG). An "esterase-free" Ultraflo preparation greatly enhanced the ability of a feruloyl esterase from Aspergillus niger, AnFAEA, to release FA (from 23 to 47%) and its dimeric forms, especially the 8,5' benzofuran form, from BSG. While total release of these phenolic acids was not observed, this synergistic enhancement of ferulate release demonstrates that FA and its dimeric forms present in BSG require the addition of more than a xylanase. This suggests either that FA is not solely attached to arabinoxylan in the barley cell wall, or that the cell wall polysaccharides in BSG hinder the accessibility of enzymes to the ferulates, due to processing treatments.

Hydroxycinnamic acids and ferulic acid dehydrodimers in barley and processed barley.

Hydroxycinnamic acid content and ferulic acid dehydrodimer content were determined in 11 barley varieties after alkaline hydrolysis. Ferulic acid (FA) was the most abundant hydroxycinnamate with concentrations ranging from 359 to 624 microg/g dry weight. p-Coumaric acid (PCA) levels ranged from 79 to 260 microg/g dry weight, and caffeic acid was present at concentrations of <19 microg/g dry weight. Among the ferulic acid dehydrodimers that were identified, 8-O-4'-diFA was the most abundant (73-118 microg/g dry weight), followed by 5,5'-diFA (26-47 microg/g dry weight), the 8,5'-diFA benzofuran form (22-45 microg/g dry weight), and the 8,5'-diFA open form (10-23 microg/g dry weight). Significant variations (p < 0.05) among the different barley varieties were observed for all the compounds that were quantified. Barley grains were mechanically fractionated into three fractions: F1, fraction consisting mainly of the husk and outer layers; F2, intermediate fraction; and F3, fraction consisting mainly of the endosperm. Fraction F1 contained the highest concentration for ferulic acid (from 77.7 to 82.3% of the total amount in barley grain), p-coumaric acid (from 78.0 to 86.3%), and ferulic acid dehydrodimers (from 79.2 to 86.8%). Lower contents were found in fraction F2, whereas fraction F3 exhibited the lowest percentages (from 1.2 to 1.9% for ferulic acid, from 0.9 to 1.7% for p-coumaric acid, and <0.02% for ferulic acid dehydrodimers). The solid barley residue from the brewing process (brewer's spent grain) was approximately 5-fold richer in ferulic acid, p-coumaric acid, and ferulic acid dehydrodimers than barley grains.