Determination of Gluten in Foods Using a Monoclonal Antibody‐based Competition Enzyme Immunoassay

Quantification of Hordeins by ELISA: the correct standard makes a magnitude of difference

BACKGROUND

Coeliacs require a life-long gluten-free diet supported by accurate measurement of gluten (hordein) in gluten-free food. The gluten-free food industry, with a value in excess of $6 billion in 2011, currently depends on two ELISA protocols calibrated against standards that may not be representative of the sample being assayed.

AIM

The factors affecting the accuracy of ELISA analysis of hordeins in beer were examined.

RESULTS

A simple alcohol-dithiothreitol extraction protocol successfully extracts the majority of hordeins from barley flour and malt. Primary hordein standards were purified by FPLC. ELISA detected different classes of purified hordeins with vastly different sensitivity. The dissociation constant (Kd) for a given ELISA reaction with different hordeins varied by three orders of magnitude. The Kd of the same hordein determined by ELISA using different antibodies varied by up to two orders of magnitude. The choice of either ELISA kit or hordein standard may bias the results and confound interpretation.

CONCLUSIONS

Accurate determination of hordein requires that the hordein standard used to calibrate the ELISA reaction be identical in composition to the hordeins present in the test substance. In practice it is not feasible to isolate a representative hordein standard from each test food. We suggest that mass spectrometry is more reliable than ELISA, as ELISA enumerates only the concentration of particular amino-acid epitopes which may vary between different hordeins and may not be related to the absolute hordein concentration. MS quantification is undertaken using peptides that are specific and unique enabling the quantification of individual hordein isoforms.

Measuring hordein (gluten) in beer--a comparison of ELISA and mass spectrometry

BACKGROUND

Subjects suffering from coeliac disease, gluten allergy/intolerance must adopt a lifelong avoidance of gluten. Beer contains trace levels of hordeins (gluten) which are too high to be safely consumed by most coeliacs. Accurate measurement of trace hordeins by ELISA is problematic.

METHODS

We have compared hordein levels in sixty beers, by sandwich ELISA, with the level determined using multiple reaction monitoring mass spectrometry (MRM-MS).

RESULTS

Hordein levels measured by ELISA varied by four orders of magnitude, from zero (for known gluten-free beers) to 47,000 µg/mL (ppm; for a wheat-based beer). Half the commercial gluten-free beers were free of hordein by MS and ELISA. Two gluten-free and two low-gluten beers had zero ELISA readings, but contained significant hordein levels (p<0.05), or near average (60-140%) hordein levels, by MS, respectively. Six beers gave false negatives, with zero ELISA readings but near average hordein content by MS. Approximately 20% of commercial beers had ELISA readings less than 1 ppm, but a near average hordein content by MS. Several barley beers also contained undeclared wheat proteins.

CONCLUSIONS

ELISA results did not correlate with the relative content of hordein peptides determined by MS, with all barley based beers containing hordein. We suggest that mass spectrometry is more reliable than ELISA, as ELISA enumerates only the concentration of particular amino-acid epitopes; this may vary between different hordeins and may not be related to the absolute hordein concentration. MS quantification is undertaken using peptides that are specific and unique, enabling the quantification of individual hordein isoforms. This outlines the problem of relying solely on ELISA determination of gluten in beverages such as beer and highlights the need for the development of new sensitive and selective quantitative assay such as MS.

Analysis of the effects of heat treatment on gliadin immunochemical quantification using a panel of anti-prolamin antibodies

Antigen-labeled capture enzyme-linked immunosorbent assay with four different anti-gliadin monoclonal antibodies and an anti-gliadin serum and two different sample systems (purified gliadin fractions heat-treated in soluble phase and a model of dough simulating a baking process) were employed to study the effects of heat treatment on gliadin quantification. The analysis of purified gliadins showed that there is no particularly heat stable fraction. Remarkably, omega-gliadin did not present a differential heat stability. Reactivity varied depending on the time-temperature conditions of the treatment, the antibody employed, and the fraction analyzed. Heated dough samples showed an impairment of protein extraction depending on the intensity of the treatment. Capillary electrophoresis analysis of extracts showed that each gliadin group is affected to a different extent; omega-gliadin is less modified. Immunochemical analysis of the heat-treated samples using either of the five antibodies showed a decrease in the quantified gliadin, in concordance to the loss in the extracted proteins. Among the different sources of error in gliadin immunochemical quantification, the impairment in extraction efficiency in heat-treated samples appears as a major drawback to be overcome.

Measurement of gluten using a monoclonal antibody to a sequenced peptide of alpha-gliadin from the coeliac-activating domain I

A monoclonal antibody, raised against a sequenced 54 amino-acid peptide from the coeliac-activating N-terminal region of alpha-gliadin, was used in an assay for the measurement of gluten in foods. A double-sandwich ELISA using a polyclonal capture antibody produced standard curves for unfractionated gliadin and its alpha, beta, gamma and omega subfractions, and for rye, barley and oat prolamins. The sensitivity of the assay for unfractionated gliadin and rye prolamins was 15 ng/ml, for barley and oat prolamins 125 and 250 ng/ml, respectively. Prolamins from coeliac non-toxic rice, maize, millet and sorghum did not cross-react in the assay.