Getting to grips with gluten.

Ever since Dicke and coworkers identified gluten as the precipitating agent in coeliac disease (CD),1 2 enormous effort has been invested to identify the elements within gluten that are responsible for eliciting the enteropathy. This has been a frustrating endeavour because of the unusual chemistry and huge complexity of gluten proteins. Gluten consists of the alcohol-water extractable fraction of monomeric gliadins and the alcohol-water unextractable fraction of glutenins cross linked into polymers by intermolecular disulphide bonds. The gliadins can be subdivided on the basis of their amino acid sequences into α/β gliadins, γ gliadins, and ω gliadins, while glutenins can be subdivided into low (LMW) and high (HMW) molecular weight glutenins. Importantly, there are numerous variants of each type of gliadins and glutenins in a single wheat variety. Early feeding studies indicated that both gliadin and glutenin fractions were harmful to coeliac patients.2 3Results with glutenins, however, should be interpreted cautiously as the glutenin fraction also contains subunits with characteristic gliadin sequences but which have odd numbers of cysteine residues that results in the formation of intermolecular disulphide bonds.4 The insolubility of gluten further complicates testing of its toxicity. To bypass this problem, gluten is often fragmented, typically with proteolytic enzymes, prior to testing. Proteolytic fragmentation usually generates a vast array of distinct peptides. From our experience, studies that have obtained results with purified fragments without corroborating results with synthetic analogues should be interpreted with caution. We have often found that a minor peptide constituent can mediate the biological effect of an active fraction rather than the major peptide that is detected by sequencing. The problem of identifying “toxic” sequences has been further complicated by the lack of reliable and relevant toxicity assays. Assays including culturing of atrophic coeliac mucosa,5culturing of fetal chick or …