Epitope mapping and the effects of various factors on the immunoreactivity of main allergens in egg white.

Egg white has high protein content and numerous biological/functional properties. However, reported allergenicity for some of the proteins in egg white is an issue that needs to be paid exclusive attention. A consideration of the structure of IgE epitopes and their sequences, as well as a comprehensive understanding of the effects of various processes on epitopes and the impact of the gastrointestinal tract on them, can help target such issues. The current study focuses on the identified IgE epitopes in egg white proteins and evaluation of the effects of the gastrointestinal digestion, carbohydrate moiety, food matrix, microbial fermentation, recombinant allergen, heat treatment, Maillard reaction and combination of various processes and gastrointestinal digestion on egg white allergenicity. Although the gastrointestinal tract reduces the immunoreactivity of native egg white proteins, some of the IgE epitope-containing fragments remain intact during the digestion process. It has been found that the gastrointestinal tract can have both positive and negative impacts on the IgE binding activities of egg white proteins. Elimination of the carbohydrate moiety leads to a reduction in the immunoreactivity of ovalbumin. But, such effects from the carbohydrate parts in the IgE binding activity need to be explored further. In addition, the interaction between the egg white proteins and the food matrix leads to various effects from the gastrointestinal tract on the digestion of egg white proteins and their subsequent immunoreactivity. Further on this matter, studies have shown that both microbial fermentation and Maillard reaction can reduce the IgE binding activities of egg white proteins. Also, as an alternate approach, the thermal process can be used to treat the egg white proteins, which may result in the reduction or increase in their IgE binding activities depending on the conditions used in the process. Overall, based on the reported data, the allergenicity levels of egg white proteins can be mitigated or escalated depending on the conditions applied in the processing of the food products containing egg white. So far, no practical solutions have been reported to eliminate such allergenicity.

Mechanisms of plastein formation influence the IgE-binding activity of egg white protein hydrolysates after simulated static digestion.

Egg is the second most common food allergen among infants and young children. This work investigated the influence of plastein reaction on immunoglobulin E (IgE)-binding activities of egg white protein hydrolysates after simulated gastrointestinal (GIT) digestion. Compared to hydrolysate precursors, the IgE-binding activity of Pepsin-Plastein significantly decreased from 35 ± 7% to 8 ± 2% (P < 0.05), and Papain-Plastein from 70 ± 5% to 59 ± 4%. Further GIT hydrolysis of Pepsin-Plastein maintained the reduced IgE-binding activity (7 ± 3%) whereas Papain-Plastein digestion restored the IgE-binding reactivity to 66 ± 7%. This discrepancy is related to the different mechanisms of plastein formation. Covalent modifications (decreased free amino nitrogen and sulfhydryl contents) provided biostability for Pepsin-Plastein, whereas hydrophobic interactions (increased surface hydrophobicity) mainly contributed to Papain-Plastein formation. The latter can be destroyed during GIT digestion leading to re-exposure of hidden IgE-binding epitopes. Taken together, plastein reaction is a promising strategy for inducing structural modifications that reduce the immune reactivity of allergenic proteins.

Effect of enzyme immobilization and in vitro digestion on the immune-reactivity and sequence of IgE epitopes in egg white proteins.

The effects of hydrolysis by free and immobilized forms of Neutrase (FN, IN, respectively) and Thermolysin (FT, IT, respectively) and in vitro digestion on the degree of hydrolysis (DH) of egg white proteins, molecular weight distribution of peptides, immune-reactivity and IgE epitopes of egg white proteins were investigated. With FT and IT in the intestinal digests, the proteolysis followed by in vitro digestion produced peptides smaller than 10 kDa. Hydrolysis with the immobilized enzymes had a greater effect than the free enzymes on increasing surface hydrophobicity. The lowest IgE-binding capacity was observed for the intestinal digest of IT-derived hydrolysates (3.3 ± 1.9%). Compared to in vitro digestion, proteolysis showed a significant effect on the immune-reactivity reduction of egg white proteins. Liquid chromatography-tandem mass spectrometry data showed that the most resistant epitopes to enzymatic hydrolysis and in vitro digestion were in ovomucoid, where epitope fragments 1-10, 1-14, 1-20, 4-20, 11-20, 61-74, 71-75 and 101-105 remained intact. Overall, the IgE-binding capacities of egg white proteins were not completely removed after the enzymatic hydrolysis and in vitro digestion due to the presence of intact proteins such as lysozyme and also due to the several immunoreactive peptides derived from egg white proteins.