Molecular structure and immunological properties of food allergens

Review The number of people, especially babies and infants, plagued by food allergies has recently been on the increase, although the true prevalence of allergies caused by food is unknown. Thus, it is desirable to elucidate the mechanisms of various allergic symptoms caused by food ingestion and, more practically, to establish reliable procedures for the diagnosis and therapy of food allergies. The isolation and identification of the causative allergenic molecules in foods would considerably contribute not only to progress in such research but also to the development of hypoallergenic processing methods for the creation of special foods tolerable to allergic patients.

Food allergy (see Glossary) has been recognized by physicians and clinical immunologists for more than half a century. Through their diagnosis, several foods such as cow's milk, eggs, fish, legumes and cereals have been identified as common allergenic foods, mainly based on the clinical observation that allergic symptoms diminished when such foods were removed from the died. However, as indicated in several conflicting reports, determining which component in such foods is the allergen has been a controversial issue 2'3. The contradictory studies on food allergens may be partly due to differences in the sample preparations used and/or the purity of the isolated allergenic components, since allergic reactions are, unexpectedly, so sensitive that merely a trace of contaminating component could induce positive symptoms. If we confine the term 'allergy' to refer to a type I allergic reaction caused by antigen-specific immunogiobulin E (igE) 4'~, recent progress in developing sensitive and reliable methods for determining allergenic activity (i.e. IgE-binding activity) has made it possible to identify and isolate the food components responsible for each food allergy. Several allergenic proteins and glycoproteins have been identified or isolated by searching for antigens specifically reactive with patients' serum lgE, although the demonstration that specific lgE antibodies react with an allergen does not necessarily indicate clinical allergy to that allergen in some cases 5. Among the numerous allergenic food proteins that have been isolated and characterized are those from cow's milk, egg white, codfish, shrimp, legumes and cereals. This short review summarizes the food allergens that have been isolated and/or characterized on a molecular basis, discusses their immunological and allergenic properties, and briefly describes methods for the reduction and elimination of allergenic activity during food processing or by using genetic engineering to produce hypoailergenic foods. For further information on TsukasaMalsudaand Ryo Nakamuraare at the Department of Food Science & Technology, School of Agriculture, Nagoya University, Chikusa-ku, Nagoya, 464-01, Japan.

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Molecular structure and immunological properties of food

allergens TsukasaMatsudaand Ryo Nakamura allergy 4 p e r s e and on food allergy ~-3"5in particular the reader is advised to refer to Refs 1-5. The properties of the main food allergens discussed below are briefly summarized in Table 1.

Cow's milk allergens Several earlier studies in the medical literature concerning the relative allergenicities of milk proteins suggested that [3-1actoglobulin is the most allergenic component (for a review, see Ref. 3). On the other hand, Burgin-Wolff e t aL 6 reported that children with cow's milk allergy have high levels of serum lgE antibodies to

Glossary Anaphylaxis:Exaggeratedimmune response resulting from re-exposure to an allergen; sometimes fatal.

ConcanavalinA: Jack bean lectin. A protein that binds to glycoprotein carbohydrate chains with oc-glucosideor oc-mannosidegroups, and which agglutinates many cell types.

Epilope: The three-dimensional or sequential site on an antigenic substance (e.g. an allergen) to which an antibody or receptorsof B- or T-lymphocytes will bind. Food allergy: An adverse immunological reaction to an ingested food or food component. May occur after only a small amount of the substance has been ingested,and is not related to any physiological effect of the food or food component. 'Food allergy' is not used to describe adversereactions to foods that are due to non-immunological mechanisms. IgE: Immunoglobulin class E. One of the five classesof antibody molecules: the immunoglobulins IgM, IgD, IgG, IgE and IgA. IgE helps repel invasion by parasitic worms. The IgE content of a normal person's blood is very low, only at the ng/ml level, but it is high in the sera of some allergic patients for unknown reasons.

Type I allergy:An IgE-mediatedallergic reaction, which most frequently occurs in individuals with the inherited ability to form IgE to specific substances in the environment. IgE antibodies produced against a certain substance (allergen) bind to receptors on the surfaces of mast cells; this induces mast cell degranulation and results in the releaseof pharmacologically active substances such as histamine, leukotrienes,etc., which leads to various allergic symptoms.

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Table 1. Structural and immunological properties of major food allergens Sourcefeed

Potentallergen

Structural and immunological properties

Ref.

Cow's milk

Otsl-Casein

23 kDa phosphoprotein with sequential epitopes

6-9

Milk proteins react with lactose through the amino-carbonyl reaction (Maillard reaction) and produce browning products during pro18 kDa protein belonging to the lipocalin 6-8 13-Lactoglobulin cessing and storage. Bleumink and family; not present in human milk Berrens z° first reported that such a 11-15 Maillard adducts Amino-carbonyl reaction products between coupling of lactose to amino groups lactose and protein amino groups in milk proteins increases the skin reactivity of patients wi~.h cow's 43 kDa phosphoglycoprotein belonging 16, 20, Eggwhite Ovalbumin milk allergy. The specific activity to the serpin family; amino acid region 323-339 21, 23 of 13-1actoglobulin-lactose conjugates contains allergenic and antigenic epitope(s) is 10- to 100-fold greater, as deter28 kDa glycoprotein; trypsin inhibitor; 16-19, Ovomucoid mined by direct intradermal tests, heat-stable allergen 23 than that of the native protein. Otani and Tokita ~ clearly proved that the -320-360 kDa legumin-like protein 25,26 Soybean Glycinin sugar moiety in the browning prodcomposed of 6 acidic and 6 basic subunits ucts between 13-1actoglobulin and Mixture of low molecular weight proteins 24-26 2S-globulins lactose contributes to the antigenic including trypsin inhibitors determinant of the protein-lactose 32 kDa allergen Oil-body-associated allergen with sequence 27, 28 conjugate, by demonstrating the resimilarity to a house dust mite allergen activity of ovalbumin-lactose browning products with antibodies raised 65 kDa allergen Concanavalin A-reactive glycoprotein; 29, 33 Peanut against the 13-1actoglobulin-lactose heat-stable allergen browning product. Later, Matsuda 63.5 kDa allergen Verysimilarto 65 kDa allergen 31 et al. ~2 suggested that antibody (Arah I) but concanavalin A-negative response to the sugar moiety in lactose-protein amino-carbonyl prod17 kDa allergen Immunologicallycross-reactive 32 (Ara h II) with Ara h I; glycoproteinrich in ucts is significantly higher than that Glu/GIn of the sugar moiety in aminocarbonyl products with several other Castor bean 2S storage proteins 11-12 kDa glutamine-richalbumins 2, 34 sugars, such as glucose, galactose belonging to the amylase/trypsininhibitor and maltose. Mouse monoclonal antifamily bodies specific for the sugar moiety in the lactose-protein conjugates Rice 16 kDa allergen 14-16 klSa albumins belonging to the 35-37 have recently been obtained, and it amylase~trypsininhibitor family has been suggested that these antiCodfish bodies recognize e-N-deoxylactulosylAllergen M 12 kDa protein belonging to the calcium39 binding parvalbumin family; amino acid L-lysine, which is a relatively stable region 41-64 contains allergenic epitope(s) and dominant intermediate produced at an early stage of the amino-Shrimp Tropomyosin 34-38 kDa water-soluble proteins with 41,42, carbonyl reaction ~3. Further experacidic isoelectric point (pl 4.5-5.8) 44 iments are required to determine whether such monoclonai antibodies casein; Otani et al. 7 recently suggested that out of the share common epitopes with IgE from patients with three casein subunits, (Xs~-casein most frequently gives a milk allergy. positive reaction against lgE from infants and children Commercial reagent-grade lactose has been reported with clinically allergic symptoms, although other casein to contain traces of antigenic components that are not subunits and 13-1actoglobulin also cause positive re- identifiable with known milk proteins (for a review, see actions in some patients. Ref. 2). Some components of the brownish-colored nonCow's milk proteins such as casein and 13-1actoglobu- dialysable residue of commercial lactose have been isolin have been extensively studied and their molecular lated and chemically characterized. These components structure, including details of their amino acid sequence, caused positive intradermal skin reactions on patients phosphorylated and glycosylated sites, and sugar chain allergic to cow's milktL It has not yet been proved structures, have been determined (for a review, see whether the increased skin reactivity of the 13-1actoRef. 8). The immunological properties of (Xs~-casein, globulin-lactose conjugate reported by Bleumink and such as the epitope structures that interact with surface Young ~5was caused by such unidentified non-dialysable receptors of mouse B- and T-lymphocytes, have been antigens found in commercial lactose. However, the intensively studied by the group of Kaminogawa 9, non-dialysable antigens may share common antigenic though the epitope structures recognized by the IgE of structures with the lactose-protein conjugates formed allergic patients have yet to be investigated. by the amino-carbonyl reaction, since the monocional 290

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antibodies specific for the sugar moiety in the lactoseprotein conjugates also bind to the non-dialysable antigens in commercial lactose (Matsuda, T., unpublished).

Eggwhite allergens Egg white is a common cause of allergic reactions to hen's egg, though it has been suggested that egg yolk proteins bind IgE from some patients allergic to egg. Earlier reports suggested that ovomucoid is the major skin-reactive allergen in hen's egg white (for a review, see Ref. 3). Hoffman ~6reported that ovalbumin and ovomucoid are potent allergens, based on their reactivity with lgE; ovotransferrin is also cited as an allergen for some patients, but iysozyme is only a weak allergen. Ovalbumin and a small amount of ovotransferrin are detectable, in immunologically active form, in boiled egg white. Furthermore, egg allergy patients retain considerable immunoreactivity of lgE antibody to ovomucoid, a heat-stable glycoprotein, even in hard-boiled eggs. The remaining allergenicity of ovomucoid after boiling tends to be greater in fresh eggs than in stored eggs ~7. Ovomucoid, the major egg-white allergen, consists of three well-separated domains crosslinked by three intradomain disulfide bonds. Each domain still retains most of the antigenic activity of the parent molecule even after enzymatic or chemical cleavage. However, the ability to induce mouse IgE and immunoglobulin G (IgG) responses is reduced to a remarkable degree by proteolytic degradation in each domain iS. Heat denaturation markedly reduces both the antigenic and the immunogenic activities of ovomucoid ~s. Ovomucoid contains 20-25% carbohydrates (N-linked complex carbohydrate chains). It has been suggested that the carbohydrate moiety contributes to the allergenic structure of ovomucoid, though it is still uncertain whether lgE antibodies of allergic patients bind directly to the carbohydrate chains themselves or whether they recognize epitopes, including both the carbohydrate and the polypeptide chains of ovomucoid ~. An allergenic and antigenic epitope of ovalbumin has recently been identified by Johnsen and Elsayed 2° and by Kahlert et al. ~-j A synthetic peptide for amino acid region 323-339 of ovalbumin was reported to be reactive with both the lgE of egg allergy patients and rabbit anti-ovalbumin IgG (Ref. 20). The contribution of this region to the allergenic and antigenic structure was later confirmed by a demonstration that cyanogen bromide fragments of ovalbumin, corresponding to residues 41-172 and 301-385, were reactive with patients' lgE and with mouse monoclonai antibodies raised against ovalbumin2L Furthermore, these findings suggest that in some cases antigenic epitopes involved in lgG responses in animals are similar or identical to allergenic epitopes involved in IgE responses in humans. The possible contribution to egg allergy of several other minor components in egg white remains to be determined. Ovoinhibitor may be another allergenic component, since it is structurally similar and immunologically cross-reactive with ovomucoid ~-2.The molecu-

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lar structure and properties of egg white proteins have been previously described in detail 23.

Legume allergens Soybean allergens Kunitz soybean trypsin inhibitor appears to be a strong allergen in certain patients with anaphylaxic reaction to soybean 24. On the other hand, Herian et al. 25 reported that serum IgE from patients allergic to both peanuts and soybeans recognized several proteins with molecular weights in the 50-60 kDa range, while IgE from patients allergic to soybeans, but not peanuts, binds strongly to a protein(s) with a molecular mass of 20 kDa (2S-globulin). On the basis of this observation and the known sequence homology between soybean and peanut proteins, they suggested the presence of cross-reactive epitopes for IgE in soybeans and peanuts. Djurtoft et al. 26 investigated the reactivity of soybean glycinin (l IS globulin) with IgE from eight patients allergic to soybeans. The IgE from four patients showed pronounced binding to subunit A4 of the protein, whereas the lgE from the other four showed more pronounced binding to native glycinin than to the subunits. Ogawa et aL 27 have recently reported the detection in 7S and 2S globulins and whey fractions of at least 16 soybean proteins, with molecular weights in the range 14-70 kDa, that were reactive with the lgE of soybeansensitive patients. The lgE antibodies in sera from 45 out of 69 soybean-allergic patients were shown to bind a minor protein with a molecular weight of ~30kDa in the 7S-globulin fraction -~8.

Peanut allergens Peanuts sometimes cause severe, lgE-mediated type I hypersensitive reactions. Peanuts contain two major storage proteins, arachin and conarachin, and it has been reported that the major peanut allergen is found in a subfraction of either arachin or conarachin2'L Barnett and Howden 3° showed that a concanavalin A-reactive glycoprotein is a major allergenic component in peanuts, though arachin, conarachin, peanut lectin and phospholipase D could also be recognized by patients' IgE antibodies. The major allergy,n-was identified as a heat-stable glycoprotein with a molecular mass of -65 kDa and a pl of 4.6. It was reported to contain a few percent (by mass) carbohydrate, consisting of xylose, mannose, glucose and N-acetylglucosamine; chemical removal of the carbohydrate moiety did not eliminate reactivity with specific IgE. Burks et al. later isolated a similar allergen with a molecular mass of 63.5 kDa, which did not bind concanavalin A (Ref. 31), and a low molecular weight glycoprotein as a second major allergen 32. Trace amounts of peanut allergens have been detected in various food-processing materials and finished foods by a modified radioimmunoassay using sera from peanut-sensitive subjects 33.

Castor bean allergens Potent allergens in castor bean ( R i c i n u s c o m m u n i s ) have been intensively studied by Spies's group (for a

291

review see Ref. 2). The castor bean allergens are a group of low molecular weight proteins with microheterogeneity in their chemical properties, and have been identified as the low molecular weight, glutaminerich albumin storage proteins found in the matrix of protein bodies in the endosperm. During the course of amino acid sequencing, Sharief and Li 34 found that the castor bean storage protein consists of two different polypeptide chains linked by disulfide bond(s); the protein shows partial sequence similarity to other seed proteins such as the Bowman-Birk proteinase inhibitor from lima bean.

Cereal allergens IgE-reactive allergens in cereals have been studied in patients with overt symptoms caused not only by the ingestion of cereals but also by the inhalation of cereal flours. It has generally been shown that water-soluble proteins (albumins and globulins) are most relevant to lgE-mediated cereal allergy, even though they are minor components in cereal grains.

Rice allergens Shibasaki et ai. 3s first reported on the allergenic components in rice grains. They suggested that globulin proteins are more allergenic than glutelin proteins in patients allergic to rice. An allergenic protein with a molecular mass o f - 1 6 kDa, recognized by the IgE of several patients allergic to rice, was isolated from saltsoluble rice proteins containing albumin and globulin by Matsuda et aL ~6, who found that the rice albumin fraction contained several proteins with molecular masses in the range 14-16kDa, which are structurally similar and immunologically cross-reactive to the isolated allergenic protein ~6. These proteins are resistant to heat denaturation and proteolytic degradation. Their reactivity to lgE was later confirmed by using sera from 32 patients allergic to rice. These microheterogeneous albumin proteins have been shown to be the products of a multigene family a~. The complete amino acid sequence of the major allergenic protein from rice has been determined from its eDNA nucleotide sequence 37. Interestingly, the amino acid sequence showed considerable similarity to the sequences of proteins in the a-amylase/trypsininhibitor family in cereal and legume seeds. A family of allergens The major allergenic proteins in wheat and barley with reactivity to the lgE of patients with baker's asthma have been reported to be members of the or-amylase inhibitor family, though they are not ingested allergens 3~. The castor bean albumin storage proteins identified as allergens also have amino acid sequences homologous to those of this inhibitor family. Thus, several proteins belonging to the tx-amylase/trypsininhibitor family, from different species (wheat, barley, rice and castor bean), have been identified as allergenic components. Hence, proteins belonging to the tx-amylase/ trypsin-inhibitor family may be potentially prominent allergens in cereal and legume seeds. 292

Seafood allergens Codfish allergens Codfish hypersensitivity is a common form of food allergy in Scandinavian countries. The major allergen of codfish (allergen M) is one of the most wellcharacterized food allergens (for a review see Ref. 39). It is a low molecular weight muscle protein belonging to the calcium-binding parvalbumin family. It has been suggested that a major epitope recognized by IgE antibodies is located in the amino acid region 41-64, which includes several homologous tetrapeptides (one Asp-Glu-Asp-Lys peptide and two Asp-Glu--Leu-Lys peptides) interspaced by six amino acid residues. Binding inhibition studies using the IgE from the sera of children allergic to codfish suggest the presence of cross-reacting allergen(s) in cod, bass, dentex, eel, sole and tun# °. Shrimp allergens Major shrimp allergens with molecular masses of 34 kDa and/or 38 kDa have been isolated from shrimp water extracts 4L42. The IgE binding activity of these water-soluble glycoproteins is quite stable to heat, and these allergenic proteins are detectable by specific IgE binding even after boiling 4°. Lehrer et al. 43 isolated shrimp allergenic proteins and showed that most allergens had pl values of ~4.5-5.8. The heat-stable allergen has recently been identified as tropomyosin based on partial amino acid sequencing of the isolated allergenic proteins 44.

Conclusions in recent years a number of allergenic proteins have been isolated from common allergenic foods or their source plants and animals, and characterization of the structural and immunological properties of the isolated allergens (and some of the corresponding genes) is under way. However, no common universal structural or chemical property characteristic of the allergenic molecules has been found so far. Many of the food allergens identified in water- and salt-soluble fractions are relatively low molecular weight proteins or glycoproteins and they are often resistant to heat denaturation and proteolytic digestion, but many other non-allergenic proteins also have such physicochemical properties. Recent studies using sera from allergic patients have revealed that the causative allergenic molecules sometimes differ from individual to individual, even for patients with similar allergic symptoms caused by a particular allergenic food 16.-~4.27.Therefore, it seems that it will be difficult to eliminate or inactivate all allergenic components in foods containing diverse allergens recognizable by different individuals. However, if an allergen is a minor component in a food and is commonly recognized among patients, it may be possible to eliminate or reduce its level by novel food-processing techniques or the genetic engineering of crops and domestic animals. Indeed, Arai's research group has succeeded in the development of hypoallergenic rice by enzymatic treatment of the grains 45, and our group has been trying to

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develop transgenic rice with a low allergen content. Further investigation of the isolation and identification of allergenic molecules or their genes is needed. The isolated or recombinant proteins may be useful as allergen standards for clinical diagnosis, and the accumulation of knowledge about allergenic molecules in foods will perhaps give food technologists a starting point for the development of hypoallergenic food processing methods.

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Call for Nominations for Stephen S. Chang Award The American Oil Chemists' Society (AOCS) is calling for nominations for the annual Stephen S. Chang Award, established to recognize a scientist, technologist or engineer who has made distinguished and significant accomplishments in basic research resulting in the improvement or development of food products related to lipids, either by one major breakthrough or by an accumulation of publications. The award, sponsored by AOCS Past President Stephen S. Chang and his wife, Lucy D. Chang, consists of a Chinese jade galloping horse symbolizing the award and an honorarium of -$6000. Prospective recipients must agree to be present to accept the award at an AOCS Annual Meeting. Nomination must comprise a letter from the nominator describing the nominee's decisive accomplishments and how they have been utilized by industry for the improvement or development of food products related to lipids, at least three supporting letters, and biographic information of the nominee, including the nominee's curriculum vitae and list of major relevant publications (including any patents). Nominations for the 1994 award must be submitted before 15 October 1993 to Dr Thomas H. Smouse, Archer Daniels Midland Co., Lakeview Technical Center, 1001 Brush College Road, Decatur, IL 62525, USA.

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