Latin name: Penaeus aztecus
Common names: Tropomyosin
Shrimp allergen component
Shrimp are small invertebrate marine crustaceans with 10 jointed legs (decapod) on the thorax, well-developed swimmerets on the abdominal segments, and a body that is compressed laterally. They live on the floor of oceans and lakes. There are over 2,000 different species of Shrimp worldwide.
One common commercial Shrimp is of the genus Peneus.
There are several other crustacean forms that are commonly called Shrimp although they do not belong to the same order as the true Shrimp, order Decapoda (phylum Arthropoda, subphylum Crustacea, class Malacostraca), which also includes the Lobsters and Crabs.
Shrimp may be divided into 3 basic categories: cold-water or northern; warm-water, tropical, or southern; and freshwater. However the nomenclature is complicated and the term “Shrimp” sometimes applies to all crustaceans of the Natantia group, regardless of size. The terms “Prawn” and “Scampi” are often used interchangeably with Shrimp.
rPen a 1 from Brown shrimp (P. aztecus), is representative of other shrimp tropomyosin.
Recombinant allergens, which are genetically engineered isoforms resembling allergen molecules from known allergen extracts, have immunoglobulin E (IgE) antibody binding comparable to that of natural allergens and generally show excellent reactivity in in vitro and in vivo diagnostic tests (1). To date, many different recombinant allergens of pollens, molds, mites, bee venoms, foods and latex have been cloned, sequenced, and expressed. Recombinant allergens have a wide variety of uses, from the diagnosis and management of allergic patients to the development of immunotherapy to the standardisation of allergenic test products as tools in molecular allergology.
Recombinant Pen a 1 and natural Pen a 1 are structurally and immunologically identical (2).
At least 13 allergens are found in extracts of cooked Brown shrimp, and the 36-kDa muscle protein tropomyosin (Pen a 1) has been identified as a major Shrimp allergen. It is detected by sera of more than 80% of all subjects allergic to Shrimp and binds up to 75% of all Shrimp-specific IgE antibodies (2-3). Amino acid sequence identities with natural allergenic and non-allergenic tropomyosins ranged from 80% to 99% and 51% to 58%, respectively (2). Since Beef, Pork and Chicken are other tropomyosin-containing foods that are not very allergenic, tropomyosins can serve in investigations of the contribution of the varying structural properties of a protein to its allergenicity (4).
Recombinant allergens are particularly useful in addressing allergies that manifest wide cross-reactivity, such as allergies to crustaceans, Cockroaches and House dust mites (5).
Allergens from Penaeus aztecus listed by IUIS*
Pen a 1
*International Union of Immunological Societies (www.allergen.org) Jan. 2008.
Recombinant non-glycosylated protein produced in an E. coli strain carrying a cloned cDNA encoding Penaeus aztecus allergen Pen a 1
Common name: Tropomyosin
Biological function: Muscle protein
Mw: 36 kDa
Pen a 1 (4,6-7), the major protein of Brown shrimp (Penaeus aztecus), is a muscle protein tropomyosin (5,7-10). The allergen is heat-stable and is found in both raw and cooked Shrimp (11).
Tropomyosin comprises a class of highly conserved proteins with multiple isoforms found in both muscle and nonmuscle cells of all species of vertebrates and invertebrates. It is an abundant and heat-stable protein that constitutes up to 20% of total protein in the edible part of the animal. It is physically associated with actin and myosin in muscIe fibres and other motile filaments. Allergenic tropomyosins are found in invertebrates such as crustaceans (Shrimp, Lobster, Crab, Crawfish), arachnids (House dust mites), insects (Cockroaches), and mollusks (e.g., Squid); but there is a distinct lack of allergenic cross-reactivity between these tropomyosins and those from vertebrates: their tropomyosins are virtually nonallergenic (11). Tropomyosins present in vertebrate sources of food, e.g., bony fish, Beef, Pork and Chicken, are rarely allergenic to human beings, as compared to Lobster, Shrimp and Cockroach. Moreover, the invertebrate tropomyosins have high IgE cross-reactivity, and have therefore been referred to as panallergens. Tropomyosin is major allergen in crustaceans. Pen a 1 is quite important: approximately 80% of individuals sensitised to the Brown shrimp were reported to show IgE antibody reactivity to Pen a 1 (6,12).
Studies suggest that IgE-binding epitopes are restricted to certain parts of the Pen a 1 molecule and that Pen a 1 may have several similar epitopes. Pen a 1 epitopes do not appear to be located in the highly homologous parts of the molecule (9).
Studies have demonstrated that tropomyosin is an important allergen in crustaceans other than Shrimp, such as Lobster - both the Spiny lobster Panulirus stimpsoni and the American lobster Homarus americanus (Pan s 1, Hom a 1) (13-14) – the Crab Charybdis feriatus (Cha f 1) (15), mollusks such as the Squid Todarodes pacificus (Tod p 1) (16), the Snail Turbo cornutus (Tur c 1) (17), the Oyster Crassostrea gigas (Cra g 1) (18), and other invertebrates such as the House dust mites Dermatophagoides farinae (Der f 10) and Dermatophagoides pteronyssinus (Der p 10) (19-20), and the Cockroach Periplaneta americana (Per a 7) (21-22). Immunological relationships based on tropomyosin have also been demonstrated between crustaceans, Cockroaches and House dust mites, suggesting that tropomyosin is an important cross-sensitising panallergen (5).
In further evidence, rPen a 1 has been shown to extensively and specifically compete for IgE binding to extracts of other crustacean species, House dust mites and the German cockroach (6). The tropomyosins Pan s 1 (Spiny lobster, Panulirus stimpsoni) and Hom a 1 (American lobster, Homarus americanus) have been shown to have significant homology to Shrimp tropomyosin (14).
Nonetheless, studies suggest that there may be species-specific allergens in shrimps. In a comparision of the allergens from the edible Shrimp species Penaeus setifecus (White shrimp) and Penaeus aztecus (Brown shrimp) in 31 individuals with a history of immediate hypersensitivity reactions after Shrimp ingestion, skin-specific IgE to both types of extract was observed in 77% (23/30) of the subjects; 1 individual reacted to Brown shrimp extract only. Serum-specific IgE to both extracts was demonstrated in 16/31 study participants; 1 subject reacted only to White shrimp extract, and 2 subjects to Brown shrimp extract alone. Species specificity is important because it may explain the intermittent symptoms of some study subjects (23).
Recombinant Pen a 1 has been demonstrated to have allergenic activity not only similar to that of its own native allergen (6), but also very similar to that of the Greasyback shrimp (Metapenaeus ensis) tropomyosin (Met e 1) (7). Four recombinant, IgE-reactive Pen a 1 peptides, isolated in a study, showed various degrees of sequence identity with tropomyosins of other arthropods such as fruitfly (Drosophila melanogaster), House dust mite, helminths and vertebrates (4,7,11).
Tropomyosin from the Mite Blomia tropicalis (Blo t 10) was demonstrated to have cross-reactivity with Der p 10 of Dermatophagoides pteronyssinus, and to share up to 96% amino acid identity to tropomyosin of other Mites. Although Blo t 10 and Der p 10 were shown to be significantly cross-reactive, unique IgE epitopes do exist (24).
Tropomyosin has also been described in arthropods, namely Fly (Musca domestica), Moth (Ephestia spp.) and Spider (Tegenaria spp.). In a study of 100 patients allergic to household arthropods, cross-reactivity due to tropomyosin was demonstrated in a large variety of extracts obtained from insects, mites, crustaceans, mollusks and parasites (25).
While tropomyosin is a major factor in food allergy to invertebrates, it plays a less prominent role in inhalation allergy to Mites and Cockroaches. Tropomyosin has been isolated from Anisakis but does not appear to be an important allergen in Anisakis sensitisation (26). Similarly, Tur c 1, the tropomyosin from the gastropod Turbo cornutus, has an IgE-binding epitope that is dissimilar to those proposed for Cra g 1 from the Oyster Crassostrea gigas, and to Pen i 1 from the Shrimp Penaeus indicus (17).
Clinical and serological reactivity to both Mites and Snails has been described, and the development of sensitisation and allergic symptoms to Snail and Shrimp following immunotherapy treatment with Mite extract has been reported (6). IgE antibody reactivity to Shrimp can occur in an unexposed population of individuals; a study of Orthodox Jews unexposed to Shrimp demonstrated that some subjects allergic to HDM and/or Cockroach showed substantial IgE antibody reactivity to the Shrimp tropomyosin Pen a 1. Based on inhibition with Cockroach and/or Dust mite extracts, this reactivity appeared to be due to cross-reacting tropomyosins (27).
Therefore, as IgE-mediated food allergy to crustaceans and mollusks is relatively common, and affected individuals typically react to a range of different species, tropomyosin sensitivity may be useful as a diagnostic marker for allergic sensitisation to invertebrate foods. rPen a 1 has potential use as a diagnostic reagent to determine not only sensitisation specifically to Brown shrimp, but also sensitisation to and cross-reactivity with tropomyosin allergens from other species.
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Is tropomyosin an allergen in Anisakis?
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