Allergen

The latex of Carica papaya is well known for being a rich source of the cysteine endopeptidases Papain, Chymopapain, glycyl endopeptidase (also known as Papaya proteinase IV) and caricain (formerly known as Papaya proteinase III). The proteinases are synthesised as inactive precursors that convert into mature enzymes within 2 minutes of wounding of the plant, when the latex is abruptly expelled. Papaya latex also contains other enzymes as minor constituents, such as a class-II and a class-III chitinase, an inhibitor of serine proteinases and a glutaminyl cyclotransferase. Chymopapain in its native state contains two thiol functions.

Chymopapain is a proteolytic enzyme with a molecular weight of 27 kDa and is highly homologous with Papain, having considerable structural similarity with Papain and Papaya proteinase omega, including conservation of the catalytic site and of the disulphide bonding. Chymopapain is like Papaya proteinase omega in carrying 4 extra residues, but differs from both Papaya proteinases in the composition of its S2 subsite, as well as in having a second thiol group, Cys-117. Chymopapain shares 126 identical amino acid residues (58%) with Papain and 141 (65%) with Papaya proteinase omega, including the 3 disulfide bridges and the free cysteine in position 25, required for activity.

Repeated mechanical wounding of Carica papaya has been shown to result in either accumulation or activation, in the latex, of Papain, Chymopapain and Caricain. New cysteine protease activity was found to transiently accumulate in the latex collected from newly wounded fruits.

At low pH, the Papaya cysteine proteinases, Papain, caricain, Chymopapain, and glycine endopeptidase, are found to undergo a conformational transition that instantaneously converts their native forms into forms that are unstable and rapidly degraded by pepsin.

Route Of Exposure

Unexpected exposure

A dangerous risk of Chymopapain injection is anaphylaxis. Any allergic or other reaction to Chymopapain requires assessment of the patient’s exposure to Chymopapain and closely related Papain. Modes of sensitisation to Chympapain include Papaya in any form, Pineapple (Bromelain), various mixed foods (appetizer mixes, yogurt), beverages containing exotic fruits, beer, Coca-Cola, meat tenderiser, digestive aids, ENT solutions, anti-inflammatory drugs, contact lens cleaning solutions, detergents, and ointments. Occupational exposure would come through inhalation of airborne particles.

Clinical Relevance

IgE-mediated reactions

Chymopapain may commonly induce symptoms of allergy, including anaphylaxis and anaphylactoid reactions, in sensitised individuals undergoing chemonucleolysis. Although it has not been clinically investigated, there is a possibility of subsequent allergy to foods containing Chymopapain or closely related Papain. Adverse reactions to Chymopapain have rarely been reported as a consequence of oral ingestion or topical use, but there is likely to be under-reporting. A high index of suspicion is required, as indicated by adverse reactions to closely related Papain. In a study of 5 subjects (among 475 examined) with seasonal allergic disease and positive skin-specific IgE to both Papain and local pollens, double-blind placebo-Papain challenges were positive in all. Papain-induced symptoms included palatal itching, watering, itchy eyes, sneezing, rhinorrhea, abdominal cramps, diarrhea, and diaphoresis. Circulating Papain-specific IgE was detected in all the Papain-sensitive individuals. A patient developed IgE-mediated sensitisation and subsequently ocular angioedema and conjunctivitis from Papain contained in a commercial contact lens cleansing solution. Serum- and skin-specific IgE to Papain and Chymopapain were detected. When the lens solution containing Papain was stopped, there was resolution of the allergic symptoms. The authors suggested that recognition of this route of Papain-induced sensitisation may be important for those patients undergoing chemonucleolysis with Chymopapain who may be at greater risk of developing a systemic allergic reaction after injection of this enzyme. Importantly, although adverse reactions following chemonucleolysis have been attributed to Chymopain, other cysteine proteases present may have made a significant contribution. For example, Chymodiactin, a pharmaceutical preparation of Chymopapain used in chemonucleolysis, contains all 4 Papaya cysteine proteinases. This preparation was evaluated to determine what contribution each of the 4 proteinases makes to allergic responses that may occur. Chymodiactin contained 70% Chymopapain, 20% caricain, 4% glycyl endopeptidase, and 0.1% Papain. Among 12 sera containing IgE antibodies to Chymodiactin, IgE antibodies were found to all 4 proteinases in most, but in varying proportions. Antibodies to glycyl endopeptidase were predominant in 8 sera. The mean amounts of IgE directed against each protein were: glycyl endopeptidase, 4.21 IU/ml; caricain, 2.9 IU/ml; Chymopapain, 1.97 IU/ml; and Papain, 1.39 IU/ml. Total serum IgE levels showed little correlation with IgE responses to Chymodiactin. The results suggested that removal of glycyl endopeptidase and caricain from pharmaceutical preparations of Chymopapain may help reduce the incidence of allergic reactions from chemonucleolysis. Chymopapain used in chemonucleolysis may induce IgE-mediated reactions in approximately 0.18% to 2% of the patients treated for the first time. The incidence of severe anaphylactic reactions has been estimated to be between 0.18% and 0.45% after the first injection and 9 to 17% after a second injection. Eight deaths reported before 1974, 7 deaths from 1982 to 1991. Between 1982 and 1991, 121 adverse events in 135,000 patients were reported to the US Food and Drug Administration. Seven cases of fatal anaphylaxis, 24 infections, 32 bleeding problems, 32 neurologic events, and 15 miscellaneous occurrences were found.

To show examples: a report was made of 2 cases of Chymopapain allergy. In one case, a 25-year-old woman who had rhinitis, asthma, and urticaria associated with occupational health hazards was rejected for chemonucleolysis; in the other case, a 59-year-old man had a predictably severe anaphylactic reaction to Chymopapain. Both patients had IgE antibody against Chymopapain.

Many reports have been published describing anaphylaxis following the injection of Chymopapain. Although rare, anaphylaxis to Chymopapain induced by exertion has been reported.

The prevalence of anaphylaxis has been estimated to be around 1% of such cases. However, a wide variation occurs. In a report of 4,282 patients treated by intradiscal injection of Chymopapain under local anesthesia during a 12-year period from 1970 through 1981, 15 (0.35 per cent) of these patients experienced an anaphylactic reaction. Twelve patients had subjective early warning signs, including a total-body burning or tingling sensation (5 patients), a general feeling of ill health (4 patients), and diffuse pruritus (3 patients). Profound hypotension without subjective warning symptoms was the first indication of anaphylaxis in 3 patients. Ten of the 15 patients were women. Repeat chemonucleolysis is associated with a far greater risk of anaphylaxis. A study reported a 17% prevalence of anaphylaxis during chemonucleolysis among 35 patients who underwent a repeat chemonucleolysis procedure. Studies have reported the usefulness of skin-specific IgE determination for Chymopapain sensitisation. However, anaphylaxis to Chymopapain skin-specific IgE determination has been reported. A number of studies has shown that first exposure to Chymopapain through chemonucleolysis is frequently associated with subsequent sensitisation to Chympopapain. In a study group tested before chemonucleolysis, 2.3%-3.3% were shown to have Chymopapain-specific IgE on skin testing, which increased to 42.9% after chemonucleolysis. Sensitisation was time-dependent: between the 3rd and 12th week after chemonucleolysis, more than 70% of the patients developed positive skin-specific IgE to Chymopapain. Both serum IgE and IgG to Chymopapain increases after chemonucleolysis. Other studies concur. Seven hundred patients were investigated prospectively before undergoing chemonucleolysis. Based on the results obtained, the subjects were classified into 4 groups: Group 1 (225 non-atopic non-Papain-exposed subjects), Group 2 (285 non-atopic Papain- exposed subjects), Group 3 (69 atopic non-Papain-exposed subjects), and Group 4 (121 atopic Papain-exposed subjects). Latent sensitisation to Papain was observed in 0.4% of subjects in Group 1, 3.16% in Group 2, 5.8% in Group 3 and 7.4% in Group 4. Prick tests performed 6 weeks and 6 months after chemonucleolysis revealed newly acquired sensitisation in 36% of the patients. Of 104 consecutive patients who were candidates for chemonucleolysis, Chymopapain-specific IgE was found in 2 (1.9%). Sixteen patients (16%) were sensitised after chemonucleolysis. Evaluation of possible risk factors was unhelpful in predicting sensitisation, and the most appropriate way to detect Chymopapain sensitisation was through evaluation of skin- or serum- specific IgE sensitisation to Chymopapain.

Skin-specific IgE evalution for Chymopapain was performed on 3 groups of patients: Group 1 (75 patients awaiting chemonucleolysis with Chymopapain), Group 2 (42 of these 75 patients 2-3 weeks after chemonucleolysis), and Group 3 (60 atopic patients suffering from asthma and/or rhinitis with positive skin tests to at least 1 of a panel of airborne allergens). Chymopapain skin-specific IgE was found in 1 patient of Group 1, 1 patient of Group 3, and 11 patients of Group 2, confirming that chemonucleolysis has a highly significant sensitising effect. Chymopapain-specific IgE was found in 1 out of 75 patients before, and in 14 out of 45 patients after chemonucleolysis. Chymopapain-specific IgE values were studied in 91 patients prior to and for 2 months after Chymodiactin chemonucleolysis. A total of 8.8% (17/91) developed IgE antibody levels greater than or equal to 0.06 IU/ml. Those patients with detectable IgE antibody levels prior to chemonucleolysis were more likely than those with nondetectable preinjection levels (36.4% versus 4%) to develop Chymopapain-specific IgE levels greater than or equal to 0.06 IU/ml. The detection of patients at risk of allergy to Chymopapain before and after chemonucleolysis is a prerequisite. Although skin- and serum-specific IgE determination is crucial for the evalution of these patients, an independently high index of suspicion is still required for potential immune-mediated reactions during chemonucleolysis. Three of 232 (1%) of patients with no Chymopapain skin-specific IgE and 5/6 (83%) of patients with positive Chymopapain skin-specific IgE reacted to the therapeutic injection of Chymopapain.

Cross-Reactivity

Cross-reacting antibodies between Papain and Chymopapain have been described, and clinical evidence suggests that Papain and Chymopapain may share common allergenicity. Patients who become sensitised to Papain may subsequently experience an allergic reaction when they are exposed to Chymopapain. Serum samples from 6 patients who demonstrated 4+ skin-specific IgE reactions to Papain were shown to have serum-specific IgE antibodies reacting both with Papain and Chymopapain. Also, serum from 12 clinically nonreactive patients who had had chemonucleolysis with Chymopapain were shown to have serum-specific IgE to Papain as well as to Chymopapain.

Cysteine proteases are found in mite species (Der p 1m Der f 1 and Eur m 1), humans (cathepsins B, K, L, S and O), plants (Papain, Chymopapain and actinidin) and parasites (cruzain, cathepsin L-like Leishmania protease, Entamoeba ACP1 protease, Schistosoma Q26534, Q11003, and cathepsin L proteases). These cysteine proteases were shown to display an epitope corresponding to that previously identified on Der p 1, but with varying shapes and degree of accessibility. As these cysteine proteases seem to use similar accessible structures, they may be cross-reactive, but this will require further investigation.