The most important foods and food raw materials causing allergicreactions are cow’s milk, eggs, soy, wheat, peanuts, tree nuts,fish, and shellfish. Brazil nuts, belonging to this group of foodsand food raw materials, are the seeds of Bertholletia excelsa HBK.The nuts can be eaten both raw and roasted (Clark, 2002) andare also used as an ingredient in foods like bakery products (Clemente,Chambers, Lodi, Nicoletti, & Brett, 2004). Allergic reactionsin response to Brazil nut consumption can be very severe and evenlife threatening (Ewan, 1996). The most important allergen fromthese nuts, Ber e 1, is a member of the seed storage albumin group(Pastorello et al., 1998).Seed storage albumins usually are products of multigene families.For Ber e 1, so far, 6 different isoforms have been identified(NCBI accession numbers gi/839533, gi/384327, gi/112754, gi/99609, gi/384326, and gi/81557) (Moreno et al., 2004). Ber e 1 issynthesized as a 18 kDa polypeptide, which is post-translationallyprocessed into a 12 kDa polypeptide. This polypeptide is subsequentlyprocessed into two polypeptides of approximately 9 and3 kDa, linked together by 4 disulfide bridges (Altenbach, Pearson,& Sun, 1992). The protein contains 30–47% a-helices, dependingon the isoform (Moreno et al., 2004; van Boxtel et al., 2006). Oneimmunodominant conformational epitope has been identified onthe large polypeptide of the protein. The epitope corresponds toamino acids 26–63 of the large polypeptide of the protein (Swissprot
accession number P04403), which comprises a helix-turn-helix
conformation. When unfolded, this epitope binds at least four
times less IgE compared with its folded couterpart. Besides the
immunodominant conformational epitope, a linear epitope has
been identified on the small polypeptide of Ber e 1, corresponding
to amino acids 7–20 (QMQRQQMLSHCRMY) (Alcocer et al., 2004).
The denaturation temperature of Ber e 1 has been determined
at pH 2.0 to be approximately 83 C. The denaturation temperature
of Ber e 1 at neutral pH is expected to be higher than 110 C as no
change in transition was observed upon heating to 110 C (Koppelman
et al., 2004). Ber e 1, when subjected to in vitro peptic hydrolysis,
is cleaved into peptides ranging in molecular mass from <1 to
approximately 6.5 kDa. The 6.5 kDa fragment contained the region
in which the immunodominant conformational IgE epitope (Alcocer
et al., 2004) of the allergen is situated (Moreno, Mellon, Wickham,
Bottrill, & Mills, 2005). Upon reduction, this 6.5 kDa peptide
dissociates into peptides smaller than approximately 3 kDa. It
should be noted that after two hours of peptic digestion, which is considered as the average gastric transit time (Untersmayr & Jensen-
Jarolim, 2006) and generally used as the maximum time of
pepsin digestion in in vitro tests (Eiwegger et al., 2006; Fu, Abbott,
& Hatzos, 2002; Moreno et al., 2005; van Boxtel, van den Broek,
Koppelman, & Gruppen, 2007), approximately 25% of the protein
was still intact (Moreno et al., 2005). Consequently, it is assumed
that the IgE binding capacity of Ber e 1 (partly) remains after peptic
digestion (Moreno et al., 2005).
Under the same conditions, peptic digestion of reduced and
alkylated Ber e 1, compared to native Ber e 1, results in a fast degradation,
with the complete disappearance of the intact polypeptides
within 30 s (Koppelman et al., 2004). The structural
stability of the native protein thus seems to protect the protein
from peptic digestion.
The effects of heat-induced denaturation on the structure and
digestibility of Ber e 1 are not known. Therefore, the aim of our
research was to study the denaturation behaviour of Ber e 1 in
more detail, in order to assess whether heat processing could
induce changes in the digestibility of the protein and thereby
changes in its IgE binding capacity after digestion.
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