20098 Daltons (mature protein only)

28 kDa (Holen & Elsayed, 1990 [1354]; Roy et al. 2003 [1335])

Ovomucoid often runs anomalously on SDS-PAGE at 30-45 kDa (Bessler et al. 1999) [1276].

Mass spectroscopy of native ovomucoid gives a smooth distribution of masses from 25 to 31 kDa (Bessler et al. 1997) [1651]

Mine & Zhang (2002) [1650] used pooled sera from 8 egg-allergic patients and overlapping decapeptides synthesized on cellulose membranes. They identified 9 IgE epitopes, residues 32-42, 40-50, 56-66, 71-75, 80-90, 101-105, 121-130, 159-174 and 179-186. Substitution within these peptides identified some critical amino acids. IgG epitopes were also identified and were in general different. The IgE epitopes included more charged and polar amino acids than the IgG epitopes.

Holen et al. (2001) [1273] used pooled sera from 7 egg-allergic patients and 18 peptides (overlapping by 5 residues). They identified IgE binding to residues 1-14, 11-24, 31-34, 51-64, 61-74, 101-114 and 121-134. They also found 10 distinct T cell epitopes. 6 peptides were recognized by IgE and 4 peptides were exclusively T cell epitopes.

Besler et al. (1999) [1276] identified two distinct epitopes in residues 90-121 and 134-186 using chemically and enzymically cleaved ovomucoid and pooled sera from 6 egg allergic patients (RAST class 3).

Cooke & Sampson (1997) [1274] identified residues 1-20, 49-56, 85-96, 115-122 and 175-186 as IgE binding epitopes using dodecapeptides and pooled sera from 7 egg-allergic patients. The strongest IgE binding was to domain 2 i.e. residues 65-120.  

Matsuda et al. (1985) [1659] reported that human IgE binding to ovomucoid third domain was greatly reduced by deglycosylation. Matsuda et al. (1986) [1658] added that mouse anti-ovomucoid IgG bound to ovomucoid third domain after deglycosylation.

Hirose et al (2004) [1334] show that heating ovomucoid at 100°C for 30 minutes causes irreversible changes that are detected by monoclonal antibodies. However, the protein does not aggregate and human IgE from 18 patients bound to both native and denatured forms. Similarly Urisu et al (1997) [1290] also found that the main allergen of heated egg white was ovomucoid.

The concentration necessary for 50% specific IgE inhibition for denatured ovomucoid was 1700-fold higher compared with the intact and oxidized ovomucoid showing that the allergenicity of ovomucoid was substantially reduced on denaturation (Holen et al. 2001) [1273].

Ovoinhibitor, P10184, is a homologue present at lower concentration in egg white and in chicken plasma which is only 46% identical in sequence. This has not been reported as an allergen in hen's egg but a homologue may be an allergen from bird's nest soup (Ou et al, 2001 [1223]; Goh et al. 2001 [1224]).

Ovomucoid is a serine protease inhibitor. It is heavily glycosylated at up to 5 sites and is cross-linked by 9 disulfides. It comprises approximately 11% by weight of the protein in egg white.

Roy et al. (2003) [1335] describes the simultaneous purification of lysozyme, ovalbumin, and ovomucoid from hen egg white. The crude egg white extract was passed through a cation exchanger (Streamline SP). Ovalbumin and ovomucoid did not bind and ovalbumin could be precipitated by 5% trichloroacetic acid. Ovomucoid was removed from the supernatant by precipitation with ethanol and gave a single band on Commassie blue stained SDS-PAGE.

Rupa & Mine (2003) [1336] report expression and purification of recombinant ovomucoid with a histidine tag in E. coli. The antigenicity and allergenicity of recombinant ovomucoid were almost same as the native protein, which is unexpected (Matsuda et al. 1985) [1659].

The 3-D structure of the 3rd domain of turkey ovomucoid was reported by Read et al. (1983) [1220] as a complex with Streptomyces griseus proteinase B and by Fujinaga et al. (1987) [1221] as a complex with chymotrypsin. Bateman et al. (2001) [1219] reported further complexes of variants. The structures have PDB codes 1CHO, 1CTO, 1SGY, 3SGQ, 1DS2, 2SGP, 1SGP, 1SGQ, 1SGR, 3SGB. The 3D-structures of the inhibitors from Japanese quail 3OVO and silver pheasant 4OVO were reported by Musil et al. (1991) [1222]. The domains include a single alpha-helix which is cross-linked by two disulfides to the loop which binds to the protease's active site. There is also a short 3-strand anti-parallel beta-sheet often linked to the C-terminus by another disulfide.

Sequence data is available for ovomucoid from several avian species including turkey P68390 and Japanese quail P01003. These are 83% and 76% identical over the mature protein. Thus IgE cross-reactivity is likely. IgG raised against quail ovomucoid binds to hen and duck ovomucoid (Takahashi et al. 1999 [1657]).