Each stage of the protein purification process is compared against the molecular weight marker (Sigma-Aldrich, catalogue No. M3913) in Fig 1.
The high-molecular-weight-fraction (in excess of 66 kDa) in lanes 1, 2 and 3 was identified as ovotransferin. The presence of fractions with molecular weight in the range of 45 to 30 kDa (fraction C) was also determined in lanes 1, 2 and 3. The absence of lysozyme in the analyzed fraction (extract from the entire chicken egg white) and the resulting differences in electrophorogram and chromatogram results could be attributed to the clogging of gel pores and excessive ovalbumin content in the sample (ovalbumin accounts for 50% of total proteins in chicken egg white). A 28 kDa fraction (electrophoretic mobility the same as mobility of 28 kDa standard) which corresponds to the molecular weight of ovomucoid was reported in lane 4.
The trypsin inhibitor (Fig 2) and the ethanol-precipitated fraction (Fig 3) were separated by HPLC. Two peaks were reported for the trypsin inhibitor. Peak 1 with retention time of approximately 24 minutes was associated with ovomucoid and peak 2 with estimated retention time of 32 minutes was identified as lysozyme. The purified ovomucoid solution in the ion exchange column revealed one peak. Based on the obtained chromatograms, electrophoresis results and glycoprotein staining, it was identified as pure ovomucoid (peak 1 in Fig 3).
The electrophoretic separation of glycated protein vs. native protein is presented in Fig 4. As regards samples incubated at 37°C for 3 and 7 days, no significant differences were determined between the apparent molecular weight of the studied proteins (lanes 1 and 2) and native protein (lane N). An estimated c.a. 2 kDa increase in molecular weight was reported for proteins glycated over 14 days (lane 3).
In samples incubated at 60°C, an apparent molecular weight increase of 5-6 kDa was observed for all time variants (lanes 4, 5 and 5). As demonstrated in the electrophorogram, the most significant variations in molecular weight were induced by a temperature of 60°C in all glycation variants. The degree of glycation (Fig 5) was identified by determining the content of free amino groups. Statistical analyses revealed significant variations in the content of free amino groups in comparison to native protein.
The content of free amino groups in unmodified native protein (control) was used as the 100% reference. In samples glycated at 37°C, the content of free amino groups decreased by 43% in proteins incubated for 3 days, 31% in proteins incubated for 7 days and 29% in proteins incubated for 14 days in comparison with control. Protein modification at 60°C lowered the share of free amino groups by 78% in samples glycated for 3 days, 79% in samples glycated for 7 days and 76% in samples glycated for 14 days in comparison with native protein (control). The differences in the content of free amino groups were statistically significant (p≤0.01) in every analyzed variant.
Quantitative variations in ovomucoid immunoreactivity were evaluated by the competitive ELISA test. Changes in immunoreactivity were determined as percentage variations in antibody-antigen binding interactions relative to unmodified protein.
The effect of temperature on changes in ovomucoid immunoreactivity is presented in Fig 6.
At a temperature of 37°C, significant variations (p≤0.05) were observed between ovomucoid samples incubated for 7 and 14 days. As regards samples incubated at 60°C, significant variations were reported between the control and all time variants at p≤0.01 for samples incubated over 3 and 7 days and p≤0.05 for samples incubated over 14 days.
The effect of glycation on changes in ovomucoid immunoreactivity is illustrated in Fig 6. In reference to samples glycated at a temperature of 37°C, significant variations were noted between unmodified protein (control) and proteins modified for 3 days (p≤0.01). Significant differences between glycation variants lasting 3 and 7 days and 7 and 14 days were determined at p≤0.05. Statistically significant variations (p≤0.01) were also reported by comparing glycation time of 3 days and 14 days.
Statistically significant differences (p≤0.01) were observed between unmodified protein (control) and all variants modified at a temperature of 60°C. A comparison of glycation times of 3 and 14 days also revealed significant differences (p≤0.05).
A noticeable drop in ovomucoid’s electrophoretic mobility was found. Extensive glycation leads to increase of molecular weight, but observed changes of electrophoretic mobility may be enlarged by another phenomenon. Acidic character of ovomucoid could pose a problem (a higher number of acidic amino acid residues than alkaline residues and the presence of sialic acid residues). The observed molecular weight of acidic proteins may be higher than their actual weight (
Chiou and Wu, 1999). There is no published explanation of molecular mechanism of this phenomenon until now. Glycation causes modification of basic residues and may affect acid-base balance in protein molecules.
Ovomucoid possess many epitopes indicated in the Immune Epitope Database
(Vita et al., 2015). Increase of immunogenicity due to moderate glycation may be explained by the exposition of additional epitopes due to protein structure changes, induced by glycation. Some products of reaction between sugars protein form rigid structures consisting of multiple, condensed rings, connecting two lysine or arginine residues
(Ashraf et al., 2015). Additional steric repulsion, caused by formation of such moieties may lead to partial disruption of protein secondary and tertiary structure and hence to exposition of epitopes, which are normally inaccessible for antibodies. This explanation may be supported by the results of experiment described by
Ma et al., (2013). In the experiment of these authors moderate glycation led to increase of immunoreactivity of ovalbumin, associated with decrease of the b-sheet structure content and increase of surface hydrophobicity. These effects mean, that structural changes of ovalbumin provided exposition of additional epitopes, containing hydrophobic amino acid residues and buried within native protein structure. Similar phenomenon could occur due to moderate glycation of ovomucoid during the experiment, described here.
Alternative explanation of ovomucoid immunogenicity increase due to glycation at 37°C for 3 and 7 days, taking into account possibility of formation of new epitopes by modified amino acid residues itself
(Gupta et al., 2016), is less likely. There was no simple proportion between glycation degree and immunogenicity. Extensive glycation at 60°C minimized interactions between protein and antibodies. It suggests, that hindrance of such interactions by modified residues
(Gupta et al., 2016) was dominant phenomenon at high glycation degree.
Glycation is considered as a method for reduction of food protein allergenicity,but is not strongly recommended for this purpose
(Gupta et al., 2016). Influence of extensive glycation on ovomucoid immunogenicity was studied by
Jiménez-Saiz et al., (2011) and observed possibility to reduce immunogenicity via extensive glycation. Drastic reaction conditions may lead to occurrence of unpredicted and undesired products. There was necessary to include experiment with moderately glycated protein and compare its immunogenicity with this extensively glycated. Our results show, that moderate glycation (performed at relatively low temperature 37°C) does not provide reduction of ovomucoid immunoreactivity. Significant reduction of this property requires high temperature and/or long time leading to extensive glycation.
Davis et al., (2016) state that advanced glycation end products are a diverse group of compounds, which can form endogenously and glycation of molecules may negatively affect their function. The American Diabetes Association makes no recommendation regarding avoidance of advanced glycation end products, but many researchers are concerned that they may be pro-inflammatory and way worsen cardiac function and kidney function and also contribute to obesity.