- Email: [email protected]
Separation of ovotransferrin from chicken egg white without using organic solvents
Separation of ovotransferrin from chicken egg white without using organic solvents E. D. N. S. Abeyrathne,* H. y. Lee,* J. S. Ham,† and D. U. Ahn*‡1 *Department of Agricultural Biotechnology, Major in Biomodulation, Seoul National University, Seoul 151-921, Korea; †National Institute of Animal Science, Rural Development Administration, Suwon, 441-706, Korea; and ‡Department of Animal Science, Iowa State University, Ames 50010 to ultrafiltration to remove salts and concentrate the solution. The purity of the ovotransferrin was determined using SDS-PAGE, the protein identified using Western blot, and the estimated yield calculated by weighing the ovotransferrin after freeze drying. Over 85% purity and over 83% yield were obtained from the combinations of 5.0% (wt/vol) ammonium sulfate and 2.5% (wt/vol) citric acid followed by 2.0% (wt/vol) ammonium sulfate and 1.5% (wt/vol) citric acid. Activity of the ovotransferrin showed similar activity with previously separated ovotransferrin. However, this method is simpler and more cost effective than the previous method. The isolated ovotransferrin can be used as is or after modifications for various applications such as antimicrobial treatments, anticancer treatments, and iron-supplementing agents for humans.
Key words: ovotransferrin, ammonium sulfate, citric acid, egg white, separation 2013 Poultry Science 92:1091–1097 http://dx.doi.org/10.3382/ps.2012-02654
INTRODUCTION Ovotransferrin a monomeric glycoprotein consisting of 686 amino acids and 15 disulfide bonds. It was first characterized by Schade and Caroline in 1944 and was called Conalbumin. It was renamed as ovotransferrin after the finding that it can bind iron (Williams, 1968) and has the same amino acid sequence as the transferrin in human serum (Wu and Acero-Lopez, 2012). The molecular weight of ovotransferrin has 76 kDa and an isoelectric point of 6.1 (Ko and Ahn, 2008). One ovotransferrin molecule can bind 2 iron molecules and transports iron into the body (Wu and Acero-Lopez, 2012). Ovotransferrin is known to have a strong antimicrobial activity, and thus, can be used to improve the safety of foods (Valenti et al., 1982; Ibrahim et al., 2000; Ko et al., 2008). Zhang et al. (2011) reported that peptides derived from ovotransferrin also have an ©2013 Poultry Science Association Inc. Received August 1, 2012. Accepted January 14, 2013. 1 Corresponding author: [email protected]
ability to control microorganisms. Therefore, both ovotransferrin and its peptides can be used as antimicrobial agents in foods. Ovotransferrin is found in 2 main forms: apo- (iron free) and the holo- (iron bound). The chemical and physical properties of these 2 forms of ovotransferrin differ significantly (Wu and Acero-Lopez, 2012). Apoform (iron free) is colorless, whereas holo-form (iron bound) has a salmon pink color. The holo-form is more resistant to chemical and physical conditions than apoform. Iron ion (Fe3+) can be easily attached to ovotransferrin at pH >7.0, but is released at pH <4.5 (Ko and Ahn, 2008). Ovotransferrin has similar functions to lactoferrin found in milk, and both have iron scavenging and iron delivery functions (Abdallah and Chahine, 1999). Over the past years, different techniques have been developed to separate ovotransferrin from chicken egg white, but most of the methods developed were laboratory scale. Ion exchange chromatography is among the most widely used method, but has limitations for scale up preparation because of the costs involved, and separation speed and yield issues (Awade et al., 1994;
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ABSTRACT Ovotransferrin is one of the major egg white proteins that have antimicrobial activity as well as iron binding capability. The objective of this study was to develop a simple and easy method to separate ovotransferrin without using organic solvents. Egg white was separated from yolk, added in a 1:1 ratio to distilled water (DW), and then homogenized. The ovomucin in the diluted egg white was removed by centrifugation, adjusting the pH to 4.5 to 5.0. The resulting supernatant was added to different ratios of ammonium sulfate and citric acid, and then centrifuged after holding overnight at 4°C. The precipitant, which contains ovotransferrin, was dissolved in DW, and ovotransferrin was precipitated using different ratios of ammonium sulfate and citric acid. The precipitant collected after centrifugation was dissolved with DW and subjected
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Croguennec et al., 2001; Guérin-Dubiard et al., 2005). CM-Toyoperal 650M cation exchange was used for ovotransferrin separation by Tankrathok et al. (2009), but the yield was low (21%). Anion exchange chromatography was developed by Omana et al. (2010) and Wu and Acero-Lopez (2012) with an 80% yield, but the methods have difficulties in handling of resins and are not appropriate for large-scale separation of ovotransferrin due to its complexity and use of Q-sepharose chromatography columns. Recently, a large scale production of ovotransferrin was developed by Ko et al. (2008) who successfully separated ovotransferrin from egg white. They have used 43% ethanol to precipitate all the proteins in egg white except for ovotransferrin followed by 59% ethanol to precipitate ovotransferrin from the ovotransferrincontaining supernatant fraction. Because ovotransferrin is highly susceptible to extreme pH and temperature conditions, the apo-form of ovotransferrin in egg albumin was converted to holo-form before treatment with ethanol. After separating the holo-form of ovotransferrin, iron was released from ovotransferrin using pH adjustment and the released iron was removed using AG1-X2 resin. However, the removal of the released iron from ovotransferrin requires an extra step and raises the costs. Another common method used to separate ovotransferrin from egg white is precipitating it with ammonium sulfate (Fraenkel-Conrat and Feeney, 1950). Using acidic conditions along with high levels of ammonium sulfate, ovotransferrin was easily separated. However the level of purity was not reported and the amount of chemical added was very high, leading to a high separation cost. Affinity chromatography (immobilized metal affinity chromatography) has also been used to isolate ovotransferrin, but this method is very difficult to scale up (Wu and Acero-Lopez, 2012). The SDS-PAGE or immunoelectrophoresis can be used to isolate ovotransferrin, but the main problems with these methods are scaling up and denaturation of the protein (Desert et al., 2001). Therefore, developing an economical method to purify ovotransferrin in its natural form and at a large scale is vital to use ovotransferrin in the food and pharmaceutical industries. The objective of this work was to develop a simple and easy separation method of ovotransferrin (apo-form) from egg white without using organic solvent.
tific Inc., Waltham, MA). Standard ovotransferrin was purchased from Sigma-Aldrich (St. Louis, MO), and anti-ovotransferrin chicken antibody was purchased from MyBioSource (San Diego, CA). All other standards and rabbit anti-mouse IgG (H+L)-conjugated AP antibody were purchased from Bio-Rad (Bio-Rad Laboratory Inc., Richmond, CA).
MATERIALS AND METHODS
The optimum level of ammonium sulfate and citric acid was evaluated using SDS-PAGE. The SDS-PAGE was conducted under reduced conditions using a MiniProtein II cell (Bio-Rad). Ten percent SDS gel and Coomassie Brilliant Blue R-250 staining were used. A broad range molecular marker (Bio-Rad) was used as the marker. Quantification of electrophoreograms and determination of molecular weight of protein bands were conducted with a Pharmacia Phast Imagine Gel Analyzer using AlphaEase FC software (Alpha Innotech Corp., San Leandro, CA).
Chicken eggs (<7-d-old white eggs with average weight of 60 g) were purchased from a local market. Egg white from 10 eggs was pooled, and used as a replication. Each replication was prepared using 240 mL of the pooled egg white, and 3 replications were prepared. Ammonium sulfate, acetic acid, and citric acid were purchased from Fisher Scientific (Thermo Fisher Scien-
Egg white (240 mL) was diluted with 1 volume of distilled water (DW), homogenized using a hand mixer, the pH adjusted to 4.5 to 5.0 using 3 N HCl, and centrifuged at 3,400 × g for 30 min at 4°C. The supernatant was collected and treated with different levels of ammonium sulfate and acid combinations. Four different levels of ammonium sulfate ranging from 2.5 to 10% at 2.5% interval and 4 levels of citric acid from 2.0 to 3.5% at 0.5% interval were used. The samples were kept overnight at 4°C and centrifuged at 3,400 × g for 20 min at 4°C. The precipitant was measured with weighing balance, dissolved with 2 volumes of DW, and then desalted using an ultrafiltration unit (Hollow fiber, 10,000 Da cut-off-size, Flexstand, GE Healthcare Bio-Sciences Corp., Piscataway, NJ). Ovotransferrin was precipitated from the solution by again adding various levels of ammonium sulfate and acid combinations. Four different levels of ammonium sulfate ranging from 1.5 to 3% at 0.5% interval and 4 levels of citric acid from 1.0 to 2.5% at 0.5% interval were used. After storage overnight in a cold room, the samples were centrifuged at 3,400 × g for 20 min at 4°C. Samples were analyzed using SDS-PAGE to check the purity. Western blot was used to identify ovotransferrin. The whole separation procedure was replicated 3 times.
Yield of Ovotransferrin The yield obtained by using the final protocol was calculated because this protocol produced the best yield and purity. The yield was calculated using the reported value of ovotransferrin in egg white (Stadelman and Cotterill, 2001). The results of 3 replications were used for the yield calculation.
SDS-PAGE
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Materials
Separation of Ovotransferrin
SEPARATION OF OVOTRANSFERRIN FROM CHICKEN EGG WHITE
Western Blot
Statistical Analysis The data were analyzed using the Minitab 16.0 statistical software. One-way ANOVA was calculated, and Tukey’s test was employed (P < 0.05) to separate means.
RESULTS AND DISCUSSION Determining Ammonium Sulfate and Acid Conditions for the Separation of Ovotransferrin The isoelectric point of apo-ovotransferrin is pH 5.8 (Ko and Ahn, 2008). The main principle used in this study is that when the pH value of a protein solution changes, the charges on the amino acid side chains in a protein change. Therefore, the precipitation behavior of a protein in ammonium sulfate solution can be changed. Previous studies used saturated ammonium sulfate at pH 3.0 to precipitate ovotransferrin. However, the purity of ovotransferrin separated was low, and the amount of ammonium sulfate added was very high. Also, the yield was not specified in the original trial completed in 1950 (Wu and Acero-Lopez, 2012). It is known from large-scale separation of protein trials that altering the pH of a solution by acid or alkali can increase the ionic strength of a solution and change polarity of proteins, resulting in easy precipitation of proteins (Price and Nairn, 2009). Saturated ammonium sulfate with acidic condition was used to separate egg white proteins in early 1900s (Hopkins, 1900; Chick and Martin, 1913). Therefore, the first approach was testing how ovotransferrin is behaving under different ammonium sulfate and acid conditions. Acetic acid and citric acids are the 2 most common organic acids that can be added in foods, and thus the effects of acetic acid or citric acid and ammo-
nium sulfate combinations in precipitating ovotransferrin in egg white were tested. The amount and purity of proteins precipitated indicated that ammonium sulfate + citric acid performed better than ammonium sulfate + acetic acid in separating ovotransferrin (data not shown). Therefore, citric acid was used as the acid for ovotransferrin separation. Once the selection of acid was completed, the next step was to determine the optimum levels of ammonium sulfate and the selected acid (citric acid) to improve the separation of ovotransferrin from egg white. The comparison of ovotransferrin separation at different levels of ammonium sulfate and citric acid combinations indicated that increasing ammonium sulfate concentrations higher than 5% did not improve the precipitation of ovotransferrin when citric acid is at or higher than 2.5% (data not shown). In this study, ammonium sulfate granules instead of saturated ammonium sulfate solution were used to minimize the volume increase and the pH of the samples was around pH 3.5 to 4.0 when 2.5% citric acid was used. Figures 1 and 2 showed that 5.0% (wt/vol) ammonium sulfate in combination with 2.5% citric acid produced the best results among the conditions tested. The DC-protein assay (Bio-Rad) results also showed that there was no significant difference among the ammonium sulfate and citric acid combinations tested. Therefore, 5.0% (wt/vol) ammonium sulfate and 2.5% (wt/vol) citric acid combination was selected as the best conditions to isolate ovotransferrin from egg white. Ammonium sulfate has a tendency to precipitate ovalbumin, also. With 5% ammonium sulfate and 2.5% citric acid treatment, some ovalbumin was precipitated along with ovotransferrin. Thus, further purification was needed to remove ovalbumin from the crude ovotransferrin extract.
Purification of Ovotransferrin from the Crude Precipitant To remove ovalbumin coprecipitated with ovotransferrin, the precipitant was dissolved with 2 volumes of distilled water and then desalted using ultrafiltration. The desalted crude ovotransferrin was treated again with different levels of ammonium sulfate with fixed citric acid at 2.5% (wt/vol). Figure 3 showed that ammonium sulfate at 2.0% (wt/vol) was better than other ammonium sulfate levels in removing ovalbumin from ovotransferrin when 2.5% citric acid was combined. Increasing ammonium sulfate greater than 2% did not increase the yield of ovotransferrin. Figure 4 is the SDS-PAGE result with a fixed level of ammonium sulfate (2.0%, wt/vol) and varying levels of citric acid combinations. As the levels of citric acid were lowered to <1.5%, removal of ovalbumin from ovotransferrin was not as effective as those achieved at concentrations >1.5% citric acid. The DC-protein assay (Bio-Rad) results indicated that 1.5% (wt/vol) citric acid produced the highest protein content among the combinations (P < 0.05). Therefore, 1.5% citric acid and 2.0% am-
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Western blot was carried out using the method of Xie et al. (2002) with some modifications. After running SDS-PAGE, proteins in the gel were transferred to a nitrocellulose membrane (Bio-Rad), and the membrane was blocked with 5% skim milk solution in PBS with Tween (PBST). Ovotransferrin chicken antibody was added after diluting it to 1:15,000 ratios and incubated overnight at 4°C. The membrane was washed 3 times with PBST solution at 10-min intervals. Rabbit anti-mouse IgG (H+L) conjugated AP12 was diluted 1:20,000 with 5% skim milk dissolved in a PBST, added to the membrane as the secondary antibody, and incubated for 1 h at room temperature. The membrane was washed 3 times with PBST, exposed to ECL Prime diluted 1:1 with distilled water for 5 min, and checked with Chemidoc (Bio-Rad). Purified ovotransferrin was tested for its activity and purity using an ELISA kit (MyBioSource).
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monium sulfate combination was selected as the best conditions for purifying ovotransferrin from the crude precipitant. From this study, 5.0% (wt/vol) ammonium sulfate and 2.5% (wt/vol) citric acid combination at stage I and 2.0% (wt/vol) ammonium sulfate and 1.5% (wt/ vol) citric acid combination at stage II were selected as the best conditions for purifying ovotransferrin from chicken egg white. Western blot results confirmed that the protein separated was ovotransferrin (Figure 5).
Comparison with the Ethanol Precipitation Method The activity level of ovotransferrin was compared with that from the ethanol precipitation method (Ko and Ahn, 2008) because it is the latest large-scale separation method for ovotransferrin. The ELISA results of the ovotransferrin purified using the current method was 25.88 ± 3.71 ng/mL, whereas that separated using the ethanol method (Ko and Ahn, 2008) was 24.67 ±
Figure 2. Effect of citric acid on the separation of ovotransferrin from egg white. Lane 1 = marker; lane 2 = diluted egg white; lanes 3 to 6 = supernatant obtained from 2.0, 2.5, 3.0, and 3.5% citric acid, respectively, with a fixed level (5%) of ammonium sulfate; lanes 7 to 10 = dissolved precipitant from 2.0, 2.5, 3.0, and 3.5% citric acid, respectively, with 5% ammonium sulfate combinations.
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Figure 1. Effect of ammonium sulfate on the separation of ovotransferrin from egg white. Lane 1 = marker; lane 2 = diluted egg white; lanes 3 to 6 = supernatant obtained from 2.5, 5.0, 7.5, and 10.0% ammonium sulfate, respectively, with a fixed level (5%) of citric acid; lanes 7 to 10 = dissolved precipitant with 2.5, 5.0, 7.5, and 10.0% ammonium sulfate, respectively, with 5% citric acid combinations.
SEPARATION OF OVOTRANSFERRIN FROM CHICKEN EGG WHITE
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4.79 ng/mL when a 25 ng/mL sample was used (n = 3), indicating there was no difference in activity and purity between the 2 separation methods. Table 1 indicated that the yield of ovotransferrin separated using the current method was 83% (n = 3). In the ethanol precipitation method for purifying ovotransferrin from egg white (Ko and Ahn, 2008), addition of FeCl3 is a critical step. However, the current method does not require the addition of iron, and low
levels of ammonium sulfate and citric acid are needed. Therefore, this method uses fewer steps to purify ovotransferrin from egg white. In addition, apo-ovotransferrin was not denatured using our method, and conversion of ovotransferrin from the apo-form to the holo-form, and difficulties of removing released iron from ovotransferrin were not necessary. Even though the yield of ovotransferrin from the protocol developed in this study was lower than that of the ethanol method
Figure 4. Effect of citric acid on the separation of ovalbumin from ovotransferrin. Lane 1 = marker; lane 2 = diluted egg white; lanes 3 to 6 = supernatant obtained from 1.0, 1.5, 2.0, and 2.5% citric acid, respectively, with a fixed level (2.0%) of ammonium sulfate; lanes 7 to 10 = dissolved precipitant from 1.0, 1.5, 2.0, and 2.5% citric acid, respectively, with 2% ammonium sulfate combinations.
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Figure 3. Effect of ammonium sulfate on the separation of ovalbumin from ovotransferrin. Lane 1 = marker; lane 2 = diluted egg white; lanes 3 to 6 = supernatant obtained from 1.5, 2.0, 2.5, and 3.0% ammonium sulfate, respectively, with a fixed level (2.5%) of citric acid; lanes 7 to 10 = dissolved precipitant from 1.5, 2.0, 2.5, and 3.0% ammonium sulfate, respectively, with 2.5% citric acid combinations.
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(99%), this large-scale method does not use any solvent and can be an alternative method to purify ovotransferrin from egg albumin. Also, this method produced better apo-ovotransferrin yield than most of the published methods and can be scaled up easily. The highest yield of ovotransferrin reported so far was 96 to 98% when immobilized metal affinity chromatography (Cu-Sepharose 6B) was used, but this method is difficult to scale up (Wu and AceroLopez, 2012). Vachier et al. (1995) used ion exchange chromatography to separate ovotransferrin from egg white, but with the same scale-up problem. Table 1. Yield of ovotransferrin using the reported value in egg white Sample Egg white ovotransferrin1 Final ovotransferrin2
Weight (g)
Yield (%)
3.17 2.64 ± 0.17
100 83.4 ± 5.33
1The theoretical amount of ovotransferrin in 2× diluted egg white solution [egg white protein is 11% of total egg white and ovotransferrin is 12% of total egg white proteins (Stadelman and Cotterill, 2001)]. The original amount of egg white was 240 g per replication. 2Apo-ovotransferrin produced with ammonium sulfate and citric acid combination; n = 3.
The final protocol is shown in Figure 6.
Conclusion Ovotransferrin can be separated from egg white without using solvents. The best combinations for purifying ovotransferrin from chicken egg white was a 2-stage precipitation of ovotransferrin using 5.0% (wt/vol) ammonium sulfate and 2.5% (wt/vol) citric acid combination in step I and a 2.0% (wt/vol) ammonium sulfate and 1.5% (wt/vol) citric acid combination in step II. The yield of the ovotransferrin was 83%, and the purity was greater than 85%.
ACKNOWLEDGMENTS This study was supported jointly by the Cooperative Research Program for Agriculture Science & Technology Development (Project No. PJ008460) Rural Development Administration, Republic of Korea, and WCU (World Class University) program (R31-10056) through the National Research Foundation of Korea funded by the Ministry of Education, Science and Technology (Seoul, Korea).
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Figure 5. Western blot of ovotransferrin. Lane 1 = purified ovotransferrin; lane 2 = standard ovotransferrin (Sigma-Aldrich, St. Louis, MO); lane 3 = marker.
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REFERENCES Abdallah, F. B., and J. M. E. H. Chahine. 1999. Transferrins, the mechanism of iron release by ovotransferrin. Eur. J. Biochem. 263:912–920. Awadé, A. C., S. Moreau, D. Molle, G. Brule, and J. L. Maubois. 1994. Two-step chromatographic procedure for the purification of hen egg white ovomucin, lysozyme, ovotransferrin and ovalbumin and characterization of purified proteins. J. Chromatogr. A 677:279–288. Chick, H., and C. J. Martin. 1913. The precipitation of egg albumin by ammonium sulfate. A contribution to the theory of the “salting out” of proteins. Biochem. J. 7:380–398. Croguennec, T., F. Nau, S. Pezennec, M. Piot, and G. Brule. 2001. Two-step chromatographic procedure for the preparation of hen egg white ovotransferrin. Eur. Food Res. Technol. 212:296–301. Desert, C., C. G. Dubiard, F. Nau, G. Jan, F. Val, and J. Mallard. 2001. Comparison of different electrophoretic separations of hen egg white proteins. J. Agric. Food Chem. 49:4553–4561. Fraenkel-Conrat, H., and R. Feeney. 1950. The metal-binding activity of conalbumin. Arch. Biotechnol. 29:101–113. Guérin-Dubiard, C., M. Pasco, A. Hietanen, A. Quiros del Bosque, F. Nau, and T. Croguennec. 2005. Hen egg white fractionation by ion-exchange chromatography. J. Chromatogr. A 1090:58–67. Hopkins, F. G. 1900. On the separation of a pure albumin from egg white. J. Physiol. 25:306–330. Ibrahim, H. R., Y. Sugmito, and T. Akoi. 2000. Ovotransferrin antimicrobial peptide (OTAP-92) kills bacteria through a membrane damage mechanism. Biochim. Biophys. Acta 1523:196–205. Ko, K. Y., and D. U. Ahn. 2008. An economic and simple purification procedure for the large scale production of ovotransferrin from egg white. Poult. Sci. 87:1441–1450. Ko, K. Y., A. F. Mendonca, and D. U. Ahn. 2008. Effect of ethylenediaminetetraacetate and lysozyme on the antimicrobial activ-
ity of ovotransferrin against Listeria monocytogens. Poult. Sci. 87:1649–1658. Omana, D. A., J. Wang, and J. Wu. 2010. Co-extraction of egg white proteins using ion-exchange chromatography from ovomucin-removed egg white. J. Chromatogr. B Analyt. Technol. Biomed. Life Sci. 878:1771–1776. Price, N. C., and J. Nairn. 2009. Exploring Proteins: A Student’s Guide to Experimental Skills and Methods. Oxford Univ. Press, Oxford, UK. Stadelman, W. J., and O. J. Cotterill. 2001. Egg Science and Technology. 4th ed. Avi. Publ. Co., Westport, CT. Tankrathok, A., S. Daduang, R. Patramanon, T. Arakai, and S. Thammasirirak. 2009. Purification process for the preparation and characterization of hen egg white ovalbumin, lysozyme, ovotransferrin and ovomucoid. Prep. Biochem. Biotechnol. 39:380– 399. Vachier, M. C., M. Piot, and A. C. Awade. 1995. Isolation of hen egg white lysozyme, ovotransferrin and ovoalbumin, using a quaternary ammonium bound to a highly cross-linked agarose matrix. J. Chromatogr. A 664:201–210. Valenti, P., G. Antonini, M. R. Fanelli, N. Orsi, and E. Antonini. 1982. Antibacterial activity of matrix-bound ovotransferrin. Antimicrob. Agents Chemother. 21:840–841. Williams, J. 1968. A comparison of glycopeptides from the ovotransferrin and serum transferrin of the hen. Biochem. J. 108:57–67. Wu, J., and A. Acero-Lopez. 2012. Ovotransferrin: Structure, bioactivities and preparation. Food Res. Int. 46:480–487. Xie, H., G. R. Huff, W. E. Huff, J. M. Balog, P. Holt, and N. C. Rath. 2002. Identification of ovotransferrin as an acute phase protein in chickens. Poult. Sci. 81:112–120. Zhang, T., J. Zheng, H. Ye, Y. Yu, P. Zhao, and J. Liu. 2011. Purification technology and antimicrobial activity analysis of antimicrobial peptide from ovotransferrin. Chem. Res. Chin. Univ. 27:361–365.
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Figure 6. Schematic diagram for the separation of ovotransferrin from egg white. DW = distilled water. Color version available in the online PDF.
Key words: ovotransferrin, ammonium sulfate, citric acid, egg white, separation 2013 Poultry Science 92:1091–1097 http://dx.doi.org/10.3382/ps.2012-02654
INTRODUCTION Ovotransferrin a monomeric glycoprotein consisting of 686 amino acids and 15 disulfide bonds. It was first characterized by Schade and Caroline in 1944 and was called Conalbumin. It was renamed as ovotransferrin after the finding that it can bind iron (Williams, 1968) and has the same amino acid sequence as the transferrin in human serum (Wu and Acero-Lopez, 2012). The molecular weight of ovotransferrin has 76 kDa and an isoelectric point of 6.1 (Ko and Ahn, 2008). One ovotransferrin molecule can bind 2 iron molecules and transports iron into the body (Wu and Acero-Lopez, 2012). Ovotransferrin is known to have a strong antimicrobial activity, and thus, can be used to improve the safety of foods (Valenti et al., 1982; Ibrahim et al., 2000; Ko et al., 2008). Zhang et al. (2011) reported that peptides derived from ovotransferrin also have an ©2013 Poultry Science Association Inc. Received August 1, 2012. Accepted January 14, 2013. 1 Corresponding author: [email protected]
ability to control microorganisms. Therefore, both ovotransferrin and its peptides can be used as antimicrobial agents in foods. Ovotransferrin is found in 2 main forms: apo- (iron free) and the holo- (iron bound). The chemical and physical properties of these 2 forms of ovotransferrin differ significantly (Wu and Acero-Lopez, 2012). Apoform (iron free) is colorless, whereas holo-form (iron bound) has a salmon pink color. The holo-form is more resistant to chemical and physical conditions than apoform. Iron ion (Fe3+) can be easily attached to ovotransferrin at pH >7.0, but is released at pH <4.5 (Ko and Ahn, 2008). Ovotransferrin has similar functions to lactoferrin found in milk, and both have iron scavenging and iron delivery functions (Abdallah and Chahine, 1999). Over the past years, different techniques have been developed to separate ovotransferrin from chicken egg white, but most of the methods developed were laboratory scale. Ion exchange chromatography is among the most widely used method, but has limitations for scale up preparation because of the costs involved, and separation speed and yield issues (Awade et al., 1994;
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ABSTRACT Ovotransferrin is one of the major egg white proteins that have antimicrobial activity as well as iron binding capability. The objective of this study was to develop a simple and easy method to separate ovotransferrin without using organic solvents. Egg white was separated from yolk, added in a 1:1 ratio to distilled water (DW), and then homogenized. The ovomucin in the diluted egg white was removed by centrifugation, adjusting the pH to 4.5 to 5.0. The resulting supernatant was added to different ratios of ammonium sulfate and citric acid, and then centrifuged after holding overnight at 4°C. The precipitant, which contains ovotransferrin, was dissolved in DW, and ovotransferrin was precipitated using different ratios of ammonium sulfate and citric acid. The precipitant collected after centrifugation was dissolved with DW and subjected
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Croguennec et al., 2001; Guérin-Dubiard et al., 2005). CM-Toyoperal 650M cation exchange was used for ovotransferrin separation by Tankrathok et al. (2009), but the yield was low (21%). Anion exchange chromatography was developed by Omana et al. (2010) and Wu and Acero-Lopez (2012) with an 80% yield, but the methods have difficulties in handling of resins and are not appropriate for large-scale separation of ovotransferrin due to its complexity and use of Q-sepharose chromatography columns. Recently, a large scale production of ovotransferrin was developed by Ko et al. (2008) who successfully separated ovotransferrin from egg white. They have used 43% ethanol to precipitate all the proteins in egg white except for ovotransferrin followed by 59% ethanol to precipitate ovotransferrin from the ovotransferrincontaining supernatant fraction. Because ovotransferrin is highly susceptible to extreme pH and temperature conditions, the apo-form of ovotransferrin in egg albumin was converted to holo-form before treatment with ethanol. After separating the holo-form of ovotransferrin, iron was released from ovotransferrin using pH adjustment and the released iron was removed using AG1-X2 resin. However, the removal of the released iron from ovotransferrin requires an extra step and raises the costs. Another common method used to separate ovotransferrin from egg white is precipitating it with ammonium sulfate (Fraenkel-Conrat and Feeney, 1950). Using acidic conditions along with high levels of ammonium sulfate, ovotransferrin was easily separated. However the level of purity was not reported and the amount of chemical added was very high, leading to a high separation cost. Affinity chromatography (immobilized metal affinity chromatography) has also been used to isolate ovotransferrin, but this method is very difficult to scale up (Wu and Acero-Lopez, 2012). The SDS-PAGE or immunoelectrophoresis can be used to isolate ovotransferrin, but the main problems with these methods are scaling up and denaturation of the protein (Desert et al., 2001). Therefore, developing an economical method to purify ovotransferrin in its natural form and at a large scale is vital to use ovotransferrin in the food and pharmaceutical industries. The objective of this work was to develop a simple and easy separation method of ovotransferrin (apo-form) from egg white without using organic solvent.
tific Inc., Waltham, MA). Standard ovotransferrin was purchased from Sigma-Aldrich (St. Louis, MO), and anti-ovotransferrin chicken antibody was purchased from MyBioSource (San Diego, CA). All other standards and rabbit anti-mouse IgG (H+L)-conjugated AP antibody were purchased from Bio-Rad (Bio-Rad Laboratory Inc., Richmond, CA).
MATERIALS AND METHODS
The optimum level of ammonium sulfate and citric acid was evaluated using SDS-PAGE. The SDS-PAGE was conducted under reduced conditions using a MiniProtein II cell (Bio-Rad). Ten percent SDS gel and Coomassie Brilliant Blue R-250 staining were used. A broad range molecular marker (Bio-Rad) was used as the marker. Quantification of electrophoreograms and determination of molecular weight of protein bands were conducted with a Pharmacia Phast Imagine Gel Analyzer using AlphaEase FC software (Alpha Innotech Corp., San Leandro, CA).
Chicken eggs (<7-d-old white eggs with average weight of 60 g) were purchased from a local market. Egg white from 10 eggs was pooled, and used as a replication. Each replication was prepared using 240 mL of the pooled egg white, and 3 replications were prepared. Ammonium sulfate, acetic acid, and citric acid were purchased from Fisher Scientific (Thermo Fisher Scien-
Egg white (240 mL) was diluted with 1 volume of distilled water (DW), homogenized using a hand mixer, the pH adjusted to 4.5 to 5.0 using 3 N HCl, and centrifuged at 3,400 × g for 30 min at 4°C. The supernatant was collected and treated with different levels of ammonium sulfate and acid combinations. Four different levels of ammonium sulfate ranging from 2.5 to 10% at 2.5% interval and 4 levels of citric acid from 2.0 to 3.5% at 0.5% interval were used. The samples were kept overnight at 4°C and centrifuged at 3,400 × g for 20 min at 4°C. The precipitant was measured with weighing balance, dissolved with 2 volumes of DW, and then desalted using an ultrafiltration unit (Hollow fiber, 10,000 Da cut-off-size, Flexstand, GE Healthcare Bio-Sciences Corp., Piscataway, NJ). Ovotransferrin was precipitated from the solution by again adding various levels of ammonium sulfate and acid combinations. Four different levels of ammonium sulfate ranging from 1.5 to 3% at 0.5% interval and 4 levels of citric acid from 1.0 to 2.5% at 0.5% interval were used. After storage overnight in a cold room, the samples were centrifuged at 3,400 × g for 20 min at 4°C. Samples were analyzed using SDS-PAGE to check the purity. Western blot was used to identify ovotransferrin. The whole separation procedure was replicated 3 times.
Yield of Ovotransferrin The yield obtained by using the final protocol was calculated because this protocol produced the best yield and purity. The yield was calculated using the reported value of ovotransferrin in egg white (Stadelman and Cotterill, 2001). The results of 3 replications were used for the yield calculation.
SDS-PAGE
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Materials
Separation of Ovotransferrin
SEPARATION OF OVOTRANSFERRIN FROM CHICKEN EGG WHITE
Western Blot
Statistical Analysis The data were analyzed using the Minitab 16.0 statistical software. One-way ANOVA was calculated, and Tukey’s test was employed (P < 0.05) to separate means.
RESULTS AND DISCUSSION Determining Ammonium Sulfate and Acid Conditions for the Separation of Ovotransferrin The isoelectric point of apo-ovotransferrin is pH 5.8 (Ko and Ahn, 2008). The main principle used in this study is that when the pH value of a protein solution changes, the charges on the amino acid side chains in a protein change. Therefore, the precipitation behavior of a protein in ammonium sulfate solution can be changed. Previous studies used saturated ammonium sulfate at pH 3.0 to precipitate ovotransferrin. However, the purity of ovotransferrin separated was low, and the amount of ammonium sulfate added was very high. Also, the yield was not specified in the original trial completed in 1950 (Wu and Acero-Lopez, 2012). It is known from large-scale separation of protein trials that altering the pH of a solution by acid or alkali can increase the ionic strength of a solution and change polarity of proteins, resulting in easy precipitation of proteins (Price and Nairn, 2009). Saturated ammonium sulfate with acidic condition was used to separate egg white proteins in early 1900s (Hopkins, 1900; Chick and Martin, 1913). Therefore, the first approach was testing how ovotransferrin is behaving under different ammonium sulfate and acid conditions. Acetic acid and citric acids are the 2 most common organic acids that can be added in foods, and thus the effects of acetic acid or citric acid and ammo-
nium sulfate combinations in precipitating ovotransferrin in egg white were tested. The amount and purity of proteins precipitated indicated that ammonium sulfate + citric acid performed better than ammonium sulfate + acetic acid in separating ovotransferrin (data not shown). Therefore, citric acid was used as the acid for ovotransferrin separation. Once the selection of acid was completed, the next step was to determine the optimum levels of ammonium sulfate and the selected acid (citric acid) to improve the separation of ovotransferrin from egg white. The comparison of ovotransferrin separation at different levels of ammonium sulfate and citric acid combinations indicated that increasing ammonium sulfate concentrations higher than 5% did not improve the precipitation of ovotransferrin when citric acid is at or higher than 2.5% (data not shown). In this study, ammonium sulfate granules instead of saturated ammonium sulfate solution were used to minimize the volume increase and the pH of the samples was around pH 3.5 to 4.0 when 2.5% citric acid was used. Figures 1 and 2 showed that 5.0% (wt/vol) ammonium sulfate in combination with 2.5% citric acid produced the best results among the conditions tested. The DC-protein assay (Bio-Rad) results also showed that there was no significant difference among the ammonium sulfate and citric acid combinations tested. Therefore, 5.0% (wt/vol) ammonium sulfate and 2.5% (wt/vol) citric acid combination was selected as the best conditions to isolate ovotransferrin from egg white. Ammonium sulfate has a tendency to precipitate ovalbumin, also. With 5% ammonium sulfate and 2.5% citric acid treatment, some ovalbumin was precipitated along with ovotransferrin. Thus, further purification was needed to remove ovalbumin from the crude ovotransferrin extract.
Purification of Ovotransferrin from the Crude Precipitant To remove ovalbumin coprecipitated with ovotransferrin, the precipitant was dissolved with 2 volumes of distilled water and then desalted using ultrafiltration. The desalted crude ovotransferrin was treated again with different levels of ammonium sulfate with fixed citric acid at 2.5% (wt/vol). Figure 3 showed that ammonium sulfate at 2.0% (wt/vol) was better than other ammonium sulfate levels in removing ovalbumin from ovotransferrin when 2.5% citric acid was combined. Increasing ammonium sulfate greater than 2% did not increase the yield of ovotransferrin. Figure 4 is the SDS-PAGE result with a fixed level of ammonium sulfate (2.0%, wt/vol) and varying levels of citric acid combinations. As the levels of citric acid were lowered to <1.5%, removal of ovalbumin from ovotransferrin was not as effective as those achieved at concentrations >1.5% citric acid. The DC-protein assay (Bio-Rad) results indicated that 1.5% (wt/vol) citric acid produced the highest protein content among the combinations (P < 0.05). Therefore, 1.5% citric acid and 2.0% am-
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Western blot was carried out using the method of Xie et al. (2002) with some modifications. After running SDS-PAGE, proteins in the gel were transferred to a nitrocellulose membrane (Bio-Rad), and the membrane was blocked with 5% skim milk solution in PBS with Tween (PBST). Ovotransferrin chicken antibody was added after diluting it to 1:15,000 ratios and incubated overnight at 4°C. The membrane was washed 3 times with PBST solution at 10-min intervals. Rabbit anti-mouse IgG (H+L) conjugated AP12 was diluted 1:20,000 with 5% skim milk dissolved in a PBST, added to the membrane as the secondary antibody, and incubated for 1 h at room temperature. The membrane was washed 3 times with PBST, exposed to ECL Prime diluted 1:1 with distilled water for 5 min, and checked with Chemidoc (Bio-Rad). Purified ovotransferrin was tested for its activity and purity using an ELISA kit (MyBioSource).
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monium sulfate combination was selected as the best conditions for purifying ovotransferrin from the crude precipitant. From this study, 5.0% (wt/vol) ammonium sulfate and 2.5% (wt/vol) citric acid combination at stage I and 2.0% (wt/vol) ammonium sulfate and 1.5% (wt/ vol) citric acid combination at stage II were selected as the best conditions for purifying ovotransferrin from chicken egg white. Western blot results confirmed that the protein separated was ovotransferrin (Figure 5).
Comparison with the Ethanol Precipitation Method The activity level of ovotransferrin was compared with that from the ethanol precipitation method (Ko and Ahn, 2008) because it is the latest large-scale separation method for ovotransferrin. The ELISA results of the ovotransferrin purified using the current method was 25.88 ± 3.71 ng/mL, whereas that separated using the ethanol method (Ko and Ahn, 2008) was 24.67 ±
Figure 2. Effect of citric acid on the separation of ovotransferrin from egg white. Lane 1 = marker; lane 2 = diluted egg white; lanes 3 to 6 = supernatant obtained from 2.0, 2.5, 3.0, and 3.5% citric acid, respectively, with a fixed level (5%) of ammonium sulfate; lanes 7 to 10 = dissolved precipitant from 2.0, 2.5, 3.0, and 3.5% citric acid, respectively, with 5% ammonium sulfate combinations.
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Figure 1. Effect of ammonium sulfate on the separation of ovotransferrin from egg white. Lane 1 = marker; lane 2 = diluted egg white; lanes 3 to 6 = supernatant obtained from 2.5, 5.0, 7.5, and 10.0% ammonium sulfate, respectively, with a fixed level (5%) of citric acid; lanes 7 to 10 = dissolved precipitant with 2.5, 5.0, 7.5, and 10.0% ammonium sulfate, respectively, with 5% citric acid combinations.
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4.79 ng/mL when a 25 ng/mL sample was used (n = 3), indicating there was no difference in activity and purity between the 2 separation methods. Table 1 indicated that the yield of ovotransferrin separated using the current method was 83% (n = 3). In the ethanol precipitation method for purifying ovotransferrin from egg white (Ko and Ahn, 2008), addition of FeCl3 is a critical step. However, the current method does not require the addition of iron, and low
levels of ammonium sulfate and citric acid are needed. Therefore, this method uses fewer steps to purify ovotransferrin from egg white. In addition, apo-ovotransferrin was not denatured using our method, and conversion of ovotransferrin from the apo-form to the holo-form, and difficulties of removing released iron from ovotransferrin were not necessary. Even though the yield of ovotransferrin from the protocol developed in this study was lower than that of the ethanol method
Figure 4. Effect of citric acid on the separation of ovalbumin from ovotransferrin. Lane 1 = marker; lane 2 = diluted egg white; lanes 3 to 6 = supernatant obtained from 1.0, 1.5, 2.0, and 2.5% citric acid, respectively, with a fixed level (2.0%) of ammonium sulfate; lanes 7 to 10 = dissolved precipitant from 1.0, 1.5, 2.0, and 2.5% citric acid, respectively, with 2% ammonium sulfate combinations.
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Figure 3. Effect of ammonium sulfate on the separation of ovalbumin from ovotransferrin. Lane 1 = marker; lane 2 = diluted egg white; lanes 3 to 6 = supernatant obtained from 1.5, 2.0, 2.5, and 3.0% ammonium sulfate, respectively, with a fixed level (2.5%) of citric acid; lanes 7 to 10 = dissolved precipitant from 1.5, 2.0, 2.5, and 3.0% ammonium sulfate, respectively, with 2.5% citric acid combinations.
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(99%), this large-scale method does not use any solvent and can be an alternative method to purify ovotransferrin from egg albumin. Also, this method produced better apo-ovotransferrin yield than most of the published methods and can be scaled up easily. The highest yield of ovotransferrin reported so far was 96 to 98% when immobilized metal affinity chromatography (Cu-Sepharose 6B) was used, but this method is difficult to scale up (Wu and AceroLopez, 2012). Vachier et al. (1995) used ion exchange chromatography to separate ovotransferrin from egg white, but with the same scale-up problem. Table 1. Yield of ovotransferrin using the reported value in egg white Sample Egg white ovotransferrin1 Final ovotransferrin2
Weight (g)
Yield (%)
3.17 2.64 ± 0.17
100 83.4 ± 5.33
1The theoretical amount of ovotransferrin in 2× diluted egg white solution [egg white protein is 11% of total egg white and ovotransferrin is 12% of total egg white proteins (Stadelman and Cotterill, 2001)]. The original amount of egg white was 240 g per replication. 2Apo-ovotransferrin produced with ammonium sulfate and citric acid combination; n = 3.
The final protocol is shown in Figure 6.
Conclusion Ovotransferrin can be separated from egg white without using solvents. The best combinations for purifying ovotransferrin from chicken egg white was a 2-stage precipitation of ovotransferrin using 5.0% (wt/vol) ammonium sulfate and 2.5% (wt/vol) citric acid combination in step I and a 2.0% (wt/vol) ammonium sulfate and 1.5% (wt/vol) citric acid combination in step II. The yield of the ovotransferrin was 83%, and the purity was greater than 85%.
ACKNOWLEDGMENTS This study was supported jointly by the Cooperative Research Program for Agriculture Science & Technology Development (Project No. PJ008460) Rural Development Administration, Republic of Korea, and WCU (World Class University) program (R31-10056) through the National Research Foundation of Korea funded by the Ministry of Education, Science and Technology (Seoul, Korea).
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Figure 5. Western blot of ovotransferrin. Lane 1 = purified ovotransferrin; lane 2 = standard ovotransferrin (Sigma-Aldrich, St. Louis, MO); lane 3 = marker.
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Figure 6. Schematic diagram for the separation of ovotransferrin from egg white. DW = distilled water. Color version available in the online PDF.