Effects of Structural Changes in Beta-Lactoglobulin on its Allergenicity

DELORENZI, NÉSTOR; MORO, ANDREA; BUSTI, PABLO; BÁEZ, GERMÁN; BALLERINI, GRISELDA

Whey: Types, Composition and Health Implications

Nova Science Publishers

Año: 2012; p. 169 - 182

Among identified food allergens, milk exhibits a relevant allergenic potential related to the multiplicity and diversity of the involved proteins. The main whey protein, beta-lactoglobulin is usually considered to be the major cow?s milk allergen. This protein, absent in human milk, is resistant to gastric acid and to proteolytic action, and it has the ability to cross the intestinal mucosa. Food processing can alter the allergenic properties of proteins by hiding, destroying or exposing allergic epitopes through protein structural changes. Some researches have been carried out into the thermal stability of the beta-lactoglobulin allergen structure, and a significant reduction in its allergenecity was observed depending on the assayed thermal treatment. The use of microbial transglutaminase can also modify the immunogenicity of food proteins. Recently, it was suggested that the polymerization of beta-lactoglobulin catalyzed by transglutaminase might decrease the antigenic response by hiding antigenic sites due to molecular rearrangements. In our recent experiences, beta-lactoglobulin aggregates were formed by two different treatments, thermal and enzymatic. In the first one, 55 mg mL-1 beta-lactoglobulin solutions in 20 mM sodium phosphate buffer pH 6.8, were heated at 85 ºC from different periods of time: 1, 3, 5, 7, 10 and 15 min. In the enzymatic method, 25 mg mL-1 beta-lactoglobulin solutions in 20 mM sodium phosphate buffer pH 8.0, in the presence of 0.07 M cysteine, were incubated with transglutaminase (at a level of 1 U g-1 substrate), for different times: 30, 60, 120 and 180 min. Protein structural characterization was analyzed based on electrophoresis, fluorescence, circular dichroism and viscosity studies. Comparing both treatments, it was observed that heating produces a higher degree of unfolding and therefore, a higher index of surface hydrophobicity; less compact and more asymmetrical aggregates, with higher flexibility. These conformational changes can be related to the lesser degree of allergenicity observed, either by the major accessibility to a proteolytic attack, in the case of the unfolding, or by hiding epitopes, in the case of the formation of more compact protein aggregates.