Thermally induced aggregates in mixtures of alpha-lactalbumin with ovalbumins from different avian species

Interactions between alpha-lactalbumin (alpha-La) and ovalbumin (OVA) in mixed systems (1:1 ratios; 2, 4, and 8% w/w total protein, respectively) heated at pH 7 and 80 degrees C for 15 min were studied using sodium dodecyl sulfate--polyacrylamide gel electrophoresis (SDS-PAGE), gel filtration chromatography (GFC), and competitive enzyme-linked immunosorbent assay (ELISA). Although alpha-La alone did not form aggregates upon heating, it formed large aggregates when heated with OVA. The aggregated molecules eluted at the void volume had a molecular mass >300 kDa. The aggregation process was quantitatively affected by different avian OVAs from five species, possessing different numbers of free sulfhydryl groups. The amount of aggregates (M(w) > 300 kDa) increased in proportion to total protein concentration, and the amount of intermediate components (M(w) < 300 kDa) and monomeric OVA and alpha-La also changed, correlating with total protein concentration during heating. The results also indicated that the aggregates and intermediates, which contained dimeric and trimeric alpha-La, were mainly formed by the intermolecular disulfide bonds. The different interactions observed in several avian OVAs may explain heat-induced gelation of various avian OVAs as well as the enhancement of heat-induced gelation of OVA by alpha-La.