Interactions of ovalbumin with sulphated polysaccharides: effects of pH, ionic strength, heat and high pressure treatment

Ovalbumin in solution (0.5-5 mg/ml), when subjected to high pressure treatment (>400 MPa for 20 min) or thermal treatment (>70 degrees C for 1-2 min) at pH 6.5 or 7.0, was found to become denatured and some aggregates were formed. ANS studies have shown that the protein surface hydrophobicity increases by 600% after pressure treatment (600 MPa for 20 min) and 55% after thermal treatment (80 degrees C for 1 min). Addition of polysaccharide (dextran sulphate (DS) or iota-carrageenan) (1 : 0.5 by weight) to the native protein at low ionic strength leads to no change in fluorescence intensity or shapes of size exclusion chromatography profiles. The presence of DS in the thermally processed samples appears to inhibit the formation of aggregates. Pressure processing (600 MPa for 20 min) at pH 7.0 was found to lead to weak electrostatic interactions between ovalbumin and DS, which became stronger at pH 6.5. The replacement of DS with L-carrageenan in the pressure treated sample, under the same experimental conditions, was found to give no interaction at pH 7.0, but a weak interaction at pH 6.5. It appears that the strength of complexation of ovalbumin with sulphated polysaccharides is dependent on the charge density on the polysaccharide. Complexation of polysaccharide with ovalbumin at low ionic strength seems to protect the protein against pressure-induced aggregation. But, addition of 0.5 M NaCl dissociates the complex(es), and the protective effect of the polysaccharide is lost. (C) 1999 Elsevier Science Ltd. All rights reserved.