Effect of sulfated polysaccharides on heat-induced structural changes in β-lactoglobulin

The mechanism that leads to a decreased aggregation of beta-lactoglobulin in the presence of dextran sulfate and lambda-carrageenan was investigated by assessing changes in the denaturation thermodynamics and protein structure. Differential scanning calorimetry results showed that the denaturation temperature (T-p) was about 4.6 degreesC higher in the presence of dextran sulfate, as compared with beta-lactoglobulin alone, whereas in the presence of lambda-carrageenan the difference in T-p was about 1.2 degreesC. Changes in protein structure studies using near-UV circular dichroism (CD) provided support for the calorimetric results. The transition midpoint (T-m) for denaturation of beta-lactoglobulin was about 5 degreesC higher in the presence of dextran sulfate than that found with beta-lactoglobulin alone and about 2 degreesC in the presence of lambda-carrageenan. Thermal modifications of the tertiary structure of beta-lactoglobulin were irreversible at temperatures above 67 degreesC; the addition of dextran sulfate reduced the extent of such modifications. Far-UV CD studies indicated that the addition of dextran sulfate or lambda-carrageenan did not affect secondary structure changes of beta-lactoglobulin upon heating. These studies indicate that dextran sulfate and lambda-carrageenan can enhance the stability of beta-lactoglobulin and thereby inhibit heat denaturation and aggregation.