Thermal Aggregation of β-Lactoglobulin: Effect of pH, Ionic Environment, and Thiol Reagent

Heat-induced 13-lactoglobulin (1.2 mgt ml) aggregation in NaC!, CaCI2, and thiol-blocking agent (N-ethylmaleimide) solutions was determined by measuring dynamic turbidity changes at temperatures ranging from 25 to 96C. 13-Lactoglobulin in distilled water exhibited a single transition (~76C) in proteinprotein interaction (aggregation). An increase in pH from 5.50 to 6.50 suppressed the transition, whereas addition of NaCI (.02 to 1.0 M) and CaCl2 (.005 to .2 M) promoted the transition. N-Ethylmaleimide (4.0 to 10.0 mM) decreased the transition temperature and, in the absence of salts, induced a second transition. However, the combination of N-ethylmaleimide and NaCI or CaCl2 produced a single, large transition pe~. Results indicate that 13-lactoglobuhn aggregation is most sensitive to low pH, greatly depends on the type and concentration of specific salts, and involves electrostatic and possibly hydrophobic forces and sulfhydryl reactions.