Thermal Stability of alpha-Lactalbumin

Abstract

The objective of this research was to quantitatively describe the the denaturation and aggregation processes of a-lactalbumin at neutral pH in order to understand their interrelationship and effect on solution stability. Three different preparations of a-La had similar denaturation temperatures, enthalpies and % reversibility as measured by differential scanning calorimetry. However, Native PAGE reveled three non-native monomer bands that corresponded to three distinct dimer bands indicating specific intramolecular disulfide bond shuffling leads to formation of disulfide-specific dimers. The apo protein was the most thermostable to turbidity development. The Ca-La was the most thermostable holo- preparation. Turbidity development at 95 degreesC (95 degrees C) indicated pure preparations intensely associate through hydrophobic interactions through bridging by divalent phosphate and this effect was mitigated by decreasing the ionic strength, decreasing the phosphate charge to —1 (at pH 6.6) or decreasing the temperature. The aggregation behavior of a commercial a-La was investigated at neutral pH and 95'aC in a complex mineral salt environment to understand general stability factors involved in a nutritional beverage. The objective was to understand the effect of a-La lot variation, relative b-lactoglobulin concentration and excess calcium on the aggregate size development as measured by light scattering and turbidity development. The lot of holo-a-La possessing a higher intrinsic b-Lg concentration had higher solubility at pH 6.80, evolved more reactive thiol groups, had a 25% faster first order rate constant, dissociated only slightly with cooling and formed spherical aggregates with a much higher molecular weight. Aggregates intrinsic to the protein powder may play a role in aggregate growth and shape. Adding increasing quantities of b-Lg generally decreased solubility. The highest b-Lg concentrations investigated demonstrated a net thiol oxidation and, subsequently, had a diminished ability to aggregate through hydrophobic interactions. Adding excess calcium caused a dramatic loss of solubility at pH 7.0 and required an increase in pH to 7.5 to regain solubility.