Reduction of aggregation of β-lactoglobulin during heating by dihydrolipoic acid

Prevention of the heat-induced aggregation of beta-lactoglobulin (beta-Lg) would improve the heat stability of whey proteins. The effects of lipoic acid (LA, or thioctic acid), in both its oxidised and reduced form (dihydrolipoic acid, DHLA), on heat-induced unfolding and aggregation of beta-Lg were investigated. LA/DHLA was added to native beta-Lg and the mixture was heated at 70, 75, 80 or 85 degrees C for up to 30 min at pH 6.8. The samples were analysed by Polyacrylamide Gel Electrophoresis (PAGE) and Size-exclusion HPLC (SE-HPLC). LA was not as effective as DHLA in reducing the formation of aggregates of heated beta-Lg. Heating beta-Lg with DHLA resulted in formation of more beta-Lg monomers (due to dissociation of native dimers) and significantly less beta-Lg aggregates, compared with heating beta-Lg alone. The aggregates formed in the presence of DHLA were both covalently linked, via disulphide bonds, and non-covalently (hydrophobically) linked, but the amount of covalently linked aggregates was much less than when beta-Lg was heated alone. The results suggest that DHLA was able to partially trap the reactive beta-Lg monomer containing a free sulphydryl (-SH) group, by forming a 'modified monomer', and to prevent some sulphydryl - sulphydryl and sulphydryl - disulphide interactions that lead to the formation of covalently linked protein aggregates. The effects of DHLA were similar to those of N-ethylmaleimide (NEM) and dithio(bis)-p-nitrobenzoate (DTNB). However, the advantage of using DHLA over NEM and DTNB to lessen aggregation of beta-Lg is that it is a food-grade compound which occurs naturally in milk.