Thio-induced oligomerization of alpha-lactalbumin at high pressure

Denaturation and aggregation of alpha-lactalbumin at high pressure (up to 10 kbar, 1000 MPa) were studied by means of circular dichroism, gel-permeation chromatography, sodium dodecyl sulfate and gel electrophoresis. It was found that the unfolding of alpha-lactalbumin at high pressure is reversible even in basic pH and at a protein concentration as large as 10%. In these conditions only a negligible fraction of the protein is denatured irreversibly and aggregates. The rate of aggregation of alpha-lactalbumin at high pressure increases significantly in the presence of low-molecular reducing agents such as cysteine, 2-mercaptoethanol, and dithiothreitol. Maximal yield of alpha-lactalbumin oligomerization (over 90%) was achieved in the presence of cysteine at the molar cysteine/protein ratio q = 2 and at pH8.5. Apparent molecular weight of the obtained oligomers was over 500 kDa. It was shown that the size distribution of oligomers can be modulated by varying pH and reducing agent. The size distribution shifts in the direction of very large, poorly soluble particles when pH decreases. Maximal content of the insoluble fraction (about 30%) can be reached at pH 5.5 when cysteine (q = 2) is used as reducing agent. The oligomers of alpha-lactalbumin are stabilized mainly by nonnative interchain disulfide bridges. Circular dichroism measurements point to an additional mechanism of cohesion of polypeptide chains in the oligomers, which is formation of intermolecular beta-sheets.