Enzyme thermostabilization by bovine serum albumin and other proteins: evidence for hydrophobic interactions

BSA stabilizes Streptococcus thermophilus beta-galactosidase against thermal inactivation and binds to the active enzyme subunits formed on heating. The mechanism of interaction and stabilization, however, is unknown, and it was investigated using different proteins. The results show that several proteins increased the enzyme half-life (t 1/2) at 64 degrees C in the presence of the substrate lactose. The best stabilizers were BSA (9-fold) and casein (6-fold). There was a significant correlation between enzyme half-life (t 1/2) and surface hydrophobicity of the proteins (So), of the form t 1/2 varies; is directly proportional to S0.5o. The surface hydrophobicity of the enzyme increased upon heating, while that of BSA declined. Heating enzyme and BSA together caused a net loss in surface hydrophobicity, indicating hydrophobic interactions, but there was no change in the absence of heating. Stabilization of the enzyme by BSA was markedly affected by chaotropic and kosmotropic salts. Stabilization was increased by 1 M Na2SO4 and reduced by 1 M NaBr; 1 M NaCl had little effect. None of the three salts increased the stability of the enzyme itself, indicating that the effect was on the enzyme-protein interaction. The results indicate that BSA stabilized the enzyme by hydrophobic interactions with the heated enzyme and that surface hydrophobicity is a major determinant of the extent of stabilization by a protein.