WHAT MAY BE BOVINE BETA-LACTOGLOBULIN CYS121 GOOD FOR

Mixed disulphide derivatives of bovine beta-lactoglobulin B (BLG) were studied by circular dichroism (CD), gel-permeation HPLC and high-sens itivity differential scanning calorimetry (HSDSC). It was shown that m odification of Cys121 of BLG results only in tertiary and quaternary s tructure changes. At neutral pH, the equilibrium dimer-monomer of modi fied BLG is shifted to monomeric form. Thermal denaturation of modifie d BLG is reversible both at neutral and acidic pH. The values of Delta G (37 degrees C) for modified and unmodified BLG indicate substantial destabilisation of tertiary structure of the protein by modification of Cys121. The effect of destabilisation is of the same order of magni tude for disulphide derivative containing charged and neutral group. T he nature of the stabilising role of Cys121 for tertiary structure of BLG is discussed. Activation of Cys121 by thermic treatment in milk is known to induce the disulphide bond formation of BLG with kappa-casei n. Its destabilisation by high pressure in beta-lactoglobulin/alpha-la ctalbumin (ALA) mixtures induce oligomerisation of ALA, triggering thi ol double left right arrow disulfide exchange reactions. (C) 1998 Else vier Science Ltd. All rights reserved.