Effect of Pressure on the Association States of Enzyme-treated Caseins

Abstract

The effects of pressure on the association states of enzyme-treated casein molecules were studied by monitoring the turbidity of solutions under high pressures up to 3, 000 kg/cm². β-Casein molecules partially degraded with immobilized trypsin were dissociated with increasing pressure up to a critical pressure (e.g., 1.200 kg/cm² in Tris-HC1 buffer of pH 6.4 and ionic strength 1.0, at 40°C) and then reassociated under higher pressures up to 3, 000 kg/cm², following a parabola-like turbidity-pressure curve. The critical pressure for minimum turbidity increased with increasing degradation of the hydrophobic moiety of β-casein. In the case of chymosin-treated κ-casein, a similar pressure dependence of turbidity due to dissociation and reassociation was found in the same pressure region up to 3, 000kg/cm². However, chymosin-treated α_s1-casein was not reassociated even at 3, 500kg/cm². Based on these results, the pressure effects on the dissociation and reassociation of casein molecules are discussed in terms of the volume change of water around their hydrophobic moieties.