Summary The adsorption of whole casein and some of its constituent caseins by halloysite and kaolinite in the presence and absence of urea and 2-mercaptoethanol has been studied. The order of adsorption generally observed for both clays in the absence of urea and 2-mercaptoethanol was temperature sensitive casein component (TS) > α s1 -casein B > α s1 -casein A > β-casein in the pH range 7·5–9·5, confirming that these two clays have structurally similar surfaces. In the presence of urea and 2-mercaptoethanol the rate of adsorption on to halloysite increased, although the quantity adsorbed decreased and the relative affinities of the major casein components at pH 8 were altered. However, urea and 2-mercaptoethanol did not alter the relative affinities of the caseins to kaolinite, although the quantity adsorbed did decrease. The casein components are probably adsorbed as monomers, and it may be the superior dispersing properties of the urea solution that decrease the amount adsorbed. The halloysite structure was altered by absorption of urea and the altered relative affinities of the various casein components in the presence of urea can be ascribed to that effect. The results support the hypothesis that adsorption occurs because the energy of the system is lowered due to changes in protein-solvent, clay-solvent and solvent-solvent (solvent structuring), as well as to electrostatic interactions.
Summary A study of the hydrolysis of β-casein-B by crystalline rennin or rennet extract at pH 6·5, using a disk electrophoresis technique, showed that 3 bonds in β-casein are appreciably more sensitive than the others to rennin proteolysis, and that these bonds are probably located near the C-terminus of the protein. The most susceptible bond is hydrolysed, at 10°C, about 200 times faster than any other bond, whilst at 37°C it is hydrolysed 60 times faster. A study of the hydrolysis of this bond showed that its rate of hydrolysis at 37°C and pH 6·5 is decreased by either increased ionic strength or increased calcium ion concentration at constant ionic strength. Conformational changes in the substrate are probably responsible for these effects.
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