Precipitation of egg white proteins below their isoelectric points by sodium dodecyl sulphate and temperature

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The precipitating effect of sodium dodecyl sulphate (SDS) on the egg white proteins ovalbumin, conalbumin and lysozyme was studied at 25°C and at different pH values. The proteins precipitated below their respective isoelectric points, provided the detergent to protein ratio was appropriate. The pH profile of precipitation was different for the three proteins reflecting net charge differences. The binding of SDS to the proteins was studied with [ 35S]-labelled SDS and for ovalbumin a ratio of 21–28 SDS molecules/protein molecule, dependent on the concentration of SDS initially used, seem to be required for precipitation at pH 4.5. This number, however, is dependent on pH and increases with an increased positive net charge of the protein. The precipitating effect of SDS was identical for ovalbumin, conalbumin and lysozyme when compared on a gram to gram basis (0.1–0.15 g SDS/g precipitated protein). The precipitated protein was denatured as measured by differential scanning calorimetry, but was also completely redissolved if pH was increased to above the isoelectric point. The precipitating effect of SDS was also examined at elevated temperatures. The two-phase systems of the proteins induced by SDS at 25°C were heated from 25°C to 90°C at a rate of 1.25°C/min. The precipitation behaviour was similar for the three proteins upon heating. When the SDS concentration was increased the precipitation curves were transferred towards lower temperatures and the courses of precipitation became less sharp. The synergistic effect of SDS and heat on protein precipitation was differentiated by denaturation measurements and radioactive labelling. The ratio SDS to precipitated protein gradually diminished towards higher temperatures but no purely thermal precipitation was found.